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GNU tar: an archiver tool

GNU tar: an archiver tool
*************************

This manual is for GNU 'tar' (version 1.29, 4 May 2016), which creates
and extracts files from archives.

Copyright (C) 1992, 1994-1997, 1999-2001, 2003-2016 Free Software
Foundation, Inc.

Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with the Invariant Sections being "GNU General Public
License", with the Front-Cover Texts being "A GNU Manual", and with
the Back-Cover Texts as in (a) below. A copy of the license is
included in the section entitled "GNU Free Documentation License".

(a) The FSF's Back-Cover Text is: "You have the freedom to copy and
modify this GNU manual."

The first part of this master menu lists the major nodes in this Info
document. The rest of the menu lists all the lower level nodes.

1 Introduction
**************

GNU 'tar' creates and manipulates "archives" which are actually
collections of many other files; the program provides users with an
organized and systematic method for controlling a large amount of data.
The name "tar" originally came from the phrase "Tape ARchive", but
archives need not (and these days, typically do not) reside on tapes.

1.1 What this Book Contains
===========================

The first part of this chapter introduces you to various terms that will
recur throughout the book. It also tells you who has worked on GNU
'tar' and its documentation, and where you should send bug reports or
comments.

The second chapter is a tutorial (*note Tutorial::) which provides a
gentle introduction for people who are new to using 'tar'. It is meant
to be self-contained, not requiring any reading from subsequent chapters
to make sense. It moves from topic to topic in a logical, progressive
order, building on information already explained.

Although the tutorial is paced and structured to allow beginners to
learn how to use 'tar', it is not intended solely for beginners. The
tutorial explains how to use the three most frequently used operations
('create', 'list', and 'extract') as well as two frequently used options
('file' and 'verbose'). The other chapters do not refer to the tutorial
frequently; however, if a section discusses something which is a complex
variant of a basic concept, there may be a cross-reference to that basic
concept. (The entire book, including the tutorial, assumes that the
reader understands some basic concepts of using a Unix-type operating
system; *note Tutorial::.)

The third chapter presents the remaining five operations, and
information about using 'tar' options and option syntax.

The other chapters are meant to be used as a reference. Each chapter
presents everything that needs to be said about a specific topic.

One of the chapters (*note Date input formats::) exists in its
entirety in other GNU manuals, and is mostly self-contained. In
addition, one section of this manual (*note Standard::) contains a big
quote which is taken directly from 'tar' sources.

In general, we give both long and short (abbreviated) option names at
least once in each section where the relevant option is covered, so that
novice readers will become familiar with both styles. (A few options
have no short versions, and the relevant sections will indicate this.)

1.2 Some Definitions
====================

The 'tar' program is used to create and manipulate 'tar' archives. An
"archive" is a single file which contains the contents of many files,
while still identifying the names of the files, their owner(s), and so
forth. (In addition, archives record access permissions, user and
group, size in bytes, and data modification time. Some archives also
record the file names in each archived directory, as well as other file
and directory information.) You can use 'tar' to "create" a new archive
in a specified directory.

The files inside an archive are called "members". Within this
manual, we use the term "file" to refer only to files accessible in the
normal ways (by 'ls', 'cat', and so forth), and the term "member" to
refer only to the members of an archive. Similarly, a "file name" is
the name of a file, as it resides in the file system, and a "member
name" is the name of an archive member within the archive.

The term "extraction" refers to the process of copying an archive
member (or multiple members) into a file in the file system. Extracting
all the members of an archive is often called "extracting the archive".
The term "unpack" can also be used to refer to the extraction of many or
all the members of an archive. Extracting an archive does not destroy
the archive's structure, just as creating an archive does not destroy
the copies of the files that exist outside of the archive. You may also
"list" the members in a given archive (this is often thought of as
"printing" them to the standard output, or the command line), or
"append" members to a pre-existing archive. All of these operations can
be performed using 'tar'.

1.3 What 'tar' Does
===================

The 'tar' program provides the ability to create 'tar' archives, as well
as various other kinds of manipulation. For example, you can use 'tar'
on previously created archives to extract files, to store additional
files, or to update or list files which were already stored.

Initially, 'tar' archives were used to store files conveniently on
magnetic tape. The name 'tar' comes from this use; it stands for 't'ape
'ar'chiver. Despite the utility's name, 'tar' can direct its output to
available devices, files, or other programs (using pipes). 'tar' may
even access remote devices or files (as archives).

You can use 'tar' archives in many ways. We want to stress a few of
them: storage, backup, and transportation.

Storage
Often, 'tar' archives are used to store related files for
convenient file transfer over a network. For example, the GNU
Project distributes its software bundled into 'tar' archives, so
that all the files relating to a particular program (or set of
related programs) can be transferred as a single unit.

A magnetic tape can store several files in sequence. However, the
tape has no names for these files; it only knows their relative
position on the tape. One way to store several files on one tape
and retain their names is by creating a 'tar' archive. Even when
the basic transfer mechanism can keep track of names, as FTP can,
the nuisance of handling multiple files, directories, and multiple
links makes 'tar' archives useful.

Archive files are also used for long-term storage. You can think
of this as transportation from the present into the future. (It is
a science-fiction idiom that you can move through time as well as
in space; the idea here is that 'tar' can be used to move archives
in all dimensions, even time!)

Backup
Because the archive created by 'tar' is capable of preserving file
information and directory structure, 'tar' is commonly used for
performing full and incremental backups of disks. A backup puts a
collection of files (possibly pertaining to many users and
projects) together on a disk or a tape. This guards against
accidental destruction of the information in those files. GNU
'tar' has special features that allow it to be used to make
incremental and full dumps of all the files in a file system.

Transportation
You can create an archive on one system, transfer it to another
system, and extract the contents there. This allows you to
transport a group of files from one system to another.

1.4 How 'tar' Archives are Named
================================

Conventionally, 'tar' archives are given names ending with '.tar'. This
is not necessary for 'tar' to operate properly, but this manual follows
that convention in order to accustom readers to it and to make examples
more clear.

Often, people refer to 'tar' archives as "'tar' files," and archive
members as "files" or "entries". For people familiar with the operation
of 'tar', this causes no difficulty. However, in this manual, we
consistently refer to "archives" and "archive members" to make learning
to use 'tar' easier for novice users.

1.5 GNU 'tar' Authors
=====================

GNU 'tar' was originally written by John Gilmore, and modified by many
people. The GNU enhancements were written by Jay Fenlason, then Joy
Kendall, and the whole package has been further maintained by Thomas
Bushnell, n/BSG, Franc,ois Pinard, Paul Eggert, and finally Sergey
Poznyakoff with the help of numerous and kind users.

We wish to stress that 'tar' is a collective work, and owes much to
all those people who reported problems, offered solutions and other
insights, or shared their thoughts and suggestions. An impressive, yet
partial list of those contributors can be found in the 'THANKS' file
from the GNU 'tar' distribution.

Jay Fenlason put together a draft of a GNU 'tar' manual, borrowing
notes from the original man page from John Gilmore. This was withdrawn
in version 1.11. Thomas Bushnell, n/BSG and Amy Gorin worked on a
tutorial and manual for GNU 'tar'. Franc,ois Pinard put version 1.11.8
of the manual together by taking information from all these sources and
merging them. Melissa Weisshaus finally edited and redesigned the book
to create version 1.12. The book for versions from 1.14 up to 1.29 were
edited by the current maintainer, Sergey Poznyakoff.

For version 1.12, Daniel Hagerty contributed a great deal of
technical consulting. In particular, he is the primary author of *note
Backups::.

In July, 2003 GNU 'tar' was put on CVS at savannah.gnu.org (see
<http://savannah.gnu.org/projects/tar>), and active development and
maintenance work has started again. Currently GNU 'tar' is being
maintained by Paul Eggert, Sergey Poznyakoff and Jeff Bailey.

Support for POSIX archives was added by Sergey Poznyakoff.

1.6 Reporting bugs or suggestions
=================================

If you find problems or have suggestions about this program or manual,
please report them to 'bug-tar@gnu.org'.

When reporting a bug, please be sure to include as much detail as
possible, in order to reproduce it.

2 Tutorial Introduction to 'tar'
********************************

This chapter guides you through some basic examples of three 'tar'
operations: '--create', '--list', and '--extract'. If you already know
how to use some other version of 'tar', then you may not need to read
this chapter. This chapter omits most complicated details about how
'tar' works.

2.1 Assumptions this Tutorial Makes
===================================

This chapter is paced to allow beginners to learn about 'tar' slowly.
At the same time, we will try to cover all the basic aspects of these
three operations. In order to accomplish both of these tasks, we have
made certain assumptions about your knowledge before reading this
manual, and the hardware you will be using:

* Before you start to work through this tutorial, you should
understand what the terms "archive" and "archive member" mean
(*note Definitions::). In addition, you should understand
something about how Unix-type operating systems work, and you
should know how to use some basic utilities. For example, you
should know how to create, list, copy, rename, edit, and delete
files and directories; how to change between directories; and how
to figure out where you are in the file system. You should have
some basic understanding of directory structure and how files are
named according to which directory they are in. You should
understand concepts such as standard output and standard input,
what various definitions of the term 'argument' mean, and the
differences between relative and absolute file names.

* This manual assumes that you are working from your own home
directory (unless we state otherwise). In this tutorial, you will
create a directory to practice 'tar' commands in. When we show
file names, we will assume that those names are relative to your
home directory. For example, my home directory is
'/home/fsf/melissa'. All of my examples are in a subdirectory of
the directory named by that file name; the subdirectory is called
'practice'.

* In general, we show examples of archives which exist on (or can be
written to, or worked with from) a directory on a hard disk. In
most cases, you could write those archives to, or work with them on
any other device, such as a tape drive. However, some of the later
examples in the tutorial and next chapter will not work on tape
drives. Additionally, working with tapes is much more complicated
than working with hard disks. For these reasons, the tutorial does
not cover working with tape drives. *Note Media::, for complete
information on using 'tar' archives with tape drives.

2.2 Stylistic Conventions
=========================

In the examples, '$' represents a typical shell prompt. It precedes
lines you should type; to make this more clear, those lines are shown in
'this font', as opposed to lines which represent the computer's
response; those lines are shown in 'this font', or sometimes 'like
this'.

2.3 Basic 'tar' Operations and Options
======================================

'tar' can take a wide variety of arguments which specify and define the
actions it will have on the particular set of files or the archive. The
main types of arguments to 'tar' fall into one of two classes:
operations, and options.

Some arguments fall into a class called "operations"; exactly one of
these is both allowed and required for any instance of using 'tar'; you
may _not_ specify more than one. People sometimes speak of "operating
modes". You are in a particular operating mode when you have specified
the operation which specifies it; there are eight operations in total,
and thus there are eight operating modes.

The other arguments fall into the class known as "options". You are
not required to specify any options, and you are allowed to specify more
than one at a time (depending on the way you are using 'tar' at that
time). Some options are used so frequently, and are so useful for
helping you type commands more carefully that they are effectively
"required". We will discuss them in this chapter.

You can write most of the 'tar' operations and options in any of
three forms: long (mnemonic) form, short form, and old style. Some of
the operations and options have no short or "old" forms; however, the
operations and options which we will cover in this tutorial have
corresponding abbreviations. We will indicate those abbreviations
appropriately to get you used to seeing them. Note, that the "old
style" option forms exist in GNU 'tar' for compatibility with Unix
'tar'. In this book we present a full discussion of this way of writing
options and operations (*note Old Options::), and we discuss the other
two styles of writing options (*Note Long Options::, and *note Short
Options::).

In the examples and in the text of this tutorial, we usually use the
long forms of operations and options; but the "short" forms produce the
same result and can make typing long 'tar' commands easier. For
example, instead of typing

tar --create --verbose --file=afiles.tar apple angst aspic

you can type
tar -c -v -f afiles.tar apple angst aspic

or even
tar -cvf afiles.tar apple angst aspic

For more information on option syntax, see *note Advanced tar::. In
discussions in the text, when we name an option by its long form, we
also give the corresponding short option in parentheses.

The term, "option", can be confusing at times, since "operations" are
often lumped in with the actual, _optional_ "options" in certain general
class statements. For example, we just talked about "short and long
forms of options and operations". However, experienced 'tar' users
often refer to these by shorthand terms such as, "short and long
options". This term assumes that the "operations" are included, also.
Context will help you determine which definition of "options" to use.

Similarly, the term "command" can be confusing, as it is often used
in two different ways. People sometimes refer to 'tar' "commands". A
'tar' "command" is the entire command line of user input which tells
'tar' what to do -- including the operation, options, and any arguments
(file names, pipes, other commands, etc.). However, you will also
sometimes hear the term "the 'tar' command". When the word "command" is
used specifically like this, a person is usually referring to the 'tar'
_operation_, not the whole line. Again, use context to figure out which
of the meanings the speaker intends.

2.4 The Three Most Frequently Used Operations
=============================================

Here are the three most frequently used operations (both short and long
forms), as well as a brief description of their meanings. The rest of
this chapter will cover how to use these operations in detail. We will
present the rest of the operations in the next chapter.

'--create'
'-c'
Create a new 'tar' archive.
'--list'
'-t'
List the contents of an archive.
'--extract'
'-x'
Extract one or more members from an archive.

2.5 Two Frequently Used Options
===============================

To understand how to run 'tar' in the three operating modes listed
previously, you also need to understand how to use two of the options to
'tar': '--file' (which takes an archive file as an argument) and
'--verbose'. (You are usually not _required_ to specify either of these
options when you run 'tar', but they can be very useful in making things
more clear and helping you avoid errors.)

The '--file' Option
-------------------

'--file=ARCHIVE-NAME'
'-f ARCHIVE-NAME'
Specify the name of an archive file.

You can specify an argument for the '--file=ARCHIVE-NAME' ('-f
ARCHIVE-NAME') option whenever you use 'tar'; this option determines the
name of the archive file that 'tar' will work on.

If you don't specify this argument, then 'tar' will examine the
environment variable 'TAPE'. If it is set, its value will be used as
the archive name. Otherwise, 'tar' will use the default archive,
determined at compile time. Usually it is standard output or some
physical tape drive attached to your machine (you can verify what the
default is by running 'tar --show-defaults', *note defaults::). If
there is no tape drive attached, or the default is not meaningful, then
'tar' will print an error message. The error message might look roughly
like one of the following:

tar: can't open /dev/rmt8 : No such device or address
tar: can't open /dev/rsmt0 : I/O error

To avoid confusion, we recommend that you always specify an archive file
name by using '--file=ARCHIVE-NAME' ('-f ARCHIVE-NAME') when writing
your 'tar' commands. For more information on using the
'--file=ARCHIVE-NAME' ('-f ARCHIVE-NAME') option, see *note file::.

The '--verbose' Option
----------------------

'--verbose'
'-v'
Show the files being worked on as 'tar' is running.

'--verbose' ('-v') shows details about the results of running 'tar'.
This can be especially useful when the results might not be obvious.
For example, if you want to see the progress of 'tar' as it writes files
into the archive, you can use the '--verbose' option. In the beginning,
you may find it useful to use '--verbose' at all times; when you are
more accustomed to 'tar', you will likely want to use it at certain
times but not at others. We will use '--verbose' at times to help make
something clear, and we will give many examples both using and not using
'--verbose' to show the differences.

Each instance of '--verbose' on the command line increases the
verbosity level by one, so if you need more details on the output,
specify it twice.

When reading archives ('--list', '--extract', '--diff'), 'tar' by
default prints only the names of the members being extracted. Using
'--verbose' will show a full, 'ls' style member listing.

In contrast, when writing archives ('--create', '--append',
'--update'), 'tar' does not print file names by default. So, a single
'--verbose' option shows the file names being added to the archive,
while two '--verbose' options enable the full listing.

For example, to create an archive in verbose mode:

$ tar -cvf afiles.tar apple angst aspic
apple
angst
aspic

Creating the same archive with the verbosity level 2 could give:

$ tar -cvvf afiles.tar apple angst aspic
-rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
-rw-r--r-- gray/staff 11481 2006-06-09 12:06 angst
-rw-r--r-- gray/staff 23152 2006-06-09 12:06 aspic

This works equally well using short or long forms of options. Using
long forms, you would simply write out the mnemonic form of the option
twice, like this:

$ tar --create --verbose --verbose ...

Note that you must double the hyphens properly each time.

Later in the tutorial, we will give examples using
'--verbose --verbose'.

The full output consists of six fields:

* File type and permissions in symbolic form. These are displayed in
the same format as the first column of 'ls -l' output (*note
format=verbose: (fileutils)What information is listed.).

* Owner name and group separated by a slash character. If these data
are not available (for example, when listing a 'v7' format
archive), numeric ID values are printed instead.

* Size of the file, in bytes.

* File modification date in ISO 8601 format.

* File modification time.

* File name. If the name contains any special characters (white
space, newlines, etc.) these are displayed in an unambiguous form
using so called "quoting style". For the detailed discussion of
available styles and on how to use them, see *note quoting
styles::.

Depending on the file type, the name can be followed by some
additional information, described in the following table:

'-> LINK-NAME'
The file or archive member is a "symbolic link" and LINK-NAME
is the name of file it links to.

'link to LINK-NAME'
The file or archive member is a "hard link" and LINK-NAME is
the name of file it links to.

'--Long Link--'
The archive member is an old GNU format long link. You will
normally not encounter this.

'--Long Name--'
The archive member is an old GNU format long name. You will
normally not encounter this.

'--Volume Header--'
The archive member is a GNU "volume header" (*note Tape
Files::).

'--Continued at byte N--'
Encountered only at the beginning of a multi-volume archive
(*note Using Multiple Tapes::). This archive member is a
continuation from the previous volume. The number N gives the
offset where the original file was split.

'unknown file type C'
An archive member of unknown type. C is the type character
from the archive header. If you encounter such a message, it
means that either your archive contains proprietary member
types GNU 'tar' is not able to handle, or the archive is
corrupted.

For example, here is an archive listing containing most of the
special suffixes explained above:

V--------- 0/0 1536 2006-06-09 13:07 MyVolume--Volume Header--
-rw-r--r-- gray/staff 456783 2006-06-09 12:06 aspic--Continued at byte 32456--
-rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
lrwxrwxrwx gray/staff 0 2006-06-09 13:01 angst -> apple
-rw-r--r-- gray/staff 35793 2006-06-09 12:06 blues
hrw-r--r-- gray/staff 0 2006-06-09 12:06 music link to blues


Getting Help: Using the '--help' Option
---------------------------------------

'--help'

The '--help' option to 'tar' prints out a very brief list of all
operations and option available for the current version of 'tar'
available on your system.

2.6 How to Create Archives
==========================

One of the basic operations of 'tar' is '--create' ('-c'), which you use
to create a 'tar' archive. We will explain '--create' first because, in
order to learn about the other operations, you will find it useful to
have an archive available to practice on.

To make this easier, in this section you will first create a
directory containing three files. Then, we will show you how to create
an _archive_ (inside the new directory). Both the directory, and the
archive are specifically for you to practice on. The rest of this
chapter and the next chapter will show many examples using this
directory and the files you will create: some of those files may be
other directories and other archives.

The three files you will archive in this example are called 'blues',
'folk', and 'jazz'. The archive is called 'collection.tar'.

This section will proceed slowly, detailing how to use '--create' in
'verbose' mode, and showing examples using both short and long forms.
In the rest of the tutorial, and in the examples in the next chapter, we
will proceed at a slightly quicker pace. This section moves more slowly
to allow beginning users to understand how 'tar' works.

2.6.1 Preparing a Practice Directory for Examples
-------------------------------------------------

To follow along with this and future examples, create a new directory
called 'practice' containing files called 'blues', 'folk' and 'jazz'.
The files can contain any information you like: ideally, they should
contain information which relates to their names, and be of different
lengths. Our examples assume that 'practice' is a subdirectory of your
home directory.

Now 'cd' to the directory named 'practice'; 'practice' is now your
"working directory". (_Please note_: Although the full file name of
this directory is '/HOMEDIR/practice', in our examples we will refer to
this directory as 'practice'; the HOMEDIR is presumed.)

In general, you should check that the files to be archived exist
where you think they do (in the working directory) by running 'ls'.
Because you just created the directory and the files and have changed to
that directory, you probably don't need to do that this time.

It is very important to make sure there isn't already a file in the
working directory with the archive name you intend to use (in this case,
'collection.tar'), or that you don't care about its contents. Whenever
you use 'create', 'tar' will erase the current contents of the file
named by '--file=ARCHIVE-NAME' ('-f ARCHIVE-NAME') if it exists. 'tar'
will not tell you if you are about to overwrite an archive unless you
specify an option which does this (*note backup::, for the information
on how to do so). To add files to an existing archive, you need to use
a different option, such as '--append' ('-r'); see *note append:: for
information on how to do this.

2.6.2 Creating the Archive
--------------------------

To place the files 'blues', 'folk', and 'jazz' into an archive named
'collection.tar', use the following command:

$ tar --create --file=collection.tar blues folk jazz

The order of the arguments is not very important, _when using long
option forms_, however you should always remember to use option as the
first argument to tar. For example, the following is wrong:

$ tar blues -c folk -f collection.tar jazz
tar: -c: Invalid blocking factor
Try 'tar --help' or 'tar --usage' for more information.

The error message is produced because 'tar' always treats its first
argument as an option (or cluster of options), even if it does not start
with dash. This is "traditional" or "old option" style, called so
because all implementations of 'tar' have used it since the very
inception of the tar archiver in 1970s. This option style will be
explained later (*note Old Options::), for now just remember to always
place option as the first argument.

That being said, you could issue the following command:

$ tar --create folk blues --file=collection.tar jazz

However, you can see that this order is harder to understand; this is
why we will list the arguments in the order that makes the commands
easiest to understand (and we encourage you to do the same when you use
'tar', to avoid errors).

Note that the sequence '--file=collection.tar' is considered to be
_one_ argument. If you substituted any other string of characters for
'collection.tar', then that string would become the name of the archive
file you create.

The order of the options becomes more important when you begin to use
short forms. With short forms, if you type commands in the wrong order
(even if you type them correctly in all other ways), you may end up with
results you don't expect. For this reason, it is a good idea to get
into the habit of typing options in the order that makes inherent sense.
*Note short create::, for more information on this.

In this example, you type the command as shown above: '--create' is
the operation which creates the new archive ('collection.tar'), and
'--file' is the option which lets you give it the name you chose. The
files, 'blues', 'folk', and 'jazz', are now members of the archive,
'collection.tar' (they are "file name arguments" to the '--create'
operation. *Note Choosing::, for the detailed discussion on these.)
Now that they are in the archive, they are called _archive members_, not
files. (*note members: Definitions.).

When you create an archive, you _must_ specify which files you want
placed in the archive. If you do not specify any archive members, GNU
'tar' will complain.

If you now list the contents of the working directory ('ls'), you
will find the archive file listed as well as the files you saw
previously:

blues folk jazz collection.tar

Creating the archive 'collection.tar' did not destroy the copies of the
files in the directory.

Keep in mind that if you don't indicate an operation, 'tar' will not
run and will prompt you for one. If you don't name any files, 'tar'
will complain. You must have write access to the working directory, or
else you will not be able to create an archive in that directory.

_Caution_: Do not attempt to use '--create' ('-c') to add files to an
existing archive; it will delete the archive and write a new one. Use
'--append' ('-r') instead. *Note append::.

2.6.3 Running '--create' with '--verbose'
-----------------------------------------

If you include the '--verbose' ('-v') option on the command line, 'tar'
will list the files it is acting on as it is working. In verbose mode,
the 'create' example above would appear as:

$ tar --create --verbose --file=collection.tar blues folk jazz
blues
folk
jazz

This example is just like the example we showed which did not use
'--verbose', except that 'tar' generated the remaining lines.

In the rest of the examples in this chapter, we will frequently use
'verbose' mode so we can show actions or 'tar' responses that you would
otherwise not see, and which are important for you to understand.

2.6.4 Short Forms with 'create'
-------------------------------

As we said before, the '--create' ('-c') operation is one of the most
basic uses of 'tar', and you will use it countless times. Eventually,
you will probably want to use abbreviated (or "short") forms of options.
A full discussion of the three different forms that options can take
appears in *note Styles::; for now, here is what the previous example
(including the '--verbose' ('-v') option) looks like using short option
forms:

$ tar -cvf collection.tar blues folk jazz
blues
folk
jazz

As you can see, the system responds the same no matter whether you use
long or short option forms.

One difference between using short and long option forms is that,
although the exact placement of arguments following options is no more
specific when using short forms, it is easier to become confused and
make a mistake when using short forms. For example, suppose you
attempted the above example in the following way:

$ tar -cfv collection.tar blues folk jazz

In this case, 'tar' will make an archive file called 'v', containing the
files 'blues', 'folk', and 'jazz', because the 'v' is the closest "file
name" to the '-f' option, and is thus taken to be the chosen archive
file name. 'tar' will try to add a file called 'collection.tar' to the
'v' archive file; if the file 'collection.tar' did not already exist,
'tar' will report an error indicating that this file does not exist. If
the file 'collection.tar' does already exist (e.g., from a previous
command you may have run), then 'tar' will add this file to the archive.
Because the '-v' option did not get registered, 'tar' will not run under
'verbose' mode, and will not report its progress.

The end result is that you may be quite confused about what happened,
and possibly overwrite a file. To illustrate this further, we will show
you how an example we showed previously would look using short forms.

This example,

$ tar --create folk blues --file=collection.tar jazz

is confusing as it is. It becomes even more so when using short forms:

$ tar -c folk blues -f collection.tar jazz

It would be very easy to put the wrong string of characters immediately
following the '-f', but doing that could sacrifice valuable data.

For this reason, we recommend that you pay very careful attention to
the order of options and placement of file and archive names, especially
when using short option forms. Not having the option name written out
mnemonically can affect how well you remember which option does what,
and therefore where different names have to be placed.

2.6.5 Archiving Directories
---------------------------

You can archive a directory by specifying its directory name as a file
name argument to 'tar'. The files in the directory will be archived
relative to the working directory, and the directory will be re-created
along with its contents when the archive is extracted.

To archive a directory, first move to its superior directory. If you
have followed the previous instructions in this tutorial, you should
type:

$ cd ..
$

This will put you into the directory which contains 'practice', i.e.,
your home directory. Once in the superior directory, you can specify
the subdirectory, 'practice', as a file name argument. To store
'practice' in the new archive file 'music.tar', type:

$ tar --create --verbose --file=music.tar practice

'tar' should output:

practice/
practice/blues
practice/folk
practice/jazz
practice/collection.tar

Note that the archive thus created is not in the subdirectory
'practice', but rather in the current working directory--the directory
from which 'tar' was invoked. Before trying to archive a directory from
its superior directory, you should make sure you have write access to
the superior directory itself, not only the directory you are trying
archive with 'tar'. For example, you will probably not be able to store
your home directory in an archive by invoking 'tar' from the root
directory; *Note absolute::. (Note also that 'collection.tar', the
original archive file, has itself been archived. 'tar' will accept any
file as a file to be archived, regardless of its content. When
'music.tar' is extracted, the archive file 'collection.tar' will be
re-written into the file system).

If you give 'tar' a command such as

$ tar --create --file=foo.tar .

'tar' will report 'tar: ./foo.tar is the archive; not dumped'. This
happens because 'tar' creates the archive 'foo.tar' in the current
directory before putting any files into it. Then, when 'tar' attempts
to add all the files in the directory '.' to the archive, it notices
that the file './foo.tar' is the same as the archive 'foo.tar', and
skips it. (It makes no sense to put an archive into itself.) GNU 'tar'
will continue in this case, and create the archive normally, except for
the exclusion of that one file. (_Please note:_ Other implementations
of 'tar' may not be so clever; they will enter an infinite loop when
this happens, so you should not depend on this behavior unless you are
certain you are running GNU 'tar'. In general, it is wise to always
place the archive outside of the directory being dumped.)

2.7 How to List Archives
========================

Frequently, you will find yourself wanting to determine exactly what a
particular archive contains. You can use the '--list' ('-t') operation
to get the member names as they currently appear in the archive, as well
as various attributes of the files at the time they were archived. For
example, assuming 'practice' is your working directory, you can examine
the archive 'collection.tar' that you created in the last section with
the command,

$ tar --list --file=collection.tar

The output of 'tar' would then be:

blues
folk
jazz

Be sure to use a '--file=ARCHIVE-NAME' ('-f ARCHIVE-NAME') option just
as with '--create' ('-c') to specify the name of the archive.

You can specify one or more individual member names as arguments when
using 'list'. In this case, 'tar' will only list the names of members
you identify. For example, 'tar --list --file=collection.tar folk'
would only print 'folk':

$ tar --list --file=collection.tar folk
folk

If you use the '--verbose' ('-v') option with '--list', then 'tar'
will print out a listing reminiscent of 'ls -l', showing owner, file
size, and so forth. This output is described in detail in *note verbose
member listing::.

If you had used '--verbose' ('-v') mode, the example above would look
like:

$ tar --list --verbose --file=collection.tar folk
-rw-r--r-- myself/user 62 1990-05-23 10:55 folk

It is important to notice that the output of 'tar --list --verbose'
does not necessarily match that produced by 'tar --create --verbose'
while creating the archive. It is because GNU 'tar', unless told
explicitly not to do so, removes some directory prefixes from file names
before storing them in the archive (*Note absolute::, for more
information). In other words, in verbose mode GNU 'tar' shows "file
names" when creating an archive and "member names" when listing it.
Consider this example, run from your home directory:

$ tar --create --verbose --file practice.tar ~/practice
tar: Removing leading '/' from member names
/home/myself/practice/
/home/myself/practice/blues
/home/myself/practice/folk
/home/myself/practice/jazz
/home/myself/practice/collection.tar
$ tar --test --file practice.tar
home/myself/practice/
home/myself/practice/blues
home/myself/practice/folk
home/myself/practice/jazz
home/myself/practice/collection.tar

This default behavior can sometimes be inconvenient. You can force
GNU 'tar' show member names when creating archive by supplying
'--show-stored-names' option.

'--show-stored-names'
Print member (as opposed to _file_) names when creating the
archive.

With this option, both commands produce the same output:

$ tar --create --verbose --show-stored-names \
--file practice.tar ~/practice
tar: Removing leading '/' from member names
home/myself/practice/
home/myself/practice/blues
home/myself/practice/folk
home/myself/practice/jazz
home/myself/practice/collection.tar
$ tar --test --file practice.tar
home/myself/practice/
home/myself/practice/blues
home/myself/practice/folk
home/myself/practice/jazz
home/myself/practice/collection.tar

Since 'tar' preserves file names, those you wish to list must be
specified as they appear in the archive (i.e., relative to the directory
from which the archive was created). Continuing the example above:

$ tar --list --file=practice.tar folk
tar: folk: Not found in archive
tar: Exiting with failure status due to previous errors

the error message is produced because there is no member named
'folk', only one named 'home/myself/folk'.

If you are not sure of the exact file name, use "globbing patterns",
for example:

$ tar --list --file=practice.tar --wildcards '*/folk'
home/myself/practice/folk

*Note wildcards::, for a detailed discussion of globbing patterns and
related 'tar' command line options.

Listing the Contents of a Stored Directory
------------------------------------------

To get information about the contents of an archived directory, use the
directory name as a file name argument in conjunction with '--list'
('-t'). To find out file attributes, include the '--verbose' ('-v')
option.

For example, to find out about files in the directory 'practice', in
the archive file 'music.tar', type:

$ tar --list --verbose --file=music.tar practice

'tar' responds:

drwxrwxrwx myself/user 0 1990-05-31 21:49 practice/
-rw-r--r-- myself/user 42 1990-05-21 13:29 practice/blues
-rw-r--r-- myself/user 62 1990-05-23 10:55 practice/folk
-rw-r--r-- myself/user 40 1990-05-21 13:30 practice/jazz
-rw-r--r-- myself/user 10240 1990-05-31 21:49 practice/collection.tar

When you use a directory name as a file name argument, 'tar' acts on
all the files (including sub-directories) in that directory.

2.8 How to Extract Members from an Archive
==========================================

Creating an archive is only half the job--there is no point in storing
files in an archive if you can't retrieve them. The act of retrieving
members from an archive so they can be used and manipulated as
unarchived files again is called "extraction". To extract files from an
archive, use the '--extract' ('--get' or '-x') operation. As with
'--create', specify the name of the archive with '--file' ('-f') option.
Extracting an archive does not modify the archive in any way; you can
extract it multiple times if you want or need to.

Using '--extract', you can extract an entire archive, or specific
files. The files can be directories containing other files, or not. As
with '--create' ('-c') and '--list' ('-t'), you may use the short or the
long form of the operation without affecting the performance.

2.8.1 Extracting an Entire Archive
----------------------------------

To extract an entire archive, specify the archive file name only, with
no individual file names as arguments. For example,

$ tar -xvf collection.tar

produces this:

-rw-r--r-- myself/user 28 1996-10-18 16:31 jazz
-rw-r--r-- myself/user 21 1996-09-23 16:44 blues
-rw-r--r-- myself/user 20 1996-09-23 16:44 folk

2.8.2 Extracting Specific Files
-------------------------------

To extract specific archive members, give their exact member names as
arguments, as printed by '--list' ('-t'). If you had mistakenly deleted
one of the files you had placed in the archive 'collection.tar' earlier
(say, 'blues'), you can extract it from the archive without changing the
archive's structure. Its contents will be identical to the original
file 'blues' that you deleted.

First, make sure you are in the 'practice' directory, and list the
files in the directory. Now, delete the file, 'blues', and list the
files in the directory again.

You can now extract the member 'blues' from the archive file
'collection.tar' like this:

$ tar --extract --file=collection.tar blues

If you list the files in the directory again, you will see that the file
'blues' has been restored, with its original permissions, data
modification times, and owner.(1) (These parameters will be identical
to those which the file had when you originally placed it in the
archive; any changes you may have made before deleting the file from the
file system, however, will _not_ have been made to the archive member.)
The archive file, 'collection.tar', is the same as it was before you
extracted 'blues'. You can confirm this by running 'tar' with '--list'
('-t').

Remember that as with other operations, specifying the exact member
name is important (*Note failing commands::, for more examples).

You can extract a file to standard output by combining the above
options with the '--to-stdout' ('-O') option (*note Writing to Standard
Output::).

If you give the '--verbose' option, then '--extract' will print the
names of the archive members as it extracts them.

---------- Footnotes ----------

(1) This is only accidentally true, but not in general. Whereas
modification times are always restored, in most cases, one has to be
root for restoring the owner, and use a special option for restoring
permissions. Here, it just happens that the restoring user is also the
owner of the archived members, and that the current 'umask' is
compatible with original permissions.

2.8.3 Extracting Files that are Directories
-------------------------------------------

Extracting directories which are members of an archive is similar to
extracting other files. The main difference to be aware of is that if
the extracted directory has the same name as any directory already in
the working directory, then files in the extracted directory will be
placed into the directory of the same name. Likewise, if there are
files in the pre-existing directory with the same names as the members
which you extract, the files from the extracted archive will replace the
files already in the working directory (and possible subdirectories).
This will happen regardless of whether or not the files in the working
directory were more recent than those extracted (there exist, however,
special options that alter this behavior *note Writing::).

However, if a file was stored with a directory name as part of its
file name, and that directory does not exist under the working directory
when the file is extracted, 'tar' will create the directory.

We can demonstrate how to use '--extract' to extract a directory file
with an example. Change to the 'practice' directory if you weren't
there, and remove the files 'folk' and 'jazz'. Then, go back to the
parent directory and extract the archive 'music.tar'. You may either
extract the entire archive, or you may extract only the files you just
deleted. To extract the entire archive, don't give any file names as
arguments after the archive name 'music.tar'. To extract only the files
you deleted, use the following command:

$ tar -xvf music.tar practice/folk practice/jazz
practice/folk
practice/jazz

If you were to specify two '--verbose' ('-v') options, 'tar' would have
displayed more detail about the extracted files, as shown in the example
below:

$ tar -xvvf music.tar practice/folk practice/jazz
-rw-r--r-- me/user 28 1996-10-18 16:31 practice/jazz
-rw-r--r-- me/user 20 1996-09-23 16:44 practice/folk

Because you created the directory with 'practice' as part of the file
names of each of the files by archiving the 'practice' directory as
'practice', you must give 'practice' as part of the file names when you
extract those files from the archive.

2.8.4 Extracting Archives from Untrusted Sources
------------------------------------------------

Extracting files from archives can overwrite files that already exist.
If you receive an archive from an untrusted source, you should make a
new directory and extract into that directory, so that you don't have to
worry about the extraction overwriting one of your existing files. For
example, if 'untrusted.tar' came from somewhere else on the Internet,
and you don't necessarily trust its contents, you can extract it as
follows:

$ mkdir newdir
$ cd newdir
$ tar -xvf ../untrusted.tar

It is also a good practice to examine contents of the archive before
extracting it, using '--list' ('-t') option, possibly combined with
'--verbose' ('-v').

2.8.5 Commands That Will Fail
-----------------------------

Here are some sample commands you might try which will not work, and why
they won't work.

If you try to use this command,

$ tar -xvf music.tar folk jazz

you will get the following response:

tar: folk: Not found in archive
tar: jazz: Not found in archive

This is because these files were not originally _in_ the parent
directory '..', where the archive is located; they were in the
'practice' directory, and their file names reflect this:

$ tar -tvf music.tar
practice/blues
practice/folk
practice/jazz

Likewise, if you try to use this command,

$ tar -tvf music.tar folk jazz

you would get a similar response. Members with those names are not in
the archive. You must use the correct member names, or wildcards, in
order to extract the files from the archive.

If you have forgotten the correct names of the files in the archive,
use 'tar --list --verbose' to list them correctly.

To extract the member named 'practice/folk', you must specify

$ tar --extract --file=music.tar practice/folk

Notice also, that as explained above, the 'practice' directory will be
created, if it didn't already exist. There are options that allow you
to strip away a certain number of leading directory components (*note
transform::). For example,

$ tar --extract --file=music.tar --strip-components=1 folk

will extract the file 'folk' into the current working directory.

2.9 Going Further Ahead in this Manual
======================================

_(This message will disappear, once this node revised.)_

3 Invoking GNU 'tar'
********************

This chapter is about how one invokes the GNU 'tar' command, from the
command synopsis (*note Synopsis::). There are numerous options, and
many styles for writing them. One mandatory option specifies the
operation 'tar' should perform (*note Operation Summary::), other
options are meant to detail how this operation should be performed
(*note Option Summary::). Non-option arguments are not always
interpreted the same way, depending on what the operation is.

You will find in this chapter everything about option styles and
rules for writing them (*note Styles::). On the other hand, operations
and options are fully described elsewhere, in other chapters. Here, you
will find only synthetic descriptions for operations and options,
together with pointers to other parts of the 'tar' manual.

Some options are so special they are fully described right in this
chapter. They have the effect of inhibiting the normal operation of
'tar' or else, they globally alter the amount of feedback the user
receives about what is going on. These are the '--help' and '--version'
(*note help::), '--verbose' (*note verbose::) and '--interactive'
options (*note interactive::).

3.1 General Synopsis of 'tar'
=============================

The GNU 'tar' program is invoked as either one of:

tar OPTION... [NAME]...
tar LETTER... [ARGUMENT]... [OPTION]... [NAME]...

The second form is for when old options are being used.

You can use 'tar' to store files in an archive, to extract them from
an archive, and to do other types of archive manipulation. The primary
argument to 'tar', which is called the "operation", specifies which
action to take. The other arguments to 'tar' are either "options",
which change the way 'tar' performs an operation, or file names or
archive members, which specify the files or members 'tar' is to act on.

You can actually type in arguments in any order, even if in this
manual the options always precede the other arguments, to make examples
easier to understand. Further, the option stating the main operation
mode (the 'tar' main command) is usually given first.

Each NAME in the synopsis above is interpreted as an archive member
name when the main command is one of '--compare' ('--diff', '-d'),
'--delete', '--extract' ('--get', '-x'), '--list' ('-t') or '--update'
('-u'). When naming archive members, you must give the exact name of
the member in the archive, as it is printed by '--list'. For '--append'
('-r') and '--create' ('-c'), these NAME arguments specify the names of
either files or directory hierarchies to place in the archive. These
files or hierarchies should already exist in the file system, prior to
the execution of the 'tar' command.

'tar' interprets relative file names as being relative to the working
directory. 'tar' will make all file names relative (by removing leading
slashes when archiving or restoring files), unless you specify otherwise
(using the '--absolute-names' option). *Note absolute::, for more
information about '--absolute-names'.

If you give the name of a directory as either a file name or a member
name, then 'tar' acts recursively on all the files and directories
beneath that directory. For example, the name '/' identifies all the
files in the file system to 'tar'.

The distinction between file names and archive member names is
especially important when shell globbing is used, and sometimes a source
of confusion for newcomers. *Note wildcards::, for more information
about globbing. The problem is that shells may only glob using existing
files in the file system. Only 'tar' itself may glob on archive
members, so when needed, you must ensure that wildcard characters reach
'tar' without being interpreted by the shell first. Using a backslash
before '*' or '?', or putting the whole argument between quotes, is
usually sufficient for this.

Even if NAMEs are often specified on the command line, they can also
be read from a text file in the file system, using the
'--files-from=FILE-OF-NAMES' ('-T FILE-OF-NAMES') option.

If you don't use any file name arguments, '--append' ('-r'),
'--delete' and '--concatenate' ('--catenate', '-A') will do nothing,
while '--create' ('-c') will usually yield a diagnostic and inhibit
'tar' execution. The other operations of 'tar' ('--list', '--extract',
'--compare', and '--update') will act on the entire contents of the
archive.

Besides successful exits, GNU 'tar' may fail for many reasons. Some
reasons correspond to bad usage, that is, when the 'tar' command line is
improperly written. Errors may be encountered later, while processing
the archive or the files. Some errors are recoverable, in which case
the failure is delayed until 'tar' has completed all its work. Some
errors are such that it would be not meaningful, or at least risky, to
continue processing: 'tar' then aborts processing immediately. All
abnormal exits, whether immediate or delayed, should always be clearly
diagnosed on 'stderr', after a line stating the nature of the error.

Possible exit codes of GNU 'tar' are summarized in the following
table:

0
'Successful termination'.

1
'Some files differ'. If tar was invoked with '--compare'
('--diff', '-d') command line option, this means that some files in
the archive differ from their disk counterparts (*note compare::).
If tar was given '--create', '--append' or '--update' option, this
exit code means that some files were changed while being archived
and so the resulting archive does not contain the exact copy of the
file set.

2
'Fatal error'. This means that some fatal, unrecoverable error
occurred.

If 'tar' has invoked a subprocess and that subprocess exited with a
nonzero exit code, 'tar' exits with that code as well. This can happen,
for example, if 'tar' was given some compression option (*note gzip::)
and the external compressor program failed. Another example is 'rmt'
failure during backup to the remote device (*note Remote Tape Server::).

3.2 Using 'tar' Options
=======================

GNU 'tar' has a total of eight operating modes which allow you to
perform a variety of tasks. You are required to choose one operating
mode each time you employ the 'tar' program by specifying one, and only
one operation as an argument to the 'tar' command (the corresponding
options may be found at *note frequent operations:: and *note
Operations::). Depending on circumstances, you may also wish to
customize how the chosen operating mode behaves. For example, you may
wish to change the way the output looks, or the format of the files that
you wish to archive may require you to do something special in order to
make the archive look right.

You can customize and control 'tar''s performance by running 'tar'
with one or more options (such as '--verbose' ('-v'), which we used in
the tutorial). As we said in the tutorial, "options" are arguments to
'tar' which are (as their name suggests) optional. Depending on the
operating mode, you may specify one or more options. Different options
will have different effects, but in general they all change details of
the operation, such as archive format, archive name, or level of user
interaction. Some options make sense with all operating modes, while
others are meaningful only with particular modes. You will likely use
some options frequently, while you will only use others infrequently, or
not at all. (A full list of options is available in *note All
Options::.)

The 'TAR_OPTIONS' environment variable specifies default options to
be placed in front of any explicit options. For example, if
'TAR_OPTIONS' is '-v --unlink-first', 'tar' behaves as if the two
options '-v' and '--unlink-first' had been specified before any explicit
options. Option specifications are separated by whitespace. A
backslash escapes the next character, so it can be used to specify an
option containing whitespace or a backslash.

Note that 'tar' options are case sensitive. For example, the options
'-T' and '-t' are different; the first requires an argument for stating
the name of a file providing a list of NAMEs, while the second does not
require an argument and is another way to write '--list' ('-t').

In addition to the eight operations, there are many options to 'tar',
and three different styles for writing both: long (mnemonic) form, short
form, and old style. These styles are discussed below. Both the
options and the operations can be written in any of these three styles.

3.3 The Three Option Styles
===========================

There are three styles for writing operations and options to the command
line invoking 'tar'. The different styles were developed at different
times during the history of 'tar'. These styles will be presented
below, from the most recent to the oldest.

Some options must take an argument(1). Where you _place_ the
arguments generally depends on which style of options you choose. We
will detail specific information relevant to each option style in the
sections on the different option styles, below. The differences are
subtle, yet can often be very important; incorrect option placement can
cause you to overwrite a number of important files. We urge you to note
these differences, and only use the option style(s) which makes the most
sense to you until you feel comfortable with the others.

Some options _may_ take an argument. Such options may have at most
long and short forms, they do not have old style equivalent. The rules
for specifying an argument for such options are stricter than those for
specifying mandatory arguments. Please, pay special attention to them.

---------- Footnotes ----------

(1) For example, '--file' ('-f') takes the name of an archive file as
an argument. If you do not supply an archive file name, 'tar' will use
a default, but this can be confusing; thus, we recommend that you always
supply a specific archive file name.

3.3.1 Long Option Style
-----------------------

Each option has at least one "long" (or "mnemonic") name starting with
two dashes in a row, e.g., '--list'. The long names are more clear than
their corresponding short or old names. It sometimes happens that a
single long option has many different names which are synonymous, such
as '--compare' and '--diff'. In addition, long option names can be
given unique abbreviations. For example, '--cre' can be used in place
of '--create' because there is no other long option which begins with
'cre'. (One way to find this out is by trying it and seeing what
happens; if a particular abbreviation could represent more than one
option, 'tar' will tell you that that abbreviation is ambiguous and
you'll know that that abbreviation won't work. You may also choose to
run 'tar --help' to see a list of options. Be aware that if you run
'tar' with a unique abbreviation for the long name of an option you
didn't want to use, you are stuck; 'tar' will perform the command as
ordered.)

Long options are meant to be obvious and easy to remember, and their
meanings are generally easier to discern than those of their
corresponding short options (see below). For example:

$ tar --create --verbose --blocking-factor=20 --file=/dev/rmt0

gives a fairly good set of hints about what the command does, even for
those not fully acquainted with 'tar'.

Long options which require arguments take those arguments immediately
following the option name. There are two ways of specifying a mandatory
argument. It can be separated from the option name either by an equal
sign, or by any amount of white space characters. For example, the
'--file' option (which tells the name of the 'tar' archive) is given a
file such as 'archive.tar' as argument by using any of the following
notations: '--file=archive.tar' or '--file archive.tar'.

In contrast, optional arguments must always be introduced using an
equal sign. For example, the '--backup' option takes an optional
argument specifying backup type. It must be used as
'--backup=BACKUP-TYPE'.

3.3.2 Short Option Style
------------------------

Most options also have a "short option" name. Short options start with
a single dash, and are followed by a single character, e.g., '-t' (which
is equivalent to '--list'). The forms are absolutely identical in
function; they are interchangeable.

The short option names are faster to type than long option names.

Short options which require arguments take their arguments
immediately following the option, usually separated by white space. It
is also possible to stick the argument right after the short option
name, using no intervening space. For example, you might write '-f archive.tar'
or '-farchive.tar' instead of using '--file=archive.tar'. Both
'--file=ARCHIVE-NAME' and '-f ARCHIVE-NAME' denote the option which
indicates a specific archive, here named 'archive.tar'.

Short options which take optional arguments take their arguments
immediately following the option letter, _without any intervening white
space characters_.

Short options' letters may be clumped together, but you are not
required to do this (as compared to old options; see below). When short
options are clumped as a set, use one (single) dash for them all, e.g.,
''tar' -cvf'. Only the last option in such a set is allowed to have an
argument(1).

When the options are separated, the argument for each option which
requires an argument directly follows that option, as is usual for Unix
programs. For example:

$ tar -c -v -b 20 -f /dev/rmt0

If you reorder short options' locations, be sure to move any
arguments that belong to them. If you do not move the arguments
properly, you may end up overwriting files.

---------- Footnotes ----------

(1) Clustering many options, the last of which has an argument, is a
rather opaque way to write options. Some wonder if GNU 'getopt' should
not even be made helpful enough for considering such usages as invalid.

3.3.3 Old Option Style
----------------------

As far as we know, all 'tar' programs, GNU and non-GNU, support "old
options": that is, if the first argument does not start with '-', it is
assumed to specify option letters. GNU 'tar' supports old options not
only for historical reasons, but also because many people are used to
them. If the first argument does not start with a dash, you are
announcing the old option style instead of the short option style; old
options are decoded differently.

Like short options, old options are single letters. However, old
options must be written together as a single clumped set, without spaces
separating them or dashes preceding them. This set of letters must be
the first to appear on the command line, after the 'tar' program name
and some white space; old options cannot appear anywhere else. The
letter of an old option is exactly the same letter as the corresponding
short option. For example, the old option 't' is the same as the short
option '-t', and consequently, the same as the long option '--list'. So
for example, the command 'tar cv' specifies the option '-v' in addition
to the operation '-c'.

When options that need arguments are given together with the command,
all the associated arguments follow, in the same order as the options.
Thus, the example given previously could also be written in the old
style as follows:

$ tar cvbf 20 /dev/rmt0

Here, '20' is the argument of '-b' and '/dev/rmt0' is the argument of
'-f'.

The old style syntax can make it difficult to match option letters
with their corresponding arguments, and is often confusing. In the
command 'tar cvbf 20 /dev/rmt0', for example, '20' is the argument for
'-b', '/dev/rmt0' is the argument for '-f', and '-v' does not have a
corresponding argument. Even using short options like in 'tar -c -v -b 20 -f /dev/rmt0'
is clearer, putting all arguments next to the option they pertain to.

If you want to reorder the letters in the old option argument, be
sure to reorder any corresponding argument appropriately.

This old way of writing 'tar' options can surprise even experienced
users. For example, the two commands:

tar cfz archive.tar.gz file
tar -cfz archive.tar.gz file

are quite different. The first example uses 'archive.tar.gz' as the
value for option 'f' and recognizes the option 'z'. The second example,
however, uses 'z' as the value for option 'f' -- probably not what was
intended.

This second example could be corrected in many ways, among which the
following are equivalent:

tar -czf archive.tar.gz file
tar -cf archive.tar.gz -z file
tar cf archive.tar.gz -z file

3.3.4 Mixing Option Styles
--------------------------

All three styles may be intermixed in a single 'tar' command, so long as
the rules for each style are fully respected(1). Old style options and
either of the modern styles of options may be mixed within a single
'tar' command. However, old style options must be introduced as the
first arguments only, following the rule for old options (old options
must appear directly after the 'tar' command and some white space).
Modern options may be given only after all arguments to the old options
have been collected. If this rule is not respected, a modern option
might be falsely interpreted as the value of the argument to one of the
old style options.

For example, all the following commands are wholly equivalent, and
illustrate the many combinations and orderings of option styles.

tar --create --file=archive.tar
tar --create -f archive.tar
tar --create -farchive.tar
tar --file=archive.tar --create
tar --file=archive.tar -c
tar -c --file=archive.tar
tar -c -f archive.tar
tar -c -farchive.tar
tar -cf archive.tar
tar -cfarchive.tar
tar -f archive.tar --create
tar -f archive.tar -c
tar -farchive.tar --create
tar -farchive.tar -c
tar c --file=archive.tar
tar c -f archive.tar
tar c -farchive.tar
tar cf archive.tar
tar f archive.tar --create
tar f archive.tar -c
tar fc archive.tar

On the other hand, the following commands are _not_ equivalent to the
previous set:

tar -f -c archive.tar
tar -fc archive.tar
tar -fcarchive.tar
tar -farchive.tarc
tar cfarchive.tar

These last examples mean something completely different from what the
user intended (judging based on the example in the previous set which
uses long options, whose intent is therefore very clear). The first
four specify that the 'tar' archive would be a file named '-c', 'c',
'carchive.tar' or 'archive.tarc', respectively. The first two examples
also specify a single non-option, NAME argument having the value
'archive.tar'. The last example contains only old style option letters
(repeating option 'c' twice), not all of which are meaningful (eg., '.',
'h', or 'i'), with no argument value.

---------- Footnotes ----------

(1) Before GNU 'tar' version 1.11.6, a bug prevented intermixing old
style options with long options in some cases.

3.4 All 'tar' Options
=====================

The coming manual sections contain an alphabetical listing of all 'tar'
operations and options, with brief descriptions and cross-references to
more in-depth explanations in the body of the manual. They also contain
an alphabetically arranged table of the short option forms with their
corresponding long option. You can use this table as a reference for
deciphering 'tar' commands in scripts.

3.4.1 Operations
----------------

'--append'
'-r'

Appends files to the end of the archive. *Note append::.

'--catenate'
'-A'

Same as '--concatenate'. *Note concatenate::.

'--compare'
'-d'

Compares archive members with their counterparts in the file
system, and reports differences in file size, mode, owner,
modification date and contents. *Note compare::.

'--concatenate'
'-A'

Appends other 'tar' archives to the end of the archive. *Note
concatenate::.

'--create'
'-c'

Creates a new 'tar' archive. *Note create::.

'--delete'

Deletes members from the archive. Don't try this on an archive on
a tape! *Note delete::.

'--diff'
'-d'

Same '--compare'. *Note compare::.

'--extract'
'-x'

Extracts members from the archive into the file system. *Note
extract::.

'--get'
'-x'

Same as '--extract'. *Note extract::.

'--list'
'-t'

Lists the members in an archive. *Note list::.

'--update'
'-u'

Adds files to the end of the archive, but only if they are newer
than their counterparts already in the archive, or if they do not
already exist in the archive. *Note update::.

3.4.2 'tar' Options
-------------------

'--absolute-names'
'-P'

Normally when creating an archive, 'tar' strips an initial '/' from
member names, and when extracting from an archive 'tar' treats
names specially if they have initial '/' or internal '..'. This
option disables that behavior. *Note absolute::.

'--acls'
Enable POSIX ACLs support. *Note acls: Extended File Attributes.

'--after-date'

(See '--newer', *note after::)

'--anchored'
A pattern must match an initial subsequence of the name's
components. *Note controlling pattern-matching::.

'--atime-preserve'
'--atime-preserve=replace'
'--atime-preserve=system'

Attempt to preserve the access time of files when reading them.
This option currently is effective only on files that you own,
unless you have superuser privileges.

'--atime-preserve=replace' remembers the access time of a file
before reading it, and then restores the access time afterwards.
This may cause problems if other programs are reading the file at
the same time, as the times of their accesses will be lost. On
most platforms restoring the access time also requires 'tar' to
restore the data modification time too, so this option may also
cause problems if other programs are writing the file at the same
time ('tar' attempts to detect this situation, but cannot do so
reliably due to race conditions). Worse, on most platforms
restoring the access time also updates the status change time,
which means that this option is incompatible with incremental
backups.

'--atime-preserve=system' avoids changing time stamps on files,
without interfering with time stamp updates caused by other
programs, so it works better with incremental backups. However, it
requires a special 'O_NOATIME' option from the underlying operating
and file system implementation, and it also requires that searching
directories does not update their access times. As of this writing
(November 2005) this works only with Linux, and only with Linux
kernels 2.6.8 and later. Worse, there is currently no reliable way
to know whether this feature actually works. Sometimes 'tar' knows
that it does not work, and if you use '--atime-preserve=system'
then 'tar' complains and exits right away. But other times 'tar'
might think that the option works when it actually does not.

Currently '--atime-preserve' with no operand defaults to
'--atime-preserve=replace', but this may change in the future as
support for '--atime-preserve=system' improves.

If your operating or file system does not support
'--atime-preserve=system', you might be able to preserve access
times reliably by using the 'mount' command. For example, you can
mount the file system read-only, or access the file system via a
read-only loopback mount, or use the 'noatime' mount option
available on some systems. However, mounting typically requires
superuser privileges and can be a pain to manage.

'--auto-compress'
'-a'

During a '--create' operation, enables automatic compressed format
recognition based on the archive suffix. The effect of this option
is cancelled by '--no-auto-compress'. *Note gzip::.

'--backup=BACKUP-TYPE'

Rather than deleting files from the file system, 'tar' will back
them up using simple or numbered backups, depending upon
BACKUP-TYPE. *Note backup::.

'--block-number'
'-R'

With this option present, 'tar' prints error messages for read
errors with the block number in the archive file. *Note
block-number::.

'--blocking-factor=BLOCKING'
'-b BLOCKING'

Sets the blocking factor 'tar' uses to BLOCKING x 512 bytes per
record. *Note Blocking Factor::.

'--bzip2'
'-j'

This option tells 'tar' to read or write archives through 'bzip2'.
*Note gzip::.

'--check-device'
Check device numbers when creating a list of modified files for
incremental archiving. This is the default. *Note device
numbers::, for a detailed description.

'--checkpoint[=NUMBER]'

This option directs 'tar' to print periodic checkpoint messages as
it reads through the archive. It is intended for when you want a
visual indication that 'tar' is still running, but don't want to
see '--verbose' output. You can also instruct 'tar' to execute a
list of actions on each checkpoint, see '--checkpoint-action'
below. For a detailed description, see *note checkpoints::.

'--checkpoint-action=ACTION'
Instruct 'tar' to execute an action upon hitting a breakpoint.
Here we give only a brief outline. *Note checkpoints::, for a
complete description.

The ACTION argument can be one of the following:

bell
Produce an audible bell on the console.

dot
.
Print a single dot on the standard listing stream.

echo
Display a textual message on the standard error, with the
status and number of the checkpoint. This is the default.

echo=STRING
Display STRING on the standard error. Before output, the
string is subject to meta-character expansion.

exec=COMMAND
Execute the given COMMAND.

sleep=TIME
Wait for TIME seconds.

ttyout=STRING
Output STRING on the current console ('/dev/tty').

Several '--checkpoint-action' options can be specified. The
supplied actions will be executed in order of their appearance in
the command line.

Using '--checkpoint-action' without '--checkpoint' assumes default
checkpoint frequency of one checkpoint per 10 records.

'--check-links'
'-l'
If this option was given, 'tar' will check the number of links
dumped for each processed file. If this number does not match the
total number of hard links for the file, a warning message will be
output (1).

*Note hard links::.

'--compress'
'--uncompress'
'-Z'

'tar' will use the 'compress' program when reading or writing the
archive. This allows you to directly act on archives while saving
space. *Note gzip::.

'--clamp-mtime'

(See '--mtime'.)

'--confirmation'

(See '--interactive'.) *Note interactive::.

'--delay-directory-restore'

Delay setting modification times and permissions of extracted
directories until the end of extraction. *Note Directory
Modification Times and Permissions::.

'--dereference'
'-h'

When reading or writing a file to be archived, 'tar' accesses the
file that a symbolic link points to, rather than the symlink
itself. *Note dereference::.

'--directory=DIR'
'-C DIR'

When this option is specified, 'tar' will change its current
directory to DIR before performing any operations. When this
option is used during archive creation, it is order sensitive.
*Note directory::.

'--exclude=PATTERN'

When performing operations, 'tar' will skip files that match
PATTERN. *Note exclude::.

'--exclude-backups'
Exclude backup and lock files. *Note exclude-backups: exclude.

'--exclude-from=FILE'
'-X FILE'

Similar to '--exclude', except 'tar' will use the list of patterns
in the file FILE. *Note exclude::.

'--exclude-caches'

Exclude from dump any directory containing a valid cache directory
tag file, but still dump the directory node and the tag file
itself.

*Note exclude-caches: exclude.

'--exclude-caches-under'

Exclude from dump any directory containing a valid cache directory
tag file, but still dump the directory node itself.

*Note exclude::.

'--exclude-caches-all'

Exclude from dump any directory containing a valid cache directory
tag file. *Note exclude::.

'--exclude-ignore=FILE'
Before dumping a directory, 'tar' checks if it contains FILE. If
so, exclusion patterns are read from this file. The patterns
affect only the directory itself. *Note exclude::.

'--exclude-ignore-recursive=FILE'
Before dumping a directory, 'tar' checks if it contains FILE. If
so, exclusion patterns are read from this file. The patterns
affect the directory and all itssubdirectories. *Note exclude::.

'--exclude-tag=FILE'

Exclude from dump any directory containing file named FILE, but
dump the directory node and FILE itself. *Note exclude-tag:
exclude.

'--exclude-tag-under=FILE'

Exclude from dump the contents of any directory containing file
named FILE, but dump the directory node itself. *Note
exclude-tag-under: exclude.

'--exclude-tag-all=FILE'

Exclude from dump any directory containing file named FILE. *Note
exclude-tag-all: exclude.

'--exclude-vcs'

Exclude from dump directories and files, that are internal for some
widely used version control systems.

*Note exclude-vcs::.

'--exclude-vcs-ignores'
Exclude files that match patterns read from VCS-specific ignore
files. Supported files are: '.cvsignore', '.gitignore',
'.bzrignore', and '.hgignore'. The semantics of each file is the
same as for the corresponding VCS, e.g. patterns read from
'.gitignore' affect the directory and all its subdirectories.
*Note exclude-vcs-ignores::.

'--file=ARCHIVE'
'-f ARCHIVE'

'tar' will use the file ARCHIVE as the 'tar' archive it performs
operations on, rather than 'tar''s compilation dependent default.
*Note file tutorial::.

'--files-from=FILE'
'-T FILE'

'tar' will use the contents of FILE as a list of archive members or
files to operate on, in addition to those specified on the
command-line. *Note files::.

'--force-local'

Forces 'tar' to interpret the file name given to '--file' as a
local file, even if it looks like a remote tape drive name. *Note
local and remote archives::.

'--format=FORMAT'
'-H FORMAT'

Selects output archive format. FORMAT may be one of the following:

'v7'
Creates an archive that is compatible with Unix V7 'tar'.

'oldgnu'
Creates an archive that is compatible with GNU 'tar' version
1.12 or earlier.

'gnu'
Creates archive in GNU tar 1.13 format. Basically it is the
same as 'oldgnu' with the only difference in the way it
handles long numeric fields.

'ustar'
Creates a POSIX.1-1988 compatible archive.

'posix'
Creates a POSIX.1-2001 archive.

*Note Formats::, for a detailed discussion of these formats.

'--full-time'
This option instructs 'tar' to print file times to their full
resolution. Usually this means 1-second resolution, but that
depends on the underlying file system. The '--full-time' option
takes effect only when detailed output (verbosity level 2 or
higher) has been requested using the '--verbose' option, e.g., when
listing or extracting archives:

$ tar -t -v --full-time -f archive.tar

or, when creating an archive:

$ tar -c -vv --full-time -f archive.tar .

Notice, thar when creating the archive you need to specify
'--verbose' twice to get a detailed output (*note verbose
tutorial::).

'--group=GROUP'

Files added to the 'tar' archive will have a group ID of GROUP,
rather than the group from the source file. GROUP can specify a
symbolic name, or a numeric ID, or both as NAME:ID. *Note
override::.

Also see the '--group-map' option and comments for the
'--owner=USER' option.

'--group-map=FILE'

Read owner group translation map from FILE. This option allows to
translate only certain group names and/or UIDs. *Note override::,
for a detailed description. When used together with '--group'
option, the latter affects only those files whose owner group is
not listed in the FILE.

This option does not affect extraction from archives.

'--gzip'
'--gunzip'
'--ungzip'
'-z'

This option tells 'tar' to read or write archives through 'gzip',
allowing 'tar' to directly operate on several kinds of compressed
archives transparently. *Note gzip::.

'--hard-dereference'
When creating an archive, dereference hard links and store the
files they refer to, instead of creating usual hard link members.

*Note hard links::.

'--help'
'-?'

'tar' will print out a short message summarizing the operations and
options to 'tar' and exit. *Note help::.

'--hole-detection=METHOD'
Use METHOD to detect holes in sparse files. This option implies
'--sparse'. Valid methods are 'seek' and 'raw'. Default is 'seek'
with fallback to 'raw' when not applicable. *Note sparse::.

'--ignore-case'
Ignore case when matching member or file names with patterns.
*Note controlling pattern-matching::.

'--ignore-command-error'
Ignore exit codes of subprocesses. *Note Writing to an External
Program::.

'--ignore-failed-read'

Do not exit unsuccessfully merely because an unreadable file was
encountered. *Note Ignore Failed Read::.

'--ignore-zeros'
'-i'

With this option, 'tar' will ignore zeroed blocks in the archive,
which normally signals EOF. *Note Reading::.

'--incremental'
'-G'

Informs 'tar' that it is working with an old GNU-format incremental
backup archive. It is intended primarily for backwards
compatibility only. *Note Incremental Dumps::, for a detailed
discussion of incremental archives.

'--index-file=FILE'

Send verbose output to FILE instead of to standard output.

'--info-script=COMMAND'
'--new-volume-script=COMMAND'
'-F COMMAND'

When 'tar' is performing multi-tape backups, COMMAND is run at the
end of each tape. If it exits with nonzero status, 'tar' fails
immediately. *Note info-script::, for a detailed discussion of
this feature.

'--interactive'
'--confirmation'
'-w'

Specifies that 'tar' should ask the user for confirmation before
performing potentially destructive options, such as overwriting
files. *Note interactive::.

'--keep-directory-symlink'

This option changes the behavior of tar when it encounters a
symlink with the same name as the directory that it is about to
extract. By default, in this case tar would first remove the
symlink and then proceed extracting the directory.

The '--keep-directory-symlink' option disables this behavior and
instructs tar to follow symlinks to directories when extracting
from the archive.

It is mainly intended to provide compatibility with the Slackware
installation scripts.

'--keep-newer-files'

Do not replace existing files that are newer than their archive
copies when extracting files from an archive.

'--keep-old-files'
'-k'

Do not overwrite existing files when extracting files from an
archive. Return error if such files exist. See also *note
--skip-old-files::.

*Note Keep Old Files::.

'--label=NAME'
'-V NAME'

When creating an archive, instructs 'tar' to write NAME as a name
record in the archive. When extracting or listing archives, 'tar'
will only operate on archives that have a label matching the
pattern specified in NAME. *Note Tape Files::.

'--level=N'
Force incremental backup of level N. As of GNU 'tar' version 1.29,
the option '--level=0' truncates the snapshot file, thereby forcing
the level 0 dump. Other values of N are effectively ignored.
*Note --level=0::, for details and examples.

The use of this option is valid only in conjunction with the
'--listed-incremental' option. *Note Incremental Dumps::, for a
detailed description.

'--listed-incremental=SNAPSHOT-FILE'
'-g SNAPSHOT-FILE'

During a '--create' operation, specifies that the archive that
'tar' creates is a new GNU-format incremental backup, using
SNAPSHOT-FILE to determine which files to backup. With other
operations, informs 'tar' that the archive is in incremental
format. *Note Incremental Dumps::.

'--lzip'

This option tells 'tar' to read or write archives through 'lzip'.
*Note gzip::.

'--lzma'

This option tells 'tar' to read or write archives through 'lzma'.
*Note gzip::.

'--lzop'

This option tells 'tar' to read or write archives through 'lzop'.
*Note gzip::.

'--mode=PERMISSIONS'

When adding files to an archive, 'tar' will use PERMISSIONS for the
archive members, rather than the permissions from the files.
PERMISSIONS can be specified either as an octal number or as
symbolic permissions, like with 'chmod'. *Note override::.

'--mtime=DATE'

When adding files to an archive, 'tar' will use DATE as the
modification time of members when creating archives, instead of
their actual modification times. The value of DATE can be either a
textual date representation (*note Date input formats::) or a name
of the existing file, starting with '/' or '.'. In the latter
case, the modification time of that file is used. *Note
override::.

When '--clamp-mtime' is also specified, files with modification
times earlier than DATE will retain their actual modification
times, and DATE will only be used for files whose modification
times are later than DATE.

'--multi-volume'
'-M'

Informs 'tar' that it should create or otherwise operate on a
multi-volume 'tar' archive. *Note Using Multiple Tapes::.

'--new-volume-script'

(see '--info-script')

'--newer=DATE'
'--after-date=DATE'
'-N'

When creating an archive, 'tar' will only add files that have
changed since DATE. If DATE begins with '/' or '.', it is taken to
be the name of a file whose data modification time specifies the
date. *Note after::.

'--newer-mtime=DATE'

Like '--newer', but add only files whose contents have changed (as
opposed to just '--newer', which will also back up files for which
any status information has changed). *Note after::.

'--no-acls'
Disable the POSIX ACLs support. *Note acls: Extended File
Attributes.

'--no-anchored'
An exclude pattern can match any subsequence of the name's
components. *Note controlling pattern-matching::.

'--no-auto-compress'

Disables automatic compressed format recognition based on the
archive suffix. *Note --auto-compress::. *Note gzip::.

'--no-check-device'
Do not check device numbers when creating a list of modified files
for incremental archiving. *Note device numbers::, for a detailed
description.

'--no-delay-directory-restore'

Modification times and permissions of extracted directories are set
when all files from this directory have been extracted. This is
the default. *Note Directory Modification Times and Permissions::.

'--no-ignore-case'
Use case-sensitive matching. *Note controlling pattern-matching::.

'--no-ignore-command-error'
Print warnings about subprocesses that terminated with a nonzero
exit code. *Note Writing to an External Program::.

'--no-null'

If the '--null' option was given previously, this option cancels
its effect, so that any following '--files-from' options will
expect their file lists to be newline-terminated.

'--no-overwrite-dir'

Preserve metadata of existing directories when extracting files
from an archive. *Note Overwrite Old Files::.

'--no-quote-chars=STRING'
Remove characters listed in STRING from the list of quoted
characters set by the previous '--quote-chars' option (*note
quoting styles::).

'--no-recursion'

With this option, 'tar' will not recurse into directories. *Note
recurse::.

'--no-same-owner'
'-o'

When extracting an archive, do not attempt to preserve the owner
specified in the 'tar' archive. This the default behavior for
ordinary users.

'--no-same-permissions'

When extracting an archive, subtract the user's umask from files
from the permissions specified in the archive. This is the default
behavior for ordinary users.

'--no-seek'

The archive media does not support seeks to arbitrary locations.
Usually 'tar' determines automatically whether the archive can be
seeked or not. Use this option to disable this mechanism.

'--no-selinux'
Disable SELinux context support. *Note SELinux: Extended File
Attributes.

'--no-unquote'
Treat all input file or member names literally, do not interpret
escape sequences. *Note input name quoting::.

'--no-verbatim-files-from'

Instructs GNU 'tar' to treat each line read from a file list as if
it were supplied in the command line. I.e., leading and trailing
whitespace is removed and, if the result begins with a dash, it is
treated as a GNU 'tar' command line option.

This is default behavior. This option is provided as a way to
restore it after '--verbatim-files-from' option.

It is implied by the '--no-null' option.

*Note no-verbatim-files-from::.

'--no-wildcards'
Do not use wildcards. *Note controlling pattern-matching::.

'--no-wildcards-match-slash'
Wildcards do not match '/'. *Note controlling pattern-matching::.

'--no-xattrs'
Disable extended attributes support. *Note xattrs: Extended File
Attributes.

'--null'

When 'tar' is using the '--files-from' option, this option
instructs 'tar' to expect file names terminated with NUL, and to
process file names verbatim.

This means that 'tar' correctly works with file names that contain
newlines or begin with a dash.

*Note nul::.

See also *note verbatim-files-from::.

'--numeric-owner'

This option will notify 'tar' that it should use numeric user and
group IDs when creating a 'tar' file, rather than names. *Note
Attributes::.

'-o'
The function of this option depends on the action 'tar' is
performing. When extracting files, '-o' is a synonym for
'--no-same-owner', i.e., it prevents 'tar' from restoring ownership
of files being extracted.

When creating an archive, it is a synonym for '--old-archive'.
This behavior is for compatibility with previous versions of GNU
'tar', and will be removed in future releases.

*Note Changes::, for more information.

'--occurrence[=NUMBER]'

This option can be used in conjunction with one of the subcommands
'--delete', '--diff', '--extract' or '--list' when a list of files
is given either on the command line or via '-T' option.

This option instructs 'tar' to process only the NUMBERth occurrence
of each named file. NUMBER defaults to 1, so

tar -x -f archive.tar --occurrence filename

will extract the first occurrence of the member 'filename' from
'archive.tar' and will terminate without scanning to the end of the
archive.

'--old-archive'
Synonym for '--format=v7'.

'--one-file-system'
Used when creating an archive. Prevents 'tar' from recursing into
directories that are on different file systems from the current
directory.

'--one-top-level[=DIR]'
Tells 'tar' to create a new directory beneath the extraction
directory (or the one passed to '-C') and use it to guard against
tarbombs. In the absence of DIR argument, the name of the new
directory will be equal to the base name of the archive (file name
minus the archive suffix, if recognized). Any member names that do
not begin with that directory name (after transformations from
'--transform' and '--strip-components') will be prefixed with it.
Recognized file name suffixes are '.tar', and any compression
suffixes recognizable by *Note --auto-compress::.

'--overwrite'

Overwrite existing files and directory metadata when extracting
files from an archive. *Note Overwrite Old Files::.

'--overwrite-dir'

Overwrite the metadata of existing directories when extracting
files from an archive. *Note Overwrite Old Files::.

'--owner=USER'

Specifies that 'tar' should use USER as the owner of members when
creating archives, instead of the user associated with the source
file. USER can specify a symbolic name, or a numeric ID, or both
as NAME:ID. *Note override::.

This option does not affect extraction from archives. See also
'--owner-map', below.

'--owner-map=FILE'

Read owner translation map from FILE. This option allows to
translate only certain owner names or UIDs. *Note override::, for
a detailed description. When used together with '--owner' option,
the latter affects only those files whose owner is not listed in
the FILE.

This option does not affect extraction from archives.

'--pax-option=KEYWORD-LIST'
This option enables creation of the archive in POSIX.1-2001 format
(*note posix::) and modifies the way 'tar' handles the extended
header keywords. KEYWORD-LIST is a comma-separated list of keyword
options. *Note PAX keywords::, for a detailed discussion.

'--portability'
'--old-archive'
Synonym for '--format=v7'.

'--posix'
Same as '--format=posix'.

'--preserve-order'

(See '--same-order'; *note Reading::.)

'--preserve-permissions'
'--same-permissions'
'-p'

When 'tar' is extracting an archive, it normally subtracts the
users' umask from the permissions specified in the archive and uses
that number as the permissions to create the destination file.
Specifying this option instructs 'tar' that it should use the
permissions directly from the archive. *Note Setting Access
Permissions::.

'--quote-chars=STRING'
Always quote characters from STRING, even if the selected quoting
style would not quote them (*note quoting styles::).

'--quoting-style=STYLE'
Set quoting style to use when printing member and file names (*note
quoting styles::). Valid STYLE values are: 'literal', 'shell',
'shell-always', 'c', 'escape', 'locale', and 'clocale'. Default
quoting style is 'escape', unless overridden while configuring the
package.

'--read-full-records'
'-B'

Specifies that 'tar' should reblock its input, for reading from
pipes on systems with buggy implementations. *Note Reading::.

'--record-size=SIZE[SUF]'

Instructs 'tar' to use SIZE bytes per record when accessing the
archive. The argument can be suffixed with a "size suffix", e.g.
'--record-size=10K' for 10 Kilobytes. *Note Table 9.1:
size-suffixes, for a list of valid suffixes. *Note Blocking
Factor::, for a detailed description of this option.

'--recursion'

With this option, 'tar' recurses into directories (default). *Note
recurse::.

'--recursive-unlink'

Remove existing directory hierarchies before extracting directories
of the same name from the archive. *Note Recursive Unlink::.

'--remove-files'

Directs 'tar' to remove the source file from the file system after
appending it to an archive. *Note remove files::.

'--restrict'

Disable use of some potentially harmful 'tar' options. Currently
this option disables shell invocation from multi-volume menu (*note
Using Multiple Tapes::).

'--rmt-command=CMD'

Notifies 'tar' that it should use CMD instead of the default
'/usr/libexec/rmt' (*note Remote Tape Server::).

'--rsh-command=CMD'

Notifies 'tar' that is should use CMD to communicate with remote
devices. *Note Device::.

'--same-order'
'--preserve-order'
'-s'

This option is an optimization for 'tar' when running on machines
with small amounts of memory. It informs 'tar' that the list of
file arguments has already been sorted to match the order of files
in the archive. *Note Reading::.

'--same-owner'

When extracting an archive, 'tar' will attempt to preserve the
owner specified in the 'tar' archive with this option present.
This is the default behavior for the superuser; this option has an
effect only for ordinary users. *Note Attributes::.

'--same-permissions'

(See '--preserve-permissions'; *note Setting Access Permissions::.)

'--seek'
'-n'

Assume that the archive media supports seeks to arbitrary
locations. Usually 'tar' determines automatically whether the
archive can be seeked or not. This option is intended for use in
cases when such recognition fails. It takes effect only if the
archive is open for reading (e.g. with '--list' or '--extract'
options).

'--selinux'
Enable the SELinux context support. *Note selinux: Extended File
Attributes.

'--show-defaults'

Displays the default options used by 'tar' and exits successfully.
This option is intended for use in shell scripts. Here is an
example of what you can see using this option:

$ tar --show-defaults
--format=gnu -f- -b20 --quoting-style=escape
--rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh

Notice, that this option outputs only one line. The example output
above has been split to fit page boundaries. *Note defaults::.

'--show-omitted-dirs'

Instructs 'tar' to mention the directories it is skipping when
operating on a 'tar' archive. *Note show-omitted-dirs::.

'--show-snapshot-field-ranges'

Displays the range of values allowed by this version of 'tar' for
each field in the snapshot file, then exits successfully. *Note
Snapshot Files::.

'--show-transformed-names'
'--show-stored-names'

Display file or member names after applying any transformations
(*note transform::). In particular, when used in conjunction with
one of the archive creation operations it instructs 'tar' to list
the member names stored in the archive, as opposed to the actual
file names. *Note listing member and file names::.

'--skip-old-files'

Do not overwrite existing files when extracting files from an
archive. *Note Keep Old Files::.

This option differs from '--keep-old-files' in that it does not
treat such files as an error, instead it just silently avoids
overwriting them.

The '--warning=existing-file' option can be used together with this
option to produce warning messages about existing old files (*note
warnings::).

'--sort=ORDER'
Specify the directory sorting order when reading directories.
ORDER may be one of the following:

'none'
No directory sorting is performed. This is the default.

'name'
Sort the directory entries on name. The operating system may
deliver directory entries in a more or less random order, and
sorting them makes archive creation reproducible.

'inode'
Sort the directory entries on inode number. Sorting
directories on inode number may reduce the amount of disk seek
operations when creating an archive for some file systems.

'--sparse'
'-S'

Invokes a GNU extension when adding files to an archive that
handles sparse files efficiently. *Note sparse::.

'--sparse-version=VERSION'

Specifies the "format version" to use when archiving sparse files.
Implies '--sparse'. *Note sparse::. For the description of the
supported sparse formats, *Note Sparse Formats::.

'--starting-file=NAME'
'-K NAME'

This option affects extraction only; 'tar' will skip extracting
files in the archive until it finds one that matches NAME. *Note
Scarce::.

'--strip-components=NUMBER'
Strip given NUMBER of leading components from file names before
extraction. For example, if archive 'archive.tar' contained
'/some/file/name', then running

tar --extract --file archive.tar --strip-components=2

would extract this file to file 'name'.

*Note transform::.

'--suffix=SUFFIX'

Alters the suffix 'tar' uses when backing up files from the default
'~'. *Note backup::.

'--tape-length=NUM[SUF]'
'-L NUM[SUF]'

Specifies the length of tapes that 'tar' is writing as being NUM x 1024
bytes long. If optional SUF is given, it specifies a
multiplicative factor to be used instead of 1024. For example,
'-L2M' means 2 megabytes. *Note Table 9.1: size-suffixes, for a
list of allowed suffixes. *Note Using Multiple Tapes::, for a
detailed discussion of this option.

'--test-label'

Reads the volume label. If an argument is specified, test whether
it matches the volume label. *Note --test-label option::.

'--to-command=COMMAND'

During extraction 'tar' will pipe extracted files to the standard
input of COMMAND. *Note Writing to an External Program::.

'--to-stdout'
'-O'

During extraction, 'tar' will extract files to stdout rather than
to the file system. *Note Writing to Standard Output::.

'--totals[=SIGNO]'

Displays the total number of bytes transferred when processing an
archive. If an argument is given, these data are displayed on
request, when signal SIGNO is delivered to 'tar'. *Note totals::.

'--touch'
'-m'

Sets the data modification time of extracted files to the
extraction time, rather than the data modification time stored in
the archive. *Note Data Modification Times::.

'--transform=SED-EXPR'
'--xform=SED-EXPR'
Transform file or member names using 'sed' replacement expression
SED-EXPR. For example,

$ tar cf archive.tar --transform 's,^\./,usr/,' .

will add to 'archive' files from the current working directory,
replacing initial './' prefix with 'usr/'. For the detailed
discussion, *Note transform::.

To see transformed member names in verbose listings, use
'--show-transformed-names' option (*note show-transformed-names::).

'--uncompress'

(See '--compress', *note gzip::)

'--ungzip'

(See '--gzip', *note gzip::)

'--unlink-first'
'-U'

Directs 'tar' to remove the corresponding file from the file system
before extracting it from the archive. *Note Unlink First::.

'--unquote'
Enable unquoting input file or member names (default). *Note input
name quoting::.

'--use-compress-program=PROG'
'-I=PROG'

Instructs 'tar' to access the archive through PROG, which is
presumed to be a compression program of some sort. *Note gzip::.

'--utc'

Display file modification dates in UTC. This option implies
'--verbose'.

'--verbatim-files-from'

Instructs GNU 'tar' to treat each line read from a file list as a
file name, even if it starts with a dash.

File lists are supplied with the '--files-from' ('-T') option. By
default, each line read from a file list is first trimmed off the
leading and trailing whitespace and, if the result begins with a
dash, it is treated as a GNU 'tar' command line option.

Use the '--verbatim-files-from' option to disable this special
handling. This facilitates the use of 'tar' with file lists
created by 'file' command.

This option affects all '--files-from' options that occur after it
in the command line. Its effect is reverted by the
'--no-verbatim-files-from' option.

This option is implied by the '--null' option.

*Note verbatim-files-from::.

'--verbose'
'-v'

Specifies that 'tar' should be more verbose about the operations it
is performing. This option can be specified multiple times for
some operations to increase the amount of information displayed.
*Note verbose::.

'--verify'
'-W'

Verifies that the archive was correctly written when creating an
archive. *Note verify::.

'--version'

Print information about the program's name, version, origin and
legal status, all on standard output, and then exit successfully.
*Note help::.

'--volno-file=FILE'

Used in conjunction with '--multi-volume'. 'tar' will keep track
of which volume of a multi-volume archive it is working in FILE.
*Note volno-file::.

'--warning=KEYWORD'

Enable or disable warning messages identified by KEYWORD. The
messages are suppressed if KEYWORD is prefixed with 'no-'. *Note
warnings::.

'--wildcards'
Use wildcards when matching member names with patterns. *Note
controlling pattern-matching::.

'--wildcards-match-slash'
Wildcards match '/'. *Note controlling pattern-matching::.

'--xattrs'
Enable extended attributes support. *Note xattrs: Extended File
Attributes.

'--xattrs-exclude=PATTERN'
Specify exclude pattern for xattr keys. *Note xattrs-exclude:
Extended File Attributes.

'--xattrs-include=PATTERN.'
Specify include pattern for xattr keys. PATTERN is a POSIX regular
expression, e.g. '--xattrs-exclude='^user\.'' to include only
attributes from the user namespace. *Note xattrs-include: Extended
File Attributes.

'--xz'
'-J'
Use 'xz' for compressing or decompressing the archives. *Note
gzip::.

---------- Footnotes ----------

(1) Earlier versions of GNU 'tar' understood '-l' as a synonym for
'--one-file-system'. The current semantics, which complies to UNIX98,
was introduced with version 1.15.91. *Note Changes::, for more
information.

3.4.3 Short Options Cross Reference
-----------------------------------

Here is an alphabetized list of all of the short option forms, matching
them with the equivalent long option.

Short Option Reference

--------------------------------------------------------------------------
-A *note --concatenate::.

-B *note --read-full-records::.

-C *note --directory::.

-F *note --info-script::.

-G *note --incremental::.

-J *note --xz::.

-K *note --starting-file::.

-L *note --tape-length::.

-M *note --multi-volume::.

-N *note --newer::.

-O *note --to-stdout::.

-P *note --absolute-names::.

-R *note --block-number::.

-S *note --sparse::.

-T *note --files-from::.

-U *note --unlink-first::.

-V *note --label::.

-W *note --verify::.

-X *note --exclude-from::.

-Z *note --compress::.

-b *note --blocking-factor::.

-c *note --create::.

-d *note --compare::.

-f *note --file::.

-g *note --listed-incremental::.

-h *note --dereference::.

-i *note --ignore-zeros::.

-j *note --bzip2::.

-k *note --keep-old-files::.

-l *note --check-links::.

-m *note --touch::.

-o When creating, *note --no-same-owner::, when extracting
-- *note --portability::.

The latter usage is deprecated. It is retained for
compatibility with the earlier versions of GNU 'tar'.
In future releases '-o' will be equivalent to
'--no-same-owner' only.

-p *note --preserve-permissions::.

-r *note --append::.

-s *note --same-order::.

-t *note --list::.

-u *note --update::.

-v *note --verbose::.

-w *note --interactive::.

-x *note --extract::.

-z *note --gzip::.


3.4.4 Position-Sensitive Options
--------------------------------

Some GNU 'tar' options can be used multiple times in the same invocation
and affect all arguments that appear after them. These are options that
control how file names are selected and what kind of pattern matching is
used.

The most obvious example is the '-C' option. It instructs 'tar' to
change to the directory given as its argument prior to processing the
rest of command line (*note directory::). Thus, in the following
command:

tar -c -f a.tar -C /etc passwd -C /var log spool

the file 'passwd' will be searched in the directory '/etc', and files
'log' and 'spool' - in '/var'.

These options can also be used in a file list supplied with the
'--files-from' ('-T') option (*note files::). In that case they affect
all files (patterns) appearing in that file after them and remain in
effect for any arguments processed after that file. For example, if the
file 'list.txt' contained:

README
-C src
main.c

and 'tar' were invoked as follows:

tar -c -f a.tar -T list.txt Makefile

then the file 'README' would be looked up in the current working
directory, and files 'main.c' and 'Makefile' would be looked up in the
directory 'src'.

Many options can be prefixed with '--no-' to cancel the effect of the
original option.

For example, the '--recursion' option controls whether to recurse in
the subdirectories. It's counterpart '--no-recursion' disables this.
Consider the command below. It will store in the archive the directory
'/usr' with all files and directories that are located in it as well as
any files and directories in '/var', without recursing into them(1):

tar -cf a.tar --recursion /usr --no-recursion /var/*

The following table summarizes all position-sensitive options.

'--directory=DIR'
'-C DIR'
*Note directory::.

'--null'
'--no-null'
*Note nul::.

'--unquote'
'--no-unquote'
*Note input name quoting::.

'--verbatim-files-from'
'--no-verbatim-files-from'
*Note verbatim-files-from::.

'--recursion'
'--no-recursion'
*Note recurse::.

'--anchored'
'--no-anchored'
*Note anchored patterns::.

'--ignore-case'
'--no-ignore-case'
*Note case-insensitive matches::.

'--wildcards'
'--no-wildcards'
*Note controlling pattern-matching::.

'--wildcards-match-slash'
'--no-wildcards-match-slash'
*Note controlling pattern-matching::.

'--exclude'
*Note exclude::.

'--exclude-from'
'-X'
'--exclude-caches'
'--exclude-caches-under'
'--exclude-caches-all'
'--exclude-tag'
'--exclude-ignore'
'--exclude-ignore-recursive'
'--exclude-tag-under'
'--exclude-tag-all'
'--exclude-vcs'
'--exclude-vcs-ignores'
'--exclude-backups'
*Note exclude::.

---------- Footnotes ----------

(1) The '--recursion' option is the default and is used here for
clarity. The same example can be written as:

tar -cf a.tar /usr --no-recursion /var/*

3.5 GNU 'tar' documentation
===========================

Being careful, the first thing is really checking that you are using GNU
'tar', indeed. The '--version' option causes 'tar' to print information
about its name, version, origin and legal status, all on standard
output, and then exit successfully. For example, 'tar --version' might
print:

tar (GNU tar) 1.29
Copyright (C) 2013-2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by John Gilmore and Jay Fenlason.

The first occurrence of 'tar' in the result above is the program name in
the package (for example, 'rmt' is another program), while the second
occurrence of 'tar' is the name of the package itself, containing
possibly many programs. The package is currently named 'tar', after the
name of the main program it contains(1).

Another thing you might want to do is checking the spelling or
meaning of some particular 'tar' option, without resorting to this
manual, for once you have carefully read it. GNU 'tar' has a short help
feature, triggerable through the '--help' option. By using this option,
'tar' will print a usage message listing all available options on
standard output, then exit successfully, without doing anything else and
ignoring all other options. Even if this is only a brief summary, it
may be several screens long. So, if you are not using some kind of
scrollable window, you might prefer to use something like:

$ tar --help | less

presuming, here, that you like using 'less' for a pager. Other popular
pagers are 'more' and 'pg'. If you know about some KEYWORD which
interests you and do not want to read all the '--help' output, another
common idiom is doing:

tar --help | grep KEYWORD

for getting only the pertinent lines. Notice, however, that some 'tar'
options have long description lines and the above command will list only
the first of them.

The exact look of the option summary displayed by 'tar --help' is
configurable. *Note Configuring Help Summary::, for a detailed
description.

If you only wish to check the spelling of an option, running 'tar
--usage' may be a better choice. This will display a terse list of
'tar' options without accompanying explanations.

The short help output is quite succinct, and you might have to get
back to the full documentation for precise points. If you are reading
this paragraph, you already have the 'tar' manual in some form. This
manual is available in a variety of forms from
<http://www.gnu.org/software/tar/manual>. It may be printed out of the
GNU 'tar' distribution, provided you have TeX already installed
somewhere, and a laser printer around. Just configure the distribution,
execute the command 'make dvi', then print 'doc/tar.dvi' the usual way
(contact your local guru to know how). If GNU 'tar' has been
conveniently installed at your place, this manual is also available in
interactive, hypertextual form as an Info file. Just call 'info tar'
or, if you do not have the 'info' program handy, use the Info reader
provided within GNU Emacs, calling 'tar' from the main Info menu.

There is currently no 'man' page for GNU 'tar'. If you observe such
a 'man' page on the system you are running, either it does not belong to
GNU 'tar', or it has not been produced by GNU. Some package maintainers
convert 'tar --help' output to a man page, using 'help2man'. In any
case, please bear in mind that the authoritative source of information
about GNU 'tar' is this Texinfo documentation.

---------- Footnotes ----------

(1) There are plans to merge the 'cpio' and 'tar' packages into a
single one which would be called 'paxutils'. So, who knows if, one of
this days, the '--version' would not output 'tar (GNU paxutils) 3.2'.

3.6 Obtaining GNU 'tar' default values
======================================

GNU 'tar' has some predefined defaults that are used when you do not
explicitly specify another values. To obtain a list of such defaults,
use '--show-defaults' option. This will output the values in the form
of 'tar' command line options:

$ tar --show-defaults
--format=gnu -f- -b20 --quoting-style=escape
--rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh

Notice, that this option outputs only one line. The example output
above has been split to fit page boundaries.

The above output shows that this version of GNU 'tar' defaults to using
'gnu' archive format (*note Formats::), it uses standard output as the
archive, if no '--file' option has been given (*note file tutorial::),
the default blocking factor is 20 (*note Blocking Factor::). It also
shows the default locations where 'tar' will look for 'rmt' and 'rsh'
binaries.

3.7 Checking 'tar' progress
===========================

Typically, 'tar' performs most operations without reporting any
information to the user except error messages. When using 'tar' with
many options, particularly ones with complicated or difficult-to-predict
behavior, it is possible to make serious mistakes. 'tar' provides
several options that make observing 'tar' easier. These options cause
'tar' to print information as it progresses in its job, and you might
want to use them just for being more careful about what is going on, or
merely for entertaining yourself. If you have encountered a problem
when operating on an archive, however, you may need more information
than just an error message in order to solve the problem. The following
options can be helpful diagnostic tools.

Normally, the '--list' ('-t') command to list an archive prints just
the file names (one per line) and the other commands are silent. When
used with most operations, the '--verbose' ('-v') option causes 'tar' to
print the name of each file or archive member as it is processed. This
and the other options which make 'tar' print status information can be
useful in monitoring 'tar'.

With '--create' or '--extract', '--verbose' used once just prints the
names of the files or members as they are processed. Using it twice
causes 'tar' to print a longer listing (*Note verbose member listing::,
for the description) for each member. Since '--list' already prints the
names of the members, '--verbose' used once with '--list' causes 'tar'
to print an 'ls -l' type listing of the files in the archive. The
following examples both extract members with long list output:

$ tar --extract --file=archive.tar --verbose --verbose
$ tar xvvf archive.tar

Verbose output appears on the standard output except when an archive
is being written to the standard output, as with 'tar --create --file=-
--verbose' ('tar cvf -', or even 'tar cv'--if the installer let standard
output be the default archive). In that case 'tar' writes verbose
output to the standard error stream.

If '--index-file=FILE' is specified, 'tar' sends verbose output to
FILE rather than to standard output or standard error.

The '--totals' option causes 'tar' to print on the standard error the
total amount of bytes transferred when processing an archive. When
creating or appending to an archive, this option prints the number of
bytes written to the archive and the average speed at which they have
been written, e.g.:

$ tar -c -f archive.tar --totals /home
Total bytes written: 7924664320 (7.4GiB, 85MiB/s)

When reading an archive, this option displays the number of bytes
read:

$ tar -x -f archive.tar --totals
Total bytes read: 7924664320 (7.4GiB, 95MiB/s)

Finally, when deleting from an archive, the '--totals' option
displays both numbers plus number of bytes removed from the archive:

$ tar --delete -f foo.tar --totals --wildcards '*~'
Total bytes read: 9543680 (9.2MiB, 201MiB/s)
Total bytes written: 3829760 (3.7MiB, 81MiB/s)
Total bytes deleted: 1474048

You can also obtain this information on request. When '--totals' is
used with an argument, this argument is interpreted as a symbolic name
of a signal, upon delivery of which the statistics is to be printed:

'--totals=SIGNO'
Print statistics upon delivery of signal SIGNO. Valid arguments
are: 'SIGHUP', 'SIGQUIT', 'SIGINT', 'SIGUSR1' and 'SIGUSR2'.
Shortened names without 'SIG' prefix are also accepted.

Both forms of '--totals' option can be used simultaneously. Thus,
'tar -x --totals --totals=USR1' instructs 'tar' to extract all members
from its default archive and print statistics after finishing the
extraction, as well as when receiving signal 'SIGUSR1'.

The '--checkpoint' option prints an occasional message as 'tar' reads
or writes the archive. It is designed for those who don't need the more
detailed (and voluminous) output of '--block-number' ('-R'), but do want
visual confirmation that 'tar' is actually making forward progress. By
default it prints a message each 10 records read or written. This can
be changed by giving it a numeric argument after an equal sign:

$ tar -c --checkpoint=1000 /var
tar: Write checkpoint 1000
tar: Write checkpoint 2000
tar: Write checkpoint 3000

This example shows the default checkpoint message used by 'tar'. If
you place a dot immediately after the equal sign, it will print a '.' at
each checkpoint(1). For example:

$ tar -c --checkpoint=.1000 /var
...

The '--checkpoint' option provides a flexible mechanism for executing
arbitrary actions upon hitting checkpoints, see the next section (*note
checkpoints::), for more information on it.

The '--show-omitted-dirs' option, when reading an archive--with
'--list' or '--extract', for example--causes a message to be printed for
each directory in the archive which is skipped. This happens regardless
of the reason for skipping: the directory might not have been named on
the command line (implicitly or explicitly), it might be excluded by the
use of the '--exclude=PATTERN' option, or some other reason.

If '--block-number' ('-R') is used, 'tar' prints, along with every
message it would normally produce, the block number within the archive
where the message was triggered. Also, supplementary messages are
triggered when reading blocks full of NULs, or when hitting end of file
on the archive. As of now, if the archive is properly terminated with a
NUL block, the reading of the file may stop before end of file is met,
so the position of end of file will not usually show when
'--block-number' ('-R') is used. Note that GNU 'tar' drains the archive
before exiting when reading the archive from a pipe.

This option is especially useful when reading damaged archives, since
it helps pinpoint the damaged sections. It can also be used with
'--list' ('-t') when listing a file-system backup tape, allowing you to
choose among several backup tapes when retrieving a file later, in favor
of the tape where the file appears earliest (closest to the front of the
tape). *Note backup::.

---------- Footnotes ----------

(1) This is actually a shortcut for '--checkpoint=N
--checkpoint-action=dot'. *Note dot: checkpoints.

3.8 Checkpoints
===============

A "checkpoint" is a moment of time before writing Nth record to the
archive (a "write checkpoint"), or before reading Nth record from the
archive (a "read checkpoint"). Checkpoints allow to periodically
execute arbitrary actions.

The checkpoint facility is enabled using the following option:

'--checkpoint[=N]'
Schedule checkpoints before writing or reading each Nth record.
The default value for N is 10.

A list of arbitrary "actions" can be executed at each checkpoint.
These actions include: pausing, displaying textual messages, and
executing arbitrary external programs. Actions are defined using the
'--checkpoint-action' option.

'--checkpoint-action=ACTION'
Execute an ACTION at each checkpoint.

The simplest value of ACTION is 'echo'. It instructs 'tar' to
display the default message on the standard error stream upon arriving
at each checkpoint. The default message is (in POSIX locale) 'Write
checkpoint N', for write checkpoints, and 'Read checkpoint N', for read
checkpoints. Here, N represents ordinal number of the checkpoint.

In another locales, translated versions of this message are used.

This is the default action, so running:

$ tar -c --checkpoint=1000 --checkpoint-action=echo /var

is equivalent to:

$ tar -c --checkpoint=1000 /var

The 'echo' action also allows to supply a customized message. You do
so by placing an equals sign and the message right after it, e.g.:

--checkpoint-action="echo=Hit %s checkpoint #%u"

The '%s' and '%u' in the above example are "format specifiers". The
'%s' specifier is replaced with the "type" of the checkpoint: 'write' or
'read' (or a corresponding translated version in locales other than
POSIX). The '%u' specifier is replaced with the ordinal number of the
checkpoint. Thus, the above example could produce the following output
when used with the '--create' option:

tar: Hit write checkpoint #10
tar: Hit write checkpoint #20
tar: Hit write checkpoint #30

The complete list of available format specifiers follows. Some of
them can take optional arguments. These arguments, if given, are
supplied in curly braces between the percent sign and the specifier
letter.

'%s'
Print type of the checkpoint ('write' or 'read').

'%u'
Print number of the checkpoint.

'%{r,w,d}T'
Print number of bytes transferred so far and approximate transfer
speed. Optional arguments supply prefixes to be used before number
of bytes read, written and deleted, correspondingly. If absent,
they default to 'R'. 'W', 'D'. Any or all of them can be omitted,
so, that e.g. '%{}T' means to print corresponding statistics
without any prefixes. Any surplus arguments, if present, are
silently ignored.

$ tar --delete -f f.tar --checkpoint-action=echo="#%u: %T" main.c
tar: #1: R: 0 (0B, 0B/s),W: 0 (0B, 0B/s),D: 0
tar: #2: R: 10240 (10KiB, 19MiB/s),W: 0 (0B, 0B/s),D: 10240

See also the 'totals' action, described below.

'%{FMT}t'
Output current local time using FMT as format for 'strftime' (*note
strftime: (strftime(3))strftime.). The '{FMT}' part is optional.
If not present, the default format is '%c', i.e. the preferred
date and time representation for the current locale.

'%{N}*'
Pad output with spaces to the Nth column. If the '{N}' part is
omitted, the current screen width is assumed.

'%c'
This is a shortcut for '%{%Y-%m-%d %H:%M:%S}t: %ds,
%{read,wrote}T%*\r', intended mainly for use with 'ttyout' action
(see below).

Aside from format expansion, the message string is subject to
"unquoting", during which the backslash "escape sequences" are replaced
with their corresponding ASCII characters (*note escape sequences::).
E.g. the following action will produce an audible bell and the message
described above at each checkpoint:

--checkpoint-action='echo=\aHit %s checkpoint #%u'

There is also a special action which produces an audible signal:
'bell'. It is not equivalent to 'echo='\a'', because 'bell' sends the
bell directly to the console ('/dev/tty'), whereas 'echo='\a'' sends it
to the standard error.

The 'ttyout=STRING' action outputs STRING to '/dev/tty', so it can be
used even if the standard output is redirected elsewhere. The STRING is
subject to the same modifications as with 'echo' action. In contrast to
the latter, 'ttyout' does not prepend 'tar' executable name to the
string, nor does it output a newline after it. For example, the
following action will print the checkpoint message at the same screen
line, overwriting any previous message:

--checkpoint-action="ttyout=Hit %s checkpoint #%u%*\r"

Notice the use of '%*' specifier to clear out any eventual remains of
the prior output line. As as more complex example, consider this:

--checkpoint-action=ttyout='%{%Y-%m-%d %H:%M:%S}t (%d sec): #%u, %T%*\r'

This prints the current local time, number of seconds expired since tar
was started, the checkpoint ordinal number, transferred bytes and
average computed I/O speed.

Another available checkpoint action is 'dot' (or '.'). It instructs
'tar' to print a single dot on the standard listing stream, e.g.:

$ tar -c --checkpoint=1000 --checkpoint-action=dot /var
...

For compatibility with previous GNU 'tar' versions, this action can
be abbreviated by placing a dot in front of the checkpoint frequency, as
shown in the previous section.

The 'totals' action prints the total number of bytes transferred so
far. The format of the data is the same as for the '--totals' option
(*note totals::). See also '%T' format specifier of the 'echo' or
'ttyout' action.

Yet another action, 'sleep', pauses 'tar' for a specified amount of
seconds. The following example will stop for 30 seconds at each
checkpoint:

$ tar -c --checkpoint=1000 --checkpoint-action=sleep=30

Finally, the 'exec' action executes a given external command. For
example:

$ tar -c --checkpoint=1000 --checkpoint-action=exec=/sbin/cpoint

The supplied command can be any valid command invocation, with or
without additional command line arguments. If it does contain
arguments, don't forget to quote it to prevent it from being split by
the shell. *Note Running External Commands: external, for more detail.

The command gets a copy of 'tar''s environment plus the following
variables:

'TAR_VERSION'
GNU 'tar' version number.

'TAR_ARCHIVE'
The name of the archive 'tar' is processing.

'TAR_BLOCKING_FACTOR'
Current blocking factor (*note Blocking::).

'TAR_CHECKPOINT'
Number of the checkpoint.

'TAR_SUBCOMMAND'
A short option describing the operation 'tar' is executing. *Note
Operations::, for a complete list of subcommand options.

'TAR_FORMAT'
Format of the archive being processed. *Note Formats::, for a
complete list of archive format names.

These environment variables can also be passed as arguments to the
command, provided that they are properly escaped, for example:

tar -c -f arc.tar \
--checkpoint-action='exec=/sbin/cpoint $TAR_CHECKPOINT'

Notice single quotes to prevent variable names from being expanded by
the shell when invoking 'tar'.

Any number of actions can be defined, by supplying several
'--checkpoint-action' options in the command line. For example, the
command below displays two messages, pauses execution for 30 seconds and
executes the '/sbin/cpoint' script:

$ tar -c -f arc.tar \
--checkpoint-action='\aecho=Hit %s checkpoint #%u' \
--checkpoint-action='echo=Sleeping for 30 seconds' \
--checkpoint-action='sleep=30' \
--checkpoint-action='exec=/sbin/cpoint'

This example also illustrates the fact that '--checkpoint-action' can
be used without '--checkpoint'. In this case, the default checkpoint
frequency (at each 10th record) is assumed.

3.9 Controlling Warning Messages
================================

Sometimes, while performing the requested task, GNU 'tar' notices some
conditions that are not exactly errors, but which the user should be
aware of. When this happens, 'tar' issues a "warning message"
describing the condition. Warning messages are output to the standard
error and they do not affect the exit code of 'tar' command.

GNU 'tar' allows the user to suppress some or all of its warning
messages:

'--warning=KEYWORD'
Control display of the warning messages identified by KEYWORD. If
KEYWORD starts with the prefix 'no-', such messages are suppressed.
Otherwise, they are enabled.

Multiple '--warning' messages accumulate.

The tables below list allowed values for KEYWORD along with the
warning messages they control.

Keywords controlling 'tar' operation
------------------------------------

all
Enable all warning messages. This is the default.
none
Disable all warning messages.
filename-with-nuls
'%s: file name read contains nul character'
alone-zero-block
'A lone zero block at %s'

Keywords applicable for 'tar --create'
--------------------------------------

cachedir
'%s: contains a cache directory tag %s; %s'
file-shrank
'%s: File shrank by %s bytes; padding with zeros'
xdev
'%s: file is on a different filesystem; not dumped'
file-ignored
'%s: Unknown file type; file ignored'
'%s: socket ignored'
'%s: door ignored'
file-unchanged
'%s: file is unchanged; not dumped'
ignore-archive
'%s: file is the archive; not dumped'
file-removed
'%s: File removed before we read it'
file-changed
'%s: file changed as we read it'

Keywords applicable for 'tar --extract'
---------------------------------------

existing-file
'%s: skipping existing file'
timestamp
'%s: implausibly old time stamp %s'
'%s: time stamp %s is %s s in the future'
contiguous-cast
'Extracting contiguous files as regular files'
symlink-cast
'Attempting extraction of symbolic links as hard links'
unknown-cast
'%s: Unknown file type '%c', extracted as normal file'
ignore-newer
'Current %s is newer or same age'
unknown-keyword
'Ignoring unknown extended header keyword '%s''
decompress-program
Controls verbose description of failures occurring when trying to
run alternative decompressor programs (*note alternative
decompression programs::). This warning is disabled by default
(unless '--verbose' is used). A common example of what you can get
when using this warning is:

$ tar --warning=decompress-program -x -f archive.Z
tar (child): cannot run compress: No such file or directory
tar (child): trying gzip

This means that 'tar' first tried to decompress 'archive.Z' using
'compress', and, when that failed, switched to 'gzip'.
record-size
'Record size = %lu blocks'

Keywords controlling incremental extraction:
--------------------------------------------

rename-directory
'%s: Directory has been renamed from %s'
'%s: Directory has been renamed'
new-directory
'%s: Directory is new'
xdev
'%s: directory is on a different device: not purging'
bad-dumpdir
'Malformed dumpdir: 'X' never used'

3.10 Asking for Confirmation During Operations
==============================================

Typically, 'tar' carries out a command without stopping for further
instructions. In some situations however, you may want to exclude some
files and archive members from the operation (for instance if disk or
storage space is tight). You can do this by excluding certain files
automatically (*note Choosing::), or by performing an operation
interactively, using the '--interactive' ('-w') option. 'tar' also
accepts '--confirmation' for this option.

When the '--interactive' ('-w') option is specified, before reading,
writing, or deleting files, 'tar' first prints a message for each such
file, telling what operation it intends to take, then asks for
confirmation on the terminal. The actions which require confirmation
include adding a file to the archive, extracting a file from the
archive, deleting a file from the archive, and deleting a file from
disk. To confirm the action, you must type a line of input beginning
with 'y'. If your input line begins with anything other than 'y', 'tar'
skips that file.

If 'tar' is reading the archive from the standard input, 'tar' opens
the file '/dev/tty' to support the interactive communications.

Verbose output is normally sent to standard output, separate from
other error messages. However, if the archive is produced directly on
standard output, then verbose output is mixed with errors on 'stderr'.
Producing the archive on standard output may be used as a way to avoid
using disk space, when the archive is soon to be consumed by another
process reading it, say. Some people felt the need of producing an
archive on stdout, still willing to segregate between verbose output and
error output. A possible approach would be using a named pipe to
receive the archive, and having the consumer process to read from that
named pipe. This has the advantage of letting standard output free to
receive verbose output, all separate from errors.

3.11 Running External Commands
==============================

Certain GNU 'tar' operations imply running external commands that you
supply on the command line. One of such operations is checkpointing,
described above (*note checkpoint exec::). Another example of this
feature is the '-I' option, which allows you to supply the program to
use for compressing or decompressing the archive (*note
use-compress-program::).

Whenever such operation is requested, 'tar' first splits the supplied
command into words much like the shell does. It then treats the first
word as the name of the program or the shell script to execute and the
rest of words as its command line arguments. The program, unless given
as an absolute file name, is searched in the shell's 'PATH'.

Any additional information is normally supplied to external commands
in environment variables, specific to each particular operation. For
example, the '--checkpoint-action=exec' option, defines the
'TAR_ARCHIVE' variable to the name of the archive being worked upon.
You can, should the need be, use these variables in the command line of
the external command. For example:

$ tar -x -f archive.tar \
--checkpoint-action=exec='printf "%04d in %32s\r" $TAR_CHECKPOINT $TAR_ARCHIVE'

This command prints for each checkpoint its number and the name of the
archive, using the same output line on the screen.

Notice the use of single quotes to prevent variable names from being
expanded by the shell when invoking 'tar'.

4 GNU 'tar' Operations
**********************

4.1 Basic GNU 'tar' Operations
==============================

The basic 'tar' operations, '--create' ('-c'), '--list' ('-t') and
'--extract' ('--get', '-x'), are currently presented and described in
the tutorial chapter of this manual. This section provides some
complementary notes for these operations.

'--create'
'-c'

Creating an empty archive would have some kind of elegance. One
can initialize an empty archive and later use '--append' ('-r') for
adding all members. Some applications would not welcome making an
exception in the way of adding the first archive member. On the
other hand, many people reported that it is dangerously too easy
for 'tar' to destroy a magnetic tape with an empty archive(1). The
two most common errors are:

1. Mistakingly using 'create' instead of 'extract', when the
intent was to extract the full contents of an archive. This
error is likely: keys 'c' and 'x' are right next to each other
on the QWERTY keyboard. Instead of being unpacked, the
archive then gets wholly destroyed. When users speak about
"exploding" an archive, they usually mean something else :-).

2. Forgetting the argument to 'file', when the intent was to
create an archive with a single file in it. This error is
likely because a tired user can easily add the 'f' key to the
cluster of option letters, by the mere force of habit, without
realizing the full consequence of doing so. The usual
consequence is that the single file, which was meant to be
saved, is rather destroyed.

So, recognizing the likelihood and the catastrophic nature of these
errors, GNU 'tar' now takes some distance from elegance, and
cowardly refuses to create an archive when '--create' option is
given, there are no arguments besides options, and '--files-from'
('-T') option is _not_ used. To get around the cautiousness of GNU
'tar' and nevertheless create an archive with nothing in it, one
may still use, as the value for the '--files-from' option, a file
with no names in it, as shown in the following commands:

tar --create --file=empty-archive.tar --files-from=/dev/null
tar -cf empty-archive.tar -T /dev/null

'--extract'
'--get'
'-x'

A socket is stored, within a GNU 'tar' archive, as a pipe.

'--list (-t)'

GNU 'tar' now shows dates as '1996-08-30', while it used to show
them as 'Aug 30 1996'. Preferably, people should get used to ISO
8601 dates. Local American dates should be made available again
with full date localization support, once ready. In the meantime,
programs not being localizable for dates should prefer
international dates, that's really the way to go.

Look up <http://www.cl.cam.ac.uk/~mgk25/iso-time.html> if you are
curious, it contains a detailed explanation of the ISO 8601
standard.

---------- Footnotes ----------

(1) This is well described in 'Unix-haters Handbook', by Simson
Garfinkel, Daniel Weise & Steven Strassmann, IDG Books, ISBN
1-56884-203-1.

4.2 Advanced GNU 'tar' Operations
=================================

Now that you have learned the basics of using GNU 'tar', you may want to
learn about further ways in which 'tar' can help you.

This chapter presents five, more advanced operations which you
probably won't use on a daily basis, but which serve more specialized
functions. We also explain the different styles of options and why you
might want to use one or another, or a combination of them in your 'tar'
commands. Additionally, this chapter includes options which allow you
to define the output from 'tar' more carefully, and provide help and
error correction in special circumstances.

4.2.1 The Five Advanced 'tar' Operations
----------------------------------------

In the last chapter, you learned about the first three operations to
'tar'. This chapter presents the remaining five operations to 'tar':
'--append', '--update', '--concatenate', '--delete', and '--compare'.

You are not likely to use these operations as frequently as those
covered in the last chapter; however, since they perform specialized
functions, they are quite useful when you do need to use them. We will
give examples using the same directory and files that you created in the
last chapter. As you may recall, the directory is called 'practice',
the files are 'jazz', 'blues', 'folk', and the two archive files you
created are 'collection.tar' and 'music.tar'.

We will also use the archive files 'afiles.tar' and 'bfiles.tar'.
The archive 'afiles.tar' contains the members 'apple', 'angst', and
'aspic'; 'bfiles.tar' contains the members './birds', 'baboon', and
'./box'.

Unless we state otherwise, all practicing you do and examples you
follow in this chapter will take place in the 'practice' directory that
you created in the previous chapter; see *note prepare for examples::.
(Below in this section, we will remind you of the state of the examples
where the last chapter left them.)

The five operations that we will cover in this chapter are:

'--append'
'-r'
Add new entries to an archive that already exists.
'--update'
'-u'
Add more recent copies of archive members to the end of an archive,
if they exist.
'--concatenate'
'--catenate'
'-A'
Add one or more pre-existing archives to the end of another
archive.
'--delete'
Delete items from an archive (does not work on tapes).
'--compare'
'--diff'
'-d'
Compare archive members to their counterparts in the file system.

4.2.2 How to Add Files to Existing Archives: '--append'
-------------------------------------------------------

If you want to add files to an existing archive, you don't need to
create a new archive; you can use '--append' ('-r'). The archive must
already exist in order to use '--append'. (A related operation is the
'--update' operation; you can use this to add newer versions of archive
members to an existing archive. To learn how to do this with
'--update', *note update::.)

If you use '--append' to add a file that has the same name as an
archive member to an archive containing that archive member, then the
old member is not deleted. What does happen, however, is somewhat
complex. 'tar' _allows_ you to have infinite number of files with the
same name. Some operations treat these same-named members no
differently than any other set of archive members: for example, if you
view an archive with '--list' ('-t'), you will see all of those members
listed, with their data modification times, owners, etc.

Other operations don't deal with these members as perfectly as you
might prefer; if you were to use '--extract' to extract the archive,
only the most recently added copy of a member with the same name as
other members would end up in the working directory. This is because
'--extract' extracts an archive in the order the members appeared in the
archive; the most recently archived members will be extracted last.
Additionally, an extracted member will _replace_ a file of the same name
which existed in the directory already, and 'tar' will not prompt you
about this(1). Thus, only the most recently archived member will end up
being extracted, as it will replace the one extracted before it, and so
on.

There exists a special option that allows you to get around this
behavior and extract (or list) only a particular copy of the file. This
is '--occurrence' option. If you run 'tar' with this option, it will
extract only the first copy of the file. You may also give this option
an argument specifying the number of copy to be extracted. Thus, for
example if the archive 'archive.tar' contained three copies of file
'myfile', then the command

tar --extract --file archive.tar --occurrence=2 myfile

would extract only the second copy. *Note --occurrence: Option Summary,
for the description of '--occurrence' option.

If you want to replace an archive member, use '--delete' to delete
the member you want to remove from the archive, and then use '--append'
to add the member you want to be in the archive. Note that you can not
change the order of the archive; the most recently added member will
still appear last. In this sense, you cannot truly "replace" one member
with another. (Replacing one member with another will not work on
certain types of media, such as tapes; see *note delete:: and *note
Media::, for more information.)

---------- Footnotes ----------

(1) Unless you give it '--keep-old-files' (or '--skip-old-files')
option, or the disk copy is newer than the one in the archive and you
invoke 'tar' with '--keep-newer-files' option.

4.2.2.1 Appending Files to an Archive
.....................................

The simplest way to add a file to an already existing archive is the
'--append' ('-r') operation, which writes specified files into the
archive whether or not they are already among the archived files.

When you use '--append', you _must_ specify file name arguments, as
there is no default. If you specify a file that already exists in the
archive, another copy of the file will be added to the end of the
archive. As with other operations, the member names of the newly added
files will be exactly the same as their names given on the command line.
The '--verbose' ('-v') option will print out the names of the files as
they are written into the archive.

'--append' cannot be performed on some tape drives, unfortunately,
due to deficiencies in the formats those tape drives use. The archive
must be a valid 'tar' archive, or else the results of using this
operation will be unpredictable. *Note Media::.

To demonstrate using '--append' to add a file to an archive, create a
file called 'rock' in the 'practice' directory. Make sure you are in
the 'practice' directory. Then, run the following 'tar' command to add
'rock' to 'collection.tar':

$ tar --append --file=collection.tar rock

If you now use the '--list' ('-t') operation, you will see that 'rock'
has been added to the archive:

$ tar --list --file=collection.tar
-rw-r--r-- me/user 28 1996-10-18 16:31 jazz
-rw-r--r-- me/user 21 1996-09-23 16:44 blues
-rw-r--r-- me/user 20 1996-09-23 16:44 folk
-rw-r--r-- me/user 20 1996-09-23 16:44 rock

4.2.2.2 Multiple Members with the Same Name
...........................................

You can use '--append' ('-r') to add copies of files which have been
updated since the archive was created. (However, we do not recommend
doing this since there is another 'tar' option called '--update'; *Note
update::, for more information. We describe this use of '--append' here
for the sake of completeness.) When you extract the archive, the older
version will be effectively lost. This works because files are
extracted from an archive in the order in which they were archived.
Thus, when the archive is extracted, a file archived later in time will
replace a file of the same name which was archived earlier, even though
the older version of the file will remain in the archive unless you
delete all versions of the file.

Supposing you change the file 'blues' and then append the changed
version to 'collection.tar'. As you saw above, the original 'blues' is
in the archive 'collection.tar'. If you change the file and append the
new version of the file to the archive, there will be two copies in the
archive. When you extract the archive, the older version of the file
will be extracted first, and then replaced by the newer version when it
is extracted.

You can append the new, changed copy of the file 'blues' to the
archive in this way:

$ tar --append --verbose --file=collection.tar blues
blues

Because you specified the '--verbose' option, 'tar' has printed the name
of the file being appended as it was acted on. Now list the contents of
the archive:

$ tar --list --verbose --file=collection.tar
-rw-r--r-- me/user 28 1996-10-18 16:31 jazz
-rw-r--r-- me/user 21 1996-09-23 16:44 blues
-rw-r--r-- me/user 20 1996-09-23 16:44 folk
-rw-r--r-- me/user 20 1996-09-23 16:44 rock
-rw-r--r-- me/user 58 1996-10-24 18:30 blues

The newest version of 'blues' is now at the end of the archive (note the
different creation dates and file sizes). If you extract the archive,
the older version of the file 'blues' will be replaced by the newer
version. You can confirm this by extracting the archive and running
'ls' on the directory.

If you wish to extract the first occurrence of the file 'blues' from
the archive, use '--occurrence' option, as shown in the following
example:

$ tar --extract -vv --occurrence --file=collection.tar blues
-rw-r--r-- me/user 21 1996-09-23 16:44 blues

*Note Writing::, for more information on '--extract' and see *note
-occurrence: Option Summary, for a description of '--occurrence' option.

4.2.3 Updating an Archive
-------------------------

In the previous section, you learned how to use '--append' to add a file
to an existing archive. A related operation is '--update' ('-u'). The
'--update' operation updates a 'tar' archive by comparing the date of
the specified archive members against the date of the file with the same
name. If the file has been modified more recently than the archive
member, then the newer version of the file is added to the archive (as
with '--append').

Unfortunately, you cannot use '--update' with magnetic tape drives.
The operation will fail.

Both '--update' and '--append' work by adding to the end of the
archive. When you extract a file from the archive, only the version
stored last will wind up in the file system, unless you use the
'--backup' option. *Note multiple::, for a detailed discussion.

4.2.3.1 How to Update an Archive Using '--update'
.................................................

You must use file name arguments with the '--update' ('-u') operation.
If you don't specify any files, 'tar' won't act on any files and won't
tell you that it didn't do anything (which may end up confusing you).

To see the '--update' option at work, create a new file, 'classical',
in your practice directory, and some extra text to the file 'blues',
using any text editor. Then invoke 'tar' with the 'update' operation
and the '--verbose' ('-v') option specified, using the names of all the
files in the 'practice' directory as file name arguments:

$ tar --update -v -f collection.tar blues folk rock classical
blues
classical
$

Because we have specified verbose mode, 'tar' prints out the names of
the files it is working on, which in this case are the names of the
files that needed to be updated. If you run 'tar --list' and look at
the archive, you will see 'blues' and 'classical' at its end. There
will be a total of two versions of the member 'blues'; the one at the
end will be newer and larger, since you added text before updating it.

The reason 'tar' does not overwrite the older file when updating it
is because writing to the middle of a section of tape is a difficult
process. Tapes are not designed to go backward. *Note Media::, for
more information about tapes.

'--update' ('-u') is not suitable for performing backups for two
reasons: it does not change directory content entries, and it lengthens
the archive every time it is used. The GNU 'tar' options intended
specifically for backups are more efficient. If you need to run
backups, please consult *note Backups::.

4.2.4 Combining Archives with '--concatenate'
---------------------------------------------

Sometimes it may be convenient to add a second archive onto the end of
an archive rather than adding individual files to the archive. To add
one or more archives to the end of another archive, you should use the
'--concatenate' ('--catenate', '-A') operation.

To use '--concatenate', give the first archive with '--file' option
and name the rest of archives to be concatenated on the command line.
The members, and their member names, will be copied verbatim from those
archives to the first one(1). The new, concatenated archive will be
called by the same name as the one given with the '--file' option. As
usual, if you omit '--file', 'tar' will use the value of the environment
variable 'TAPE', or, if this has not been set, the default archive name.

To demonstrate how '--concatenate' works, create two small archives
called 'bluesrock.tar' and 'folkjazz.tar', using the relevant files from
'practice':

$ tar -cvf bluesrock.tar blues rock
blues
rock
$ tar -cvf folkjazz.tar folk jazz
folk
jazz

If you like, You can run 'tar --list' to make sure the archives contain
what they are supposed to:

$ tar -tvf bluesrock.tar
-rw-r--r-- melissa/user 105 1997-01-21 19:42 blues
-rw-r--r-- melissa/user 33 1997-01-20 15:34 rock
$ tar -tvf jazzfolk.tar
-rw-r--r-- melissa/user 20 1996-09-23 16:44 folk
-rw-r--r-- melissa/user 65 1997-01-30 14:15 jazz

We can concatenate these two archives with 'tar':

$ cd ..
$ tar --concatenate --file=bluesrock.tar jazzfolk.tar

If you now list the contents of the 'bluesrock.tar', you will see
that now it also contains the archive members of 'jazzfolk.tar':

$ tar --list --file=bluesrock.tar
blues
rock
folk
jazz

When you use '--concatenate', the source and target archives must
already exist and must have been created using compatible format
parameters. Notice, that 'tar' does not check whether the archives it
concatenates have compatible formats, it does not even check if the
files are really tar archives.

Like '--append' ('-r'), this operation cannot be performed on some
tape drives, due to deficiencies in the formats those tape drives use.

It may seem more intuitive to you to want or try to use 'cat' to
concatenate two archives instead of using the '--concatenate' operation;
after all, 'cat' is the utility for combining files.

However, 'tar' archives incorporate an end-of-file marker which must
be removed if the concatenated archives are to be read properly as one
archive. '--concatenate' removes the end-of-archive marker from the
target archive before each new archive is appended. If you use 'cat' to
combine the archives, the result will not be a valid 'tar' format
archive. If you need to retrieve files from an archive that was added
to using the 'cat' utility, use the '--ignore-zeros' ('-i') option.
*Note Ignore Zeros::, for further information on dealing with archives
improperly combined using the 'cat' shell utility.

---------- Footnotes ----------

(1) This can cause multiple members to have the same name. For
information on how this affects reading the archive, see *note
multiple::.

4.2.5 Removing Archive Members Using '--delete'
-----------------------------------------------

You can remove members from an archive by using the '--delete' option.
Specify the name of the archive with '--file' ('-f') and then specify
the names of the members to be deleted; if you list no member names,
nothing will be deleted. The '--verbose' option will cause 'tar' to
print the names of the members as they are deleted. As with
'--extract', you must give the exact member names when using 'tar
--delete'. '--delete' will remove all versions of the named file from
the archive. The '--delete' operation can run very slowly.

Unlike other operations, '--delete' has no short form.

This operation will rewrite the archive. You can only use '--delete'
on an archive if the archive device allows you to write to any point on
the media, such as a disk; because of this, it does not work on magnetic
tapes. Do not try to delete an archive member from a magnetic tape; the
action will not succeed, and you will be likely to scramble the archive
and damage your tape. There is no safe way (except by completely
re-writing the archive) to delete files from most kinds of magnetic
tape. *Note Media::.

To delete all versions of the file 'blues' from the archive
'collection.tar' in the 'practice' directory, make sure you are in that
directory, and then,

$ tar --list --file=collection.tar
blues
folk
jazz
rock
$ tar --delete --file=collection.tar blues
$ tar --list --file=collection.tar
folk
jazz
rock

The '--delete' option has been reported to work properly when 'tar'
acts as a filter from 'stdin' to 'stdout'.

4.2.6 Comparing Archive Members with the File System
----------------------------------------------------

The '--compare' ('-d'), or '--diff' operation compares specified archive
members against files with the same names, and then reports differences
in file size, mode, owner, modification date and contents. You should
_only_ specify archive member names, not file names. If you do not name
any members, then 'tar' will compare the entire archive. If a file is
represented in the archive but does not exist in the file system, 'tar'
reports a difference.

You have to specify the record size of the archive when modifying an
archive with a non-default record size.

'tar' ignores files in the file system that do not have corresponding
members in the archive.

The following example compares the archive members 'rock', 'blues'
and 'funk' in the archive 'bluesrock.tar' with files of the same name in
the file system. (Note that there is no file, 'funk'; 'tar' will report
an error message.)

$ tar --compare --file=bluesrock.tar rock blues funk
rock
blues
tar: funk not found in archive

The spirit behind the '--compare' ('--diff', '-d') option is to check
whether the archive represents the current state of files on disk, more
than validating the integrity of the archive media. For this latter
goal, see *note verify::.

4.3 Options Used by '--create'
==============================

The previous chapter described the basics of how to use '--create'
('-c') to create an archive from a set of files. *Note create::. This
section described advanced options to be used with '--create'.

4.3.1 Overriding File Metadata
------------------------------

As described above, a 'tar' archive keeps, for each member it contains,
its "metadata", such as modification time, mode and ownership of the
file. GNU 'tar' allows to replace these data with other values when
adding files to the archive. The options described in this section
affect creation of archives of any type. For POSIX archives, see also
*note PAX keywords::, for additional ways of controlling metadata,
stored in the archive.

'--mode=PERMISSIONS'

When adding files to an archive, 'tar' will use PERMISSIONS for the
archive members, rather than the permissions from the files.
PERMISSIONS can be specified either as an octal number or as
symbolic permissions, like with 'chmod' (*Note Permissions:
(fileutils)File permissions. This reference also has useful
information for those not being overly familiar with the UNIX
permission system). Using latter syntax allows for more
flexibility. For example, the value 'a+rw' adds read and write
permissions for everybody, while retaining executable bits on
directories or on any other file already marked as executable:

$ tar -c -f archive.tar --mode='a+rw' .

'--mtime=DATE'

When adding files to an archive, 'tar' will use DATE as the
modification time of members when creating archives, instead of
their actual modification times. The argument DATE can be either a
textual date representation in almost arbitrary format (*note Date
input formats::) or a name of an existing file, starting with '/'
or '.'. In the latter case, the modification time of that file
will be used.

The following example will set the modification date to 00:00:00,
January 1, 1970:

$ tar -c -f archive.tar --mtime='1970-01-01' .

When used with '--verbose' (*note verbose tutorial::) GNU 'tar'
will try to convert the specified date back to its textual
representation and compare it with the one given with '--mtime'
options. If the two dates differ, 'tar' will print a warning
saying what date it will use. This is to help user ensure he is
using the right date.

For example:

$ tar -c -f archive.tar -v --mtime=yesterday .
tar: Option --mtime: Treating date 'yesterday' as 2006-06-20
13:06:29.152478
...

When used with '--clamp-mtime' GNU 'tar' will only set the
modification date to DATE on files whose actual modification date
is later than DATE. This is to make it easy to build reproducible
archives given a common timestamp for generated files while still
retaining the original timestamps of untouched files.

$ tar -c -f archive.tar --clamp-mtime --mtime=@$SOURCE_DATE_EPOCH .

'--owner=USER'

Specifies that 'tar' should use USER as the owner of members when
creating archives, instead of the user associated with the source
file.

If USER contains a colon, it is taken to be of the form NAME:ID
where a nonempty NAME specifies the user name and a nonempty ID
specifies the decimal numeric user ID. If USER does not contain a
colon, it is taken to be a user number if it is one or more decimal
digits; otherwise it is taken to be a user name.

If a name is given but no number, the number is inferred from the
current host's user database if possible, and the file's user
number is used otherwise. If a number is given but no name, the
name is inferred from the number if possible, and an empty name is
used otherwise. If both name and number are given, the user
database is not consulted, and the name and number need not be
valid on the current host.

There is no value indicating a missing number, and '0' usually
means 'root'. Some people like to force '0' as the value to offer
in their distributions for the owner of files, because the 'root'
user is anonymous anyway, so that might as well be the owner of
anonymous archives. For example:

$ tar -c -f archive.tar --owner=0 .

or:

$ tar -c -f archive.tar --owner=root .

'--group=GROUP'

Files added to the 'tar' archive will have a group ID of GROUP,
rather than the group from the source file. As with '--owner', the
argument GROUP can be an existing group symbolic name, or a decimal
numeric group ID, or NAME:ID.

The '--owner' and '--group' options affect all files added to the
archive. GNU 'tar' provides also two options that allow for more
detailed control over owner translation:

'--owner-map=FILE'
Read UID translation map from FILE.

When reading, empty lines are ignored. The '#' sign, unless
quoted, introduces a comment, which extends to the end of the line.
Each nonempty line defines mapping for a single UID. It must
consist of two fields separated by any amount of whitespace. The
first field defines original username and UID. It can be a valid
user name or a valid UID prefixed with a plus sign. In both cases
the corresponding UID or user name is inferred from the current
host's user database.

The second field defines the UID and username to map the original
one to. Its format can be the same as described above. Otherwise,
it can have the form NEWNAME:NEWUID, in which case neither NEWNAME
nor NEWUID are required to be valid as per the user database.

For example, consider the following file:

+10 bin
smith root:0

Given this file, each input file that is owner by UID 10 will be
stored in archive with owner name 'bin' and owner UID corresponding
to 'bin'. Each file owned by user 'smith' will be stored with
owner name 'root' and owner ID 0. Other files will remain
unchanged.

When used together with '--owner-map', the '--owner' option affects
only files whose owner is not listed in the map file.

'--group-map=FILE'
Read GID translation map from FILE.

The format of FILE is the same as for '--owner-map' option:

Each nonempty line defines mapping for a single GID. It must
consist of two fields separated by any amount of whitespace. The
first field defines original group name and GID. It can be a valid
group name or a valid GID prefixed with a plus sign. In both cases
the corresponding GID or user name is inferred from the current
host's group database.

The second field defines the GID and group name to map the original
one to. Its format can be the same as described above. Otherwise,
it can have the form NEWNAME:NEWGID, in which case neither NEWNAME
nor NEWGID are required to be valid as per the group database.

When used together with '--group-map', the '--group' option affects
only files whose owner group is not rewritten using the map file.

4.3.2 Extended File Attributes
------------------------------

Extended file attributes are name-value pairs that can be associated
with each node in a file system. Despite the fact that POSIX.1e draft
which proposed them has been withdrawn, the extended file attributes are
supported by many file systems. GNU 'tar' can store extended file
attributes along with the files. This feature is controlled by the
following command line arguments:

'--xattrs'
Enable extended attributes support. When used with '--create',
this option instructs GNU 'tar' to store extended file attribute in
the created archive. This implies POSIX.1-2001 archive format
('--format=pax').

When used with '--extract', this option tells 'tar', for each file
extracted, to read stored attributes from the archive and to apply
them to the file.

'--no-xattrs'
Disable extended attributes support. This is the default.

Attribute names are strings prefixed by a "namespace" name and a dot.
Currently, four namespaces exist: 'user', 'trusted', 'security' and
'system'. By default, when '--xattr' is used, all names are stored in
the archive (or extracted, if using '--extract'). This can be
controlled using the following options:

'--xattrs-exclude=PATTERN'
Specify exclude pattern for extended attributes.

'--xattrs-include=PATTERN'
Specify include pattern for extended attributes.

Here, the PATTERN is POSIX regular expression. For example, the
following command:

$ tar --xattrs --xattrs-exclude='^user\.' -c a.tar .

will include in the archive 'a.tar' all attributes, except those from
the 'user' namespace.

Any number of these options can be given, thereby creating lists of
include and exclude patterns.

When both options are used, first '--xattrs-inlcude' is applied to
select the set of attribute names to keep, and then '--xattrs-exclude'
is applied to the resulting set. In other words, only those attributes
will be stored, whose names match one of the regexps in
'--xattrs-inlcude' and don't match any of the regexps from
'--xattrs-exclude'.

When listing the archive, if both '--xattrs' and '--verbose' options
are given, files that have extended attributes are marked with an
asterisk following their permission mask. For example:

-rw-r--r--* smith/users 110 2016-03-16 16:07 file

When two or more '--verbose' options are given, a detailed listing of
extended attributes is printed after each file entry. Each attribute is
listed on a separate line, which begins with two spaces and the letter
'x' indicating extended attribute. It is followed by a colon, length of
the attribute and its name, e.g.:

-rw-r--r--* smith/users 110 2016-03-16 16:07 file
x: 7 user.mime_type
x: 32 trusted.md5sum

File access control lists ("ACL") are another actively used feature
proposed by the POSIX.1e standard. Each ACL consists of a set of ACL
entries, each of which describes the access permissions on the file for
an individual user or a group of users as a combination of read, write
and search/execute permissions.

Whether or not to use ACLs is controlled by the following two
options:

'--acls'
Enable POSIX ACLs support. When used with '--create', this option
instructs GNU 'tar' to store ACLs in the created archive. This
implies POSIX.1-2001 archive format ('--format=pax').

When used with '--extract', this option tells 'tar', to restore
ACLs for each file extracted (provided they are present in the
archive).

'--no-acls'
Disable POSIX ACLs support. This is the default.

When listing the archive, if both '--acls' and '--verbose' options
are given, files that have ACLs are marked with a plus sing following
their permission mask. For example:

-rw-r--r--+ smith/users 110 2016-03-16 16:07 file

When two or more '--verbose' options are given, a detailed listing of
ACL is printed after each file entry:

-rw-r--r--+ smith/users 110 2016-03-16 16:07 file
a: user::rw-,user:gray:-w-,group::r--,mask::rw-,other::r--

"Security-Enhanced Linux" ("SELinux" for short) is a Linux kernel
security module that provides a mechanism for supporting access control
security policies, including so-called mandatory access controls
("MAC"). Support for SELinux attributes is controlled by the following
command line options:

'--selinux'
Enable the SELinux context support.

'--no-selinux'
Disable SELinux context support.

4.3.3 Ignore Fail Read
----------------------

'--ignore-failed-read'
Do not exit with nonzero on unreadable files or directories.

4.4 Options Used by '--extract'
===============================

The previous chapter showed how to use '--extract' to extract an archive
into the file system. Various options cause 'tar' to extract more
information than just file contents, such as the owner, the permissions,
the modification date, and so forth. This section presents options to
be used with '--extract' when certain special considerations arise. You
may review the information presented in *note extract:: for more basic
information about the '--extract' operation.

4.4.1 Options to Help Read Archives
-----------------------------------

Normally, 'tar' will request data in full record increments from an
archive storage device. If the device cannot return a full record,
'tar' will report an error. However, some devices do not always return
full records, or do not require the last record of an archive to be
padded out to the next record boundary. To keep reading until you
obtain a full record, or to accept an incomplete record if it contains
an end-of-archive marker, specify the '--read-full-records' ('-B')
option in conjunction with the '--extract' or '--list' operations.
*Note Blocking::.

The '--read-full-records' ('-B') option is turned on by default when
'tar' reads an archive from standard input, or from a remote machine.
This is because on BSD Unix systems, attempting to read a pipe returns
however much happens to be in the pipe, even if it is less than was
requested. If this option were not enabled, 'tar' would fail as soon as
it read an incomplete record from the pipe.

If you're not sure of the blocking factor of an archive, you can read
the archive by specifying '--read-full-records' ('-B') and
'--blocking-factor=512-SIZE' ('-b 512-SIZE'), using a blocking factor
larger than what the archive uses. This lets you avoid having to
determine the blocking factor of an archive. *Note Blocking Factor::.

Reading Full Records
....................

'--read-full-records'
'-B'
Use in conjunction with '--extract' ('--get', '-x') to read an
archive which contains incomplete records, or one which has a
blocking factor less than the one specified.

Ignoring Blocks of Zeros
........................

Normally, 'tar' stops reading when it encounters a block of zeros
between file entries (which usually indicates the end of the archive).
'--ignore-zeros' ('-i') allows 'tar' to completely read an archive which
contains a block of zeros before the end (i.e., a damaged archive, or
one that was created by concatenating several archives together).

The '--ignore-zeros' ('-i') option is turned off by default because
many versions of 'tar' write garbage after the end-of-archive entry,
since that part of the media is never supposed to be read. GNU 'tar'
does not write after the end of an archive, but seeks to maintain
compatibility among archiving utilities.

'--ignore-zeros'
'-i'
To ignore blocks of zeros (i.e., end-of-archive entries) which may
be encountered while reading an archive. Use in conjunction with
'--extract' or '--list'.

4.4.2 Changing How 'tar' Writes Files
-------------------------------------

_(This message will disappear, once this node revised.)_

Options Controlling the Overwriting of Existing Files
.....................................................

When extracting files, if 'tar' discovers that the extracted file
already exists, it normally replaces the file by removing it before
extracting it, to prevent confusion in the presence of hard or symbolic
links. (If the existing file is a symbolic link, it is removed, not
followed.) However, if a directory cannot be removed because it is
nonempty, 'tar' normally overwrites its metadata (ownership, permission,
etc.). The '--overwrite-dir' option enables this default behavior. To
be more cautious and preserve the metadata of such a directory, use the
'--no-overwrite-dir' option.

To be even more cautious and prevent existing files from being
replaced, use the '--keep-old-files' ('-k') option. It causes 'tar' to
refuse to replace or update a file that already exists, i.e., a file
with the same name as an archive member prevents extraction of that
archive member. Instead, it reports an error. For example:

$ ls
blues
$ tar -x -k -f archive.tar
tar: blues: Cannot open: File exists
tar: Exiting with failure status due to previous errors

If you wish to preserve old files untouched, but don't want 'tar' to
treat them as errors, use the '--skip-old-files' option. This option
causes 'tar' to silently skip extracting over existing files.

To be more aggressive about altering existing files, use the
'--overwrite' option. It causes 'tar' to overwrite existing files and
to follow existing symbolic links when extracting.

Some people argue that GNU 'tar' should not hesitate to overwrite
files with other files when extracting. When extracting a 'tar'
archive, they expect to see a faithful copy of the state of the file
system when the archive was created. It is debatable that this would
always be a proper behavior. For example, suppose one has an archive in
which 'usr/local' is a link to 'usr/local2'. Since then, maybe the site
removed the link and renamed the whole hierarchy from '/usr/local2' to
'/usr/local'. Such things happen all the time. I guess it would not be
welcome at all that GNU 'tar' removes the whole hierarchy just to make
room for the link to be reinstated (unless it _also_ simultaneously
restores the full '/usr/local2', of course!) GNU 'tar' is indeed able
to remove a whole hierarchy to reestablish a symbolic link, for example,
but _only if_ '--recursive-unlink' is specified to allow this behavior.
In any case, single files are silently removed.

Finally, the '--unlink-first' ('-U') option can improve performance
in some cases by causing 'tar' to remove files unconditionally before
extracting them.

Overwrite Old Files
...................

'--overwrite'
Overwrite existing files and directory metadata when extracting
files from an archive.

This causes 'tar' to write extracted files into the file system
without regard to the files already on the system; i.e., files with
the same names as archive members are overwritten when the archive
is extracted. It also causes 'tar' to extract the ownership,
permissions, and time stamps onto any preexisting files or
directories. If the name of a corresponding file name is a
symbolic link, the file pointed to by the symbolic link will be
overwritten instead of the symbolic link itself (if this is
possible). Moreover, special devices, empty directories and even
symbolic links are automatically removed if they are in the way of
extraction.

Be careful when using the '--overwrite' option, particularly when
combined with the '--absolute-names' ('-P') option, as this
combination can change the contents, ownership or permissions of
any file on your system. Also, many systems do not take kindly to
overwriting files that are currently being executed.

'--overwrite-dir'
Overwrite the metadata of directories when extracting files from an
archive, but remove other files before extracting.

Keep Old Files
..............

GNU 'tar' provides two options to control its actions in a situation
when it is about to extract a file which already exists on disk.

'--keep-old-files'
'-k'
Do not replace existing files from archive. When such a file is
encountered, 'tar' issues an error message. Upon end of
extraction, 'tar' exits with code 2 (*note exit status::).

'--skip-old-files'
Do not replace existing files from archive, but do not treat that
as error. Such files are silently skipped and do not affect 'tar'
exit status.

Additional verbosity can be obtained using
'--warning=existing-file' together with that option (*note
warnings::).

Keep Newer Files
................

'--keep-newer-files'
Do not replace existing files that are newer than their archive
copies. This option is meaningless with '--list' ('-t').

Unlink First
............

'--unlink-first'
'-U'
Remove files before extracting over them. This can make 'tar' run
a bit faster if you know in advance that the extracted files all
need to be removed. Normally this option slows 'tar' down
slightly, so it is disabled by default.

Recursive Unlink
................

'--recursive-unlink'
When this option is specified, try removing files and directory
hierarchies before extracting over them. _This is a dangerous
option!_

If you specify the '--recursive-unlink' option, 'tar' removes
_anything_ that keeps you from extracting a file as far as current
permissions will allow it. This could include removal of the contents
of a full directory hierarchy.

Setting Data Modification Times
...............................

Normally, 'tar' sets the data modification times of extracted files to
the corresponding times recorded for the files in the archive, but
limits the permissions of extracted files by the current 'umask'
setting.

To set the data modification times of extracted files to the time
when the files were extracted, use the '--touch' ('-m') option in
conjunction with '--extract' ('--get', '-x').

'--touch'
'-m'
Sets the data modification time of extracted archive members to the
time they were extracted, not the time recorded for them in the
archive. Use in conjunction with '--extract' ('--get', '-x').

Setting Access Permissions
..........................

To set the modes (access permissions) of extracted files to those
recorded for those files in the archive, use '--same-permissions' in
conjunction with the '--extract' ('--get', '-x') operation.

'--preserve-permissions'
'--same-permissions'
'-p'
Set modes of extracted archive members to those recorded in the
archive, instead of current umask settings. Use in conjunction
with '--extract' ('--get', '-x').

Directory Modification Times and Permissions
............................................

After successfully extracting a file member, GNU 'tar' normally restores
its permissions and modification times, as described in the previous
sections. This cannot be done for directories, because after extracting
a directory 'tar' will almost certainly extract files into that
directory and this will cause the directory modification time to be
updated. Moreover, restoring that directory permissions may not permit
file creation within it. Thus, restoring directory permissions and
modification times must be delayed at least until all files have been
extracted into that directory. GNU 'tar' restores directories using the
following approach.

The extracted directories are created with the mode specified in the
archive, as modified by the umask of the user, which gives sufficient
permissions to allow file creation. The meta-information about the
directory is recorded in the temporary list of directories. When
preparing to extract next archive member, GNU 'tar' checks if the
directory prefix of this file contains the remembered directory. If it
does not, the program assumes that all files have been extracted into
that directory, restores its modification time and permissions and
removes its entry from the internal list. This approach allows to
correctly restore directory meta-information in the majority of cases,
while keeping memory requirements sufficiently small. It is based on
the fact, that most 'tar' archives use the predefined order of members:
first the directory, then all the files and subdirectories in that
directory.

However, this is not always true. The most important exception are
incremental archives (*note Incremental Dumps::). The member order in
an incremental archive is reversed: first all directory members are
stored, followed by other (non-directory) members. So, when extracting
from incremental archives, GNU 'tar' alters the above procedure. It
remembers all restored directories, and restores their meta-data only
after the entire archive has been processed. Notice, that you do not
need to specify any special options for that, as GNU 'tar' automatically
detects archives in incremental format.

There may be cases, when such processing is required for normal
archives too. Consider the following example:

$ tar --no-recursion -cvf archive \
foo foo/file1 bar bar/file foo/file2
foo/
foo/file1
bar/
bar/file
foo/file2

During the normal operation, after encountering 'bar' GNU 'tar' will
assume that all files from the directory 'foo' were already extracted
and will therefore restore its timestamp and permission bits. However,
after extracting 'foo/file2' the directory timestamp will be offset
again.

To correctly restore directory meta-information in such cases, use
the '--delay-directory-restore' command line option:

'--delay-directory-restore'
Delays restoring of the modification times and permissions of
extracted directories until the end of extraction. This way,
correct meta-information is restored even if the archive has
unusual member ordering.

'--no-delay-directory-restore'
Cancel the effect of the previous '--delay-directory-restore'. Use
this option if you have used '--delay-directory-restore' in
'TAR_OPTIONS' variable (*note TAR_OPTIONS::) and wish to
temporarily disable it.

Writing to Standard Output
..........................

To write the extracted files to the standard output, instead of creating
the files on the file system, use '--to-stdout' ('-O') in conjunction
with '--extract' ('--get', '-x'). This option is useful if you are
extracting files to send them through a pipe, and do not need to
preserve them in the file system. If you extract multiple members, they
appear on standard output concatenated, in the order they are found in
the archive.

'--to-stdout'
'-O'
Writes files to the standard output. Use only in conjunction with
'--extract' ('--get', '-x'). When this option is used, instead of
creating the files specified, 'tar' writes the contents of the
files extracted to its standard output. This may be useful if you
are only extracting the files in order to send them through a pipe.
This option is meaningless with '--list' ('-t').

This can be useful, for example, if you have a tar archive containing
a big file and don't want to store the file on disk before processing
it. You can use a command like this:

tar -xOzf foo.tgz bigfile | process

or even like this if you want to process the concatenation of the
files:

tar -xOzf foo.tgz bigfile1 bigfile2 | process

However, '--to-command' may be more convenient for use with multiple
files. See the next section.

Writing to an External Program
..............................

You can instruct 'tar' to send the contents of each extracted file to
the standard input of an external program:

'--to-command=COMMAND'
Extract files and pipe their contents to the standard input of
COMMAND. When this option is used, instead of creating the files
specified, 'tar' invokes COMMAND and pipes the contents of the
files to its standard output. The COMMAND may contain command line
arguments (see *note Running External Commands: external, for more
detail).

Notice, that COMMAND is executed once for each regular file
extracted. Non-regular files (directories, etc.) are ignored when
this option is used.

The command can obtain the information about the file it processes
from the following environment variables:

'TAR_FILETYPE'
Type of the file. It is a single letter with the following
meaning:

f Regular file
d Directory
l Symbolic link
h Hard link
b Block device
c Character device

Currently only regular files are supported.

'TAR_MODE'
File mode, an octal number.

'TAR_FILENAME'
The name of the file.

'TAR_REALNAME'
Name of the file as stored in the archive.

'TAR_UNAME'
Name of the file owner.

'TAR_GNAME'
Name of the file owner group.

'TAR_ATIME'
Time of last access. It is a decimal number, representing seconds
since the Epoch. If the archive provides times with nanosecond
precision, the nanoseconds are appended to the timestamp after a
decimal point.

'TAR_MTIME'
Time of last modification.

'TAR_CTIME'
Time of last status change.

'TAR_SIZE'
Size of the file.

'TAR_UID'
UID of the file owner.

'TAR_GID'
GID of the file owner.

Additionally, the following variables contain information about tar
mode and the archive being processed:

'TAR_VERSION'
GNU 'tar' version number.

'TAR_ARCHIVE'
The name of the archive 'tar' is processing.

'TAR_BLOCKING_FACTOR'
Current blocking factor (*note Blocking::).

'TAR_VOLUME'
Ordinal number of the volume 'tar' is processing.

'TAR_FORMAT'
Format of the archive being processed. *Note Formats::, for a
complete list of archive format names.

These variables are defined prior to executing the command, so you
can pass them as arguments, if you prefer. For example, if the command
PROC takes the member name and size as its arguments, then you could do:

$ tar -x -f archive.tar \
--to-command='proc $TAR_FILENAME $TAR_SIZE'

Notice single quotes to prevent variable names from being expanded by
the shell when invoking 'tar'.

If COMMAND exits with a non-0 status, 'tar' will print an error
message similar to the following:

tar: 2345: Child returned status 1

Here, '2345' is the PID of the finished process.

If this behavior is not wanted, use '--ignore-command-error':

'--ignore-command-error'
Ignore exit codes of subprocesses. Notice that if the program
exits on signal or otherwise terminates abnormally, the error
message will be printed even if this option is used.

'--no-ignore-command-error'
Cancel the effect of any previous '--ignore-command-error' option.
This option is useful if you have set '--ignore-command-error' in
'TAR_OPTIONS' (*note TAR_OPTIONS::) and wish to temporarily cancel
it.

Removing Files
..............

'--remove-files'
Remove files after adding them to the archive.

4.4.3 Coping with Scarce Resources
----------------------------------

_(This message will disappear, once this node revised.)_

Starting File
.............

'--starting-file=NAME'
'-K NAME'
Starts an operation in the middle of an archive. Use in
conjunction with '--extract' ('--get', '-x') or '--list' ('-t').

If a previous attempt to extract files failed due to lack of disk
space, you can use '--starting-file=NAME' ('-K NAME') to start
extracting only after member NAME of the archive. This assumes, of
course, that there is now free space, or that you are now extracting
into a different file system. (You could also choose to suspend 'tar',
remove unnecessary files from the file system, and then resume the same
'tar' operation. In this case, '--starting-file' is not necessary.)
See also *note interactive::, and *note exclude::.

Same Order
..........

'--same-order'
'--preserve-order'
'-s'
To process large lists of file names on machines with small amounts
of memory. Use in conjunction with '--compare' ('--diff', '-d'),
'--list' ('-t') or '--extract' ('--get', '-x').

The '--same-order' ('--preserve-order', '-s') option tells 'tar' that
the list of file names to be listed or extracted is sorted in the same
order as the files in the archive. This allows a large list of names to
be used, even on a small machine that would not otherwise be able to
hold all the names in memory at the same time. Such a sorted list can
easily be created by running 'tar -t' on the archive and editing its
output.

This option is probably never needed on modern computer systems.

4.5 Backup options
==================

GNU 'tar' offers options for making backups of files before writing new
versions. These options control the details of these backups. They may
apply to the archive itself before it is created or rewritten, as well
as individual extracted members. Other GNU programs ('cp', 'install',
'ln', and 'mv', for example) offer similar options.

Backup options may prove unexpectedly useful when extracting archives
containing many members having identical name, or when extracting
archives on systems having file name limitations, making different
members appear as having similar names through the side-effect of name
truncation.

When any existing file is backed up before being overwritten by
extraction, then clashing files are automatically be renamed to be
unique, and the true name is kept for only the last file of a series of
clashing files. By using verbose mode, users may track exactly what
happens.

At the detail level, some decisions are still experimental, and may
change in the future, we are waiting comments from our users. So,
please do not learn to depend blindly on the details of the backup
features. For example, currently, directories themselves are never
renamed through using these options, so, extracting a file over a
directory still has good chances to fail. Also, backup options apply to
created archives, not only to extracted members. For created archives,
backups will not be attempted when the archive is a block or character
device, or when it refers to a remote file.

For the sake of simplicity and efficiency, backups are made by
renaming old files prior to creation or extraction, and not by copying.
The original name is restored if the file creation fails. If a failure
occurs after a partial extraction of a file, both the backup and the
partially extracted file are kept.

'--backup[=METHOD]'
Back up files that are about to be overwritten or removed. Without
this option, the original versions are destroyed.

Use METHOD to determine the type of backups made. If METHOD is not
specified, use the value of the 'VERSION_CONTROL' environment
variable. And if 'VERSION_CONTROL' is not set, use the 'existing'
method.

This option corresponds to the Emacs variable 'version-control';
the same values for METHOD are accepted as in Emacs. This option
also allows more descriptive names. The valid METHODs are:

't'
'numbered'
Always make numbered backups.

'nil'
'existing'
Make numbered backups of files that already have them, simple
backups of the others.

'never'
'simple'
Always make simple backups.

'--suffix=SUFFIX'
Append SUFFIX to each backup file made with '--backup'. If this
option is not specified, the value of the 'SIMPLE_BACKUP_SUFFIX'
environment variable is used. And if 'SIMPLE_BACKUP_SUFFIX' is not
set, the default is '~', just as in Emacs.

4.6 Notable 'tar' Usages
========================

_(This message will disappear, once this node revised.)_

You can easily use archive files to transport a group of files from
one system to another: put all relevant files into an archive on one
computer system, transfer the archive to another system, and extract the
contents there. The basic transfer medium might be magnetic tape,
Internet FTP, or even electronic mail (though you must encode the
archive with 'uuencode' in order to transport it properly by mail).
Both machines do not have to use the same operating system, as long as
they both support the 'tar' program.

For example, here is how you might copy a directory's contents from
one disk to another, while preserving the dates, modes, owners and
link-structure of all the files therein. In this case, the transfer
medium is a "pipe":

$ (cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)

You can avoid subshells by using '-C' option:

$ tar -C sourcedir -cf - . | tar -C targetdir -xf -

The command also works using long option forms:

$ (cd sourcedir; tar --create --file=- . ) \
| (cd targetdir; tar --extract --file=-)

or

$ tar --directory sourcedir --create --file=- . \
| tar --directory targetdir --extract --file=-

This is one of the easiest methods to transfer a 'tar' archive.

4.7 Looking Ahead: The Rest of this Manual
==========================================

You have now seen how to use all eight of the operations available to
'tar', and a number of the possible options. The next chapter explains
how to choose and change file and archive names, how to use files to
store names of other files which you can then call as arguments to 'tar'
(this can help you save time if you expect to archive the same list of
files a number of times), and so forth.

If there are too many files to conveniently list on the command line,
you can list the names in a file, and 'tar' will read that file. *Note
files::.

There are various ways of causing 'tar' to skip over some files, and
not archive them. *Note Choosing::.

5 Performing Backups and Restoring Files
****************************************

GNU 'tar' is distributed along with the scripts for performing backups
and restores. Even if there is a good chance those scripts may be
satisfying to you, they are not the only scripts or methods available
for doing backups and restore. You may well create your own, or use
more sophisticated packages dedicated to that purpose.

Some users are enthusiastic about 'Amanda' (The Advanced Maryland
Automatic Network Disk Archiver), a backup system developed by James da
Silva 'jds@cs.umd.edu' and available on many Unix systems. This is free
software, and it is available from <http://www.amanda.org>.

This chapter documents both the provided shell scripts and 'tar'
options which are more specific to usage as a backup tool.

To "back up" a file system means to create archives that contain all
the files in that file system. Those archives can then be used to
restore any or all of those files (for instance if a disk crashes or a
file is accidentally deleted). File system "backups" are also called
"dumps".

5.1 Using 'tar' to Perform Full Dumps
=====================================

_(This message will disappear, once this node revised.)_

Full dumps should only be made when no other people or programs are
modifying files in the file system. If files are modified while 'tar'
is making the backup, they may not be stored properly in the archive, in
which case you won't be able to restore them if you have to. (Files not
being modified are written with no trouble, and do not corrupt the
entire archive.)

You will want to use the '--label=ARCHIVE-LABEL' ('-V ARCHIVE-LABEL')
option to give the archive a volume label, so you can tell what this
archive is even if the label falls off the tape, or anything like that.

Unless the file system you are dumping is guaranteed to fit on one
volume, you will need to use the '--multi-volume' ('-M') option. Make
sure you have enough tapes on hand to complete the backup.

If you want to dump each file system separately you will need to use
the '--one-file-system' option to prevent 'tar' from crossing file
system boundaries when storing (sub)directories.

The '--incremental' ('-G') (*note Incremental Dumps::) option is not
needed, since this is a complete copy of everything in the file system,
and a full restore from this backup would only be done onto a completely
empty disk.

Unless you are in a hurry, and trust the 'tar' program (and your
tapes), it is a good idea to use the '--verify' ('-W') option, to make
sure your files really made it onto the dump properly. This will also
detect cases where the file was modified while (or just after) it was
being archived. Not all media (notably cartridge tapes) are capable of
being verified, unfortunately.

5.2 Using 'tar' to Perform Incremental Dumps
============================================

"Incremental backup" is a special form of GNU 'tar' archive that stores
additional metadata so that exact state of the file system can be
restored when extracting the archive.

GNU 'tar' currently offers two options for handling incremental
backups: '--listed-incremental=SNAPSHOT-FILE' ('-g SNAPSHOT-FILE') and
'--incremental' ('-G').

The option '--listed-incremental' instructs tar to operate on an
incremental archive with additional metadata stored in a standalone
file, called a "snapshot file". The purpose of this file is to help
determine which files have been changed, added or deleted since the last
backup, so that the next incremental backup will contain only modified
files. The name of the snapshot file is given as an argument to the
option:

'--listed-incremental=FILE'
'-g FILE'
Handle incremental backups with snapshot data in FILE.

To create an incremental backup, you would use '--listed-incremental'
together with '--create' (*note create::). For example:

$ tar --create \
--file=archive.1.tar \
--listed-incremental=/var/log/usr.snar \
/usr

This will create in 'archive.1.tar' an incremental backup of the
'/usr' file system, storing additional metadata in the file
'/var/log/usr.snar'. If this file does not exist, it will be created.
The created archive will then be a "level 0 backup"; please see the next
section for more on backup levels.

Otherwise, if the file '/var/log/usr.snar' exists, it determines
which files are modified. In this case only these files will be stored
in the archive. Suppose, for example, that after running the above
command, you delete file '/usr/doc/old' and create directory
'/usr/local/db' with the following contents:

$ ls /usr/local/db
/usr/local/db/data
/usr/local/db/index

Some time later you create another incremental backup. You will then
see:

$ tar --create \
--file=archive.2.tar \
--listed-incremental=/var/log/usr.snar \
/usr
tar: usr/local/db: Directory is new
usr/local/db/
usr/local/db/data
usr/local/db/index

The created archive 'archive.2.tar' will contain only these three
members. This archive is called a "level 1 backup". Notice that
'/var/log/usr.snar' will be updated with the new data, so if you plan to
create more 'level 1' backups, it is necessary to create a working copy
of the snapshot file before running 'tar'. The above example will then
be modified as follows:

$ cp /var/log/usr.snar /var/log/usr.snar-1
$ tar --create \
--file=archive.2.tar \
--listed-incremental=/var/log/usr.snar-1 \
/usr

You can force 'level 0' backups either by removing the snapshot file
before running 'tar', or by supplying the '--level=0' option, e.g.:

$ tar --create \
--file=archive.2.tar \
--listed-incremental=/var/log/usr.snar-0 \
--level=0 \
/usr

Incremental dumps depend crucially on time stamps, so the results are
unreliable if you modify a file's time stamps during dumping (e.g., with
the '--atime-preserve=replace' option), or if you set the clock
backwards.

Metadata stored in snapshot files include device numbers, which,
obviously are supposed to be non-volatile values. However, it turns out
that NFS devices have undependable values when an automounter gets in
the picture. This can lead to a great deal of spurious redumping in
incremental dumps, so it is somewhat useless to compare two NFS devices
numbers over time. The solution implemented currently is to consider
all NFS devices as being equal when it comes to comparing directories;
this is fairly gross, but there does not seem to be a better way to go.

Apart from using NFS, there are a number of cases where relying on
device numbers can cause spurious redumping of unmodified files. For
example, this occurs when archiving LVM snapshot volumes. To avoid
this, use '--no-check-device' option:

'--no-check-device'
Do not rely on device numbers when preparing a list of changed
files for an incremental dump.

'--check-device'
Use device numbers when preparing a list of changed files for an
incremental dump. This is the default behavior. The purpose of
this option is to undo the effect of the '--no-check-device' if it
was given in 'TAR_OPTIONS' environment variable (*note
TAR_OPTIONS::).

There is also another way to cope with changing device numbers. It
is described in detail in *note Fixing Snapshot Files::.

Note that incremental archives use 'tar' extensions and may not be
readable by non-GNU versions of the 'tar' program.

To extract from the incremental dumps, use '--listed-incremental'
together with '--extract' option (*note extracting files::). In this
case, 'tar' does not need to access snapshot file, since all the data
necessary for extraction are stored in the archive itself. So, when
extracting, you can give whatever argument to '--listed-incremental',
the usual practice is to use '--listed-incremental=/dev/null'.
Alternatively, you can use '--incremental', which needs no arguments.
In general, '--incremental' ('-G') can be used as a shortcut for
'--listed-incremental' when listing or extracting incremental backups
(for more information regarding this option, *note incremental-op::).

When extracting from the incremental backup GNU 'tar' attempts to
restore the exact state the file system had when the archive was
created. In particular, it will _delete_ those files in the file system
that did not exist in their directories when the archive was created.
If you have created several levels of incremental files, then in order
to restore the exact contents the file system had when the last level
was created, you will need to restore from all backups in turn.
Continuing our example, to restore the state of '/usr' file system, one
would do(1):

$ tar --extract \
--listed-incremental=/dev/null \
--file archive.1.tar
$ tar --extract \
--listed-incremental=/dev/null \
--file archive.2.tar

To list the contents of an incremental archive, use '--list' (*note
list::), as usual. To obtain more information about the archive, use
'--listed-incremental' or '--incremental' combined with two '--verbose'
options(2):

tar --list --incremental --verbose --verbose --file archive.tar

This command will print, for each directory in the archive, the list
of files in that directory at the time the archive was created. This
information is put out in a format which is both human-readable and
unambiguous for a program: each file name is printed as

X FILE

where X is a letter describing the status of the file: 'Y' if the file
is present in the archive, 'N' if the file is not included in the
archive, or a 'D' if the file is a directory (and is included in the
archive). *Note Dumpdir::, for the detailed description of dumpdirs and
status codes. Each such line is terminated by a newline character. The
last line is followed by an additional newline to indicate the end of
the data.

The option '--incremental' ('-G') gives the same behavior as
'--listed-incremental' when used with '--list' and '--extract' options.
When used with '--create' option, it creates an incremental archive
without creating snapshot file. Thus, it is impossible to create
several levels of incremental backups with '--incremental' option.

---------- Footnotes ----------

(1) Notice, that since both archives were created without '-P' option
(*note absolute::), these commands should be run from the root file
system.

(2) Two '--verbose' options were selected to avoid breaking usual
verbose listing output ('--list --verbose') when using in scripts.

Versions of GNU 'tar' up to 1.15.1 used to dump verbatim binary
contents of the DUMPDIR header (with terminating nulls) when
'--incremental' or '--listed-incremental' option was given, no matter
what the verbosity level. This behavior, and, especially, the binary
output it produced were considered inconvenient and were changed in
version 1.16.

5.3 Levels of Backups
=====================

An archive containing all the files in the file system is called a "full
backup" or "full dump". You could insure your data by creating a full
dump every day. This strategy, however, would waste a substantial
amount of archive media and user time, as unchanged files are daily
re-archived.

It is more efficient to do a full dump only occasionally. To back up
files between full dumps, you can use "incremental dumps". A "level
one" dump archives all the files that have changed since the last full
dump.

A typical dump strategy would be to perform a full dump once a week,
and a level one dump once a day. This means some versions of files will
in fact be archived more than once, but this dump strategy makes it
possible to restore a file system to within one day of accuracy by only
extracting two archives--the last weekly (full) dump and the last daily
(level one) dump. The only information lost would be in files changed
or created since the last daily backup. (Doing dumps more than once a
day is usually not worth the trouble.)

GNU 'tar' comes with scripts you can use to do full and level-one
(actually, even level-two and so on) dumps. Using scripts (shell
programs) to perform backups and restoration is a convenient and
reliable alternative to typing out file name lists and 'tar' commands by
hand.

Before you use these scripts, you need to edit the file
'backup-specs', which specifies parameters used by the backup scripts
and by the restore script. This file is usually located in
'/etc/backup' directory. *Note Backup Parameters::, for its detailed
description. Once the backup parameters are set, you can perform
backups or restoration by running the appropriate script.

The name of the backup script is 'backup'. The name of the restore
script is 'restore'. The following sections describe their use in
detail.

_Please Note:_ The backup and restoration scripts are designed to be
used together. While it is possible to restore files by hand from an
archive which was created using a backup script, and to create an
archive by hand which could then be extracted using the restore script,
it is easier to use the scripts. *Note Incremental Dumps::, before
making such an attempt.

5.4 Setting Parameters for Backups and Restoration
==================================================

The file 'backup-specs' specifies backup parameters for the backup and
restoration scripts provided with 'tar'. You must edit 'backup-specs'
to fit your system configuration and schedule before using these
scripts.

Syntactically, 'backup-specs' is a shell script, containing mainly
variable assignments. However, any valid shell construct is allowed in
this file. Particularly, you may wish to define functions within that
script (e.g., see 'RESTORE_BEGIN' below). For more information about
shell script syntax, please refer to the definition of the Shell Command
Language
(http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
g_02). See also *note Bash Features: (bashref)Top.

The shell variables controlling behavior of 'backup' and 'restore'
are described in the following subsections.

5.4.1 General-Purpose Variables
-------------------------------

-- Backup variable: ADMINISTRATOR
The user name of the backup administrator. 'Backup' scripts sends
a backup report to this address.

-- Backup variable: BACKUP_HOUR
The hour at which the backups are done. This can be a number from
0 to 23, or the time specification in form HOURS:MINUTES, or the
string 'now'.

This variable is used by 'backup'. Its value may be overridden
using '--time' option (*note Scripted Backups::).

-- Backup variable: TAPE_FILE

The device 'tar' writes the archive to. If TAPE_FILE is a remote
archive (*note remote-dev::), backup script will suppose that your
'mt' is able to access remote devices. If RSH (*note RSH::) is
set, '--rsh-command' option will be added to invocations of 'mt'.

-- Backup variable: BLOCKING

The blocking factor 'tar' will use when writing the dump archive.
*Note Blocking Factor::.

-- Backup variable: BACKUP_DIRS

A list of file systems to be dumped (for 'backup'), or restored
(for 'restore'). You can include any directory name in the list --
subdirectories on that file system will be included, regardless of
how they may look to other networked machines. Subdirectories on
other file systems will be ignored.

The host name specifies which host to run 'tar' on, and should
normally be the host that actually contains the file system.
However, the host machine must have GNU 'tar' installed, and must
be able to access the directory containing the backup scripts and
their support files using the same file name that is used on the
machine where the scripts are run (i.e., what 'pwd' will print when
in that directory on that machine). If the host that contains the
file system does not have this capability, you can specify another
host as long as it can access the file system through NFS.

If the list of file systems is very long you may wish to put it in
a separate file. This file is usually named '/etc/backup/dirs',
but this name may be overridden in 'backup-specs' using 'DIRLIST'
variable.

-- Backup variable: DIRLIST

The name of the file that contains a list of file systems to backup
or restore. By default it is '/etc/backup/dirs'.

-- Backup variable: BACKUP_FILES

A list of individual files to be dumped (for 'backup'), or restored
(for 'restore'). These should be accessible from the machine on
which the backup script is run.

If the list of individual files is very long you may wish to store
it in a separate file. This file is usually named
'/etc/backup/files', but this name may be overridden in
'backup-specs' using 'FILELIST' variable.

-- Backup variable: FILELIST

The name of the file that contains a list of individual files to
backup or restore. By default it is '/etc/backup/files'.

-- Backup variable: MT

Full file name of 'mt' binary.

-- Backup variable: RSH
Full file name of 'rsh' binary or its equivalent. You may wish to
set it to 'ssh', to improve security. In this case you will have
to use public key authentication.

-- Backup variable: RSH_COMMAND

Full file name of 'rsh' binary on remote machines. This will be
passed via '--rsh-command' option to the remote invocation of GNU
'tar'.

-- Backup variable: VOLNO_FILE

Name of temporary file to hold volume numbers. This needs to be
accessible by all the machines which have file systems to be
dumped.

-- Backup variable: XLIST

Name of "exclude file list". An "exclude file list" is a file
located on the remote machine and containing the list of files to
be excluded from the backup. Exclude file lists are searched in
/etc/tar-backup directory. A common use for exclude file lists is
to exclude files containing security-sensitive information (e.g.,
'/etc/shadow' from backups).

This variable affects only 'backup'.

-- Backup variable: SLEEP_TIME

Time to sleep between dumps of any two successive file systems

This variable affects only 'backup'.

-- Backup variable: DUMP_REMIND_SCRIPT

Script to be run when it's time to insert a new tape in for the
next volume. Administrators may want to tailor this script for
their site. If this variable isn't set, GNU 'tar' will display its
built-in prompt, and will expect confirmation from the console.
For the description of the default prompt, see *note change volume
prompt::.

-- Backup variable: SLEEP_MESSAGE

Message to display on the terminal while waiting for dump time.
Usually this will just be some literal text.

-- Backup variable: TAR

Full file name of the GNU 'tar' executable. If this is not set,
backup scripts will search 'tar' in the current shell path.

5.4.2 Magnetic Tape Control
---------------------------

Backup scripts access tape device using special "hook functions". These
functions take a single argument -- the name of the tape device. Their
names are kept in the following variables:

-- Backup variable: MT_BEGIN
The name of "begin" function. This function is called before
accessing the drive. By default it retensions the tape:

MT_BEGIN=mt_begin

mt_begin() {
mt -f "$1" retension
}

-- Backup variable: MT_REWIND
The name of "rewind" function. The default definition is as
follows:

MT_REWIND=mt_rewind

mt_rewind() {
mt -f "$1" rewind
}

-- Backup variable: MT_OFFLINE
The name of the function switching the tape off line. By default
it is defined as follows:

MT_OFFLINE=mt_offline

mt_offline() {
mt -f "$1" offl
}

-- Backup variable: MT_STATUS
The name of the function used to obtain the status of the archive
device, including error count. Default definition:

MT_STATUS=mt_status

mt_status() {
mt -f "$1" status
}

5.4.3 User Hooks
----------------

"User hooks" are shell functions executed before and after each 'tar'
invocation. Thus, there are "backup hooks", which are executed before
and after dumping each file system, and "restore hooks", executed before
and after restoring a file system. Each user hook is a shell function
taking four arguments:

-- User Hook Function: hook LEVEL HOST FS FSNAME
Its arguments are:

LEVEL
Current backup or restore level.

HOST
Name or IP address of the host machine being dumped or
restored.

FS
Full file name of the file system being dumped or restored.

FSNAME
File system name with directory separators replaced with
colons. This is useful, e.g., for creating unique files.

Following variables keep the names of user hook functions:

-- Backup variable: DUMP_BEGIN
Dump begin function. It is executed before dumping the file
system.

-- Backup variable: DUMP_END
Executed after dumping the file system.

-- Backup variable: RESTORE_BEGIN
Executed before restoring the file system.

-- Backup variable: RESTORE_END
Executed after restoring the file system.

5.4.4 An Example Text of 'Backup-specs'
---------------------------------------

The following is an example of 'backup-specs':

# site-specific parameters for file system backup.

ADMINISTRATOR=friedman
BACKUP_HOUR=1
TAPE_FILE=/dev/nrsmt0

# Use ssh instead of the less secure rsh
RSH=/usr/bin/ssh
RSH_COMMAND=/usr/bin/ssh

# Override MT_STATUS function:
my_status() {
mts -t $TAPE_FILE
}
MT_STATUS=my_status

# Disable MT_OFFLINE function
MT_OFFLINE=:

BLOCKING=124
BACKUP_DIRS="
albert:/fs/fsf
apple-gunkies:/gd
albert:/fs/gd2
albert:/fs/gp
geech:/usr/jla
churchy:/usr/roland
albert:/
albert:/usr
apple-gunkies:/
apple-gunkies:/usr
gnu:/hack
gnu:/u
apple-gunkies:/com/mailer/gnu
apple-gunkies:/com/archive/gnu"

BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"


5.5 Using the Backup Scripts
============================

The syntax for running a backup script is:

backup --level=LEVEL --time=TIME

The '--level' option requests the dump level. Thus, to produce a
full dump, specify '--level=0' (this is the default, so '--level' may be
omitted if its value is '0')(1).

The '--time' option determines when should the backup be run. TIME
may take three forms:

HH:MM

The dump must be run at HH hours MM minutes.

HH

The dump must be run at HH hours.

now

The dump must be run immediately.

You should start a script with a tape or disk mounted. Once you
start a script, it prompts you for new tapes or disks as it needs them.
Media volumes don't have to correspond to archive files -- a
multi-volume archive can be started in the middle of a tape that already
contains the end of another multi-volume archive. The 'restore' script
prompts for media by its archive volume, so to avoid an error message
you should keep track of which tape (or disk) contains which volume of
the archive (*note Scripted Restoration::).

The backup scripts write two files on the file system. The first is
a record file in '/etc/tar-backup/', which is used by the scripts to
store and retrieve information about which files were dumped. This file
is not meant to be read by humans, and should not be deleted by them.
*Note Snapshot Files::, for a more detailed explanation of this file.

The second file is a log file containing the names of the file
systems and files dumped, what time the backup was made, and any error
messages that were generated, as well as how much space was left in the
media volume after the last volume of the archive was written. You
should check this log file after every backup. The file name is
'log-MM-DD-YYYY-level-N', where MM-DD-YYYY represents current date, and
N represents current dump level number.

The script also prints the name of each system being dumped to the
standard output.

Following is the full list of options accepted by 'backup' script:

'-l LEVEL'
'--level=LEVEL'
Do backup level LEVEL (default 0).

'-f'
'--force'
Force backup even if today's log file already exists.

'-v[LEVEL]'
'--verbose[=LEVEL]'
Set verbosity level. The higher the level is, the more debugging
information will be output during execution. Default LEVEL is 100,
which means the highest debugging level.

'-t START-TIME'
'--time=START-TIME'
Wait till TIME, then do backup.

'-h'
'--help'
Display short help message and exit.

'-V'
'--version'
Display information about the program's name, version, origin and
legal status, all on standard output, and then exit successfully.

---------- Footnotes ----------

(1) For backward compatibility, the 'backup' will also try to deduce
the requested dump level from the name of the script itself. If the
name consists of a string 'level-' followed by a single decimal digit,
that digit is taken as the dump level number. Thus, you may create a
link from 'backup' to 'level-1' and then run 'level-1' whenever you need
to create a level one dump.

5.6 Using the Restore Script
============================

To restore files that were archived using a scripted backup, use the
'restore' script. Its usage is quite straightforward. In the simplest
form, invoke 'restore --all', it will then restore all the file systems
and files specified in 'backup-specs' (*note BACKUP_DIRS:
General-Purpose Variables.).

You may select the file systems (and/or files) to restore by giving
'restore' a list of "patterns" in its command line. For example,
running

restore 'albert:*'

will restore all file systems on the machine 'albert'. A more
complicated example:

restore 'albert:*' '*:/var'

This command will restore all file systems on the machine 'albert' as
well as '/var' file system on all machines.

By default 'restore' will start restoring files from the lowest
available dump level (usually zero) and will continue through all
available dump levels. There may be situations where such a thorough
restore is not necessary. For example, you may wish to restore only
files from the recent level one backup. To do so, use '--level' option,
as shown in the example below:

restore --level=1

The full list of options accepted by 'restore' follows:

'-a'
'--all'
Restore all file systems and files specified in 'backup-specs'.

'-l LEVEL'
'--level=LEVEL'
Start restoring from the given backup level, instead of the default
0.

'-v[LEVEL]'
'--verbose[=LEVEL]'
Set verbosity level. The higher the level is, the more debugging
information will be output during execution. Default LEVEL is 100,
which means the highest debugging level.

'-h'
'--help'
Display short help message and exit.

'-V'
'--version'
Display information about the program's name, version, origin and
legal status, all on standard output, and then exit successfully.

You should start the restore script with the media containing the
first volume of the archive mounted. The script will prompt for other
volumes as they are needed. If the archive is on tape, you don't need
to rewind the tape to to its beginning--if the tape head is positioned
past the beginning of the archive, the script will rewind the tape as
needed. *Note Tape Positioning::, for a discussion of tape positioning.

*Warning:* The script will delete files from the active file system
if they were not in the file system when the archive was made.

*Note Incremental Dumps::, for an explanation of how the script makes
that determination.

6 Choosing Files and Names for 'tar'
************************************

Certain options to 'tar' enable you to specify a name for your archive.
Other options let you decide which files to include or exclude from the
archive, based on when or whether files were modified, whether the file
names do or don't match specified patterns, or whether files are in
specified directories.

This chapter discusses these options in detail.

6.1 Choosing and Naming Archive Files
=====================================

By default, 'tar' uses an archive file name that was compiled when it
was built on the system; usually this name refers to some physical tape
drive on the machine. However, the person who installed 'tar' on the
system may not have set the default to a meaningful value as far as most
users are concerned. As a result, you will usually want to tell 'tar'
where to find (or create) the archive. The '--file=ARCHIVE-NAME' ('-f
ARCHIVE-NAME') option allows you to either specify or name a file to use
as the archive instead of the default archive file location.

'--file=ARCHIVE-NAME'
'-f ARCHIVE-NAME'
Name the archive to create or operate on. Use in conjunction with
any operation.

For example, in this 'tar' command,

$ tar -cvf collection.tar blues folk jazz

'collection.tar' is the name of the archive. It must directly follow
the '-f' option, since whatever directly follows '-f' _will_ end up
naming the archive. If you neglect to specify an archive name, you may
end up overwriting a file in the working directory with the archive you
create since 'tar' will use this file's name for the archive name.

An archive can be saved as a file in the file system, sent through a
pipe or over a network, or written to an I/O device such as a tape,
floppy disk, or CD write drive.

If you do not name the archive, 'tar' uses the value of the
environment variable 'TAPE' as the file name for the archive. If that
is not available, 'tar' uses a default, compiled-in archive name,
usually that for tape unit zero (i.e., '/dev/tu00').

If you use '-' as an ARCHIVE-NAME, 'tar' reads the archive from
standard input (when listing or extracting files), or writes it to
standard output (when creating an archive). If you use '-' as an
ARCHIVE-NAME when modifying an archive, 'tar' reads the original archive
from its standard input and writes the entire new archive to its
standard output.

The following example is a convenient way of copying directory
hierarchy from 'sourcedir' to 'targetdir'.

$ (cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)

The '-C' option allows to avoid using subshells:

$ tar -C sourcedir -cf - . | tar -C targetdir -xpf -

In both examples above, the leftmost 'tar' invocation archives the
contents of 'sourcedir' to the standard output, while the rightmost one
reads this archive from its standard input and extracts it. The '-p'
option tells it to restore permissions of the extracted files.

To specify an archive file on a device attached to a remote machine,
use the following:

--file=HOSTNAME:/DEV/FILE-NAME

'tar' will set up the remote connection, if possible, and prompt you for
a username and password. If you use '--file=@HOSTNAME:/DEV/FILE-NAME',
'tar' will attempt to set up the remote connection using your username
as the username on the remote machine.

If the archive file name includes a colon (':'), then it is assumed
to be a file on another machine. If the archive file is
'USER@HOST:FILE', then FILE is used on the host HOST. The remote host
is accessed using the 'rsh' program, with a username of USER. If the
username is omitted (along with the '@' sign), then your user name will
be used. (This is the normal 'rsh' behavior.) It is necessary for the
remote machine, in addition to permitting your 'rsh' access, to have the
'rmt' program installed (this command is included in the GNU 'tar'
distribution and by default is installed under 'PREFIX/libexec/rmt',
where PREFIX means your installation prefix). If you need to use a file
whose name includes a colon, then the remote tape drive behavior can be
inhibited by using the '--force-local' option.

When the archive is being created to '/dev/null', GNU 'tar' tries to
minimize input and output operations. The Amanda backup system, when
used with GNU 'tar', has an initial sizing pass which uses this feature.

6.2 Selecting Archive Members
=============================

"File Name arguments" specify which files in the file system 'tar'
operates on, when creating or adding to an archive, or which archive
members 'tar' operates on, when reading or deleting from an archive.
*Note Operations::.

To specify file names, you can include them as the last arguments on
the command line, as follows:
tar OPERATION [OPTION1 OPTION2 ...] [FILE NAME-1 FILE NAME-2 ...]

If a file name begins with dash ('-'), precede it with '--add-file'
option to prevent it from being treated as an option.

By default GNU 'tar' attempts to "unquote" each file or member name,
replacing "escape sequences" according to the following table:

Escape Replaced with
-----------------------------------------------------------
\a Audible bell (ASCII 7)
\b Backspace (ASCII 8)
\f Form feed (ASCII 12)
\n New line (ASCII 10)
\r Carriage return (ASCII 13)
\t Horizontal tabulation (ASCII 9)
\v Vertical tabulation (ASCII 11)
\? ASCII 127
\N ASCII N (N should be an octal number of
up to 3 digits)

A backslash followed by any other symbol is retained.

This default behavior is controlled by the following command line
option:

'--unquote'
Enable unquoting input file or member names (default).

'--no-unquote'
Disable unquoting input file or member names.

If you specify a directory name as a file name argument, all the
files in that directory are operated on by 'tar'.

If you do not specify files, 'tar' behavior differs depending on the
operation mode as described below:

When 'tar' is invoked with '--create' ('-c'), 'tar' will stop
immediately, reporting the following:

$ tar cf a.tar
tar: Cowardly refusing to create an empty archive
Try 'tar --help' or 'tar --usage' for more information.

If you specify either '--list' ('-t') or '--extract' ('--get', '-x'),
'tar' operates on all the archive members in the archive.

If run with '--diff' option, tar will compare the archive with the
contents of the current working directory.

If you specify any other operation, 'tar' does nothing.

By default, 'tar' takes file names from the command line. However,
there are other ways to specify file or member names, or to modify the
manner in which 'tar' selects the files or members upon which to
operate. In general, these methods work both for specifying the names
of files and archive members.

6.3 Reading Names from a File
=============================

Instead of giving the names of files or archive members on the command
line, you can put the names into a file, and then use the
'--files-from=FILE-OF-NAMES' ('-T FILE-OF-NAMES') option to 'tar'. Give
the name of the file which contains the list of files to include as the
argument to '--files-from'. In the list, the file names should be
separated by newlines. You will frequently use this option when you
have generated the list of files to archive with the 'find' utility.

'--files-from=FILE-NAME'
'-T FILE-NAME'
Get names to extract or create from file FILE-NAME.

If you give a single dash as a file name for '--files-from', (i.e.,
you specify either '--files-from=-' or '-T -'), then the file names are
read from standard input.

Unless you are running 'tar' with '--create', you cannot use both
'--files-from=-' and '--file=-' ('-f -') in the same command.

Any number of '-T' options can be given in the command line.

The following example shows how to use 'find' to generate a list of
files smaller than 400K in length and put that list into a file called
'small-files'. You can then use the '-T' option to 'tar' to specify the
files from that file, 'small-files', to create the archive 'little.tgz'.
(The '-z' option to 'tar' compresses the archive with 'gzip'; *note
gzip:: for more information.)

$ find . -size -400 -print > small-files
$ tar -c -v -z -T small-files -f little.tgz

By default, each line read from the file list is first stripped off any
leading and trailing whitespace. If the resulting string begins with
'-' character, it is considered a 'tar' option and is processed
accordingly(1). For example, the common use of this feature is to
change to another directory by specifying '-C' option:

$ cat list
-C/etc
passwd
hosts
-C/lib
libc.a
$ tar -c -f foo.tar --files-from list

In this example, 'tar' will first switch to '/etc' directory and add
files 'passwd' and 'hosts' to the archive. Then it will change to
'/lib' directory and will archive the file 'libc.a'. Thus, the
resulting archive 'foo.tar' will contain:

$ tar tf foo.tar
passwd
hosts
libc.a

Note, that any options used in the file list remain in effect for the
rest of the command line. For example, using the same 'list' file as
above, the following command

$ tar -c -f foo.tar --files-from list libcurses.a

will look for file 'libcurses.a' in the directory '/lib', because it was
used with the last '-C' option (*note Position-Sensitive Options::).

If such option handling is undesirable, use the
'--verbatim-files-from' option. When this option is in effect, each
line read from the file list is treated as a file name. Notice, that
this means, in particular, that no whitespace trimming is performed.

The '--verbatim-files-from' affects all '-T' options that follow it
in the command line. The default behavior can be restored using
'--no-verbatim-files-from' option.

To disable option handling for a single file name, use the
'--add-file' option, e.g.: '--add-file=--my-file'.

You can use any GNU 'tar' command line options in the file list file,
including '--files-from' option itself. This allows for including
contents of a file list into another file list file. Note however, that
options that control file list processing, such as
'--verbatim-files-from' or '--null' won't affect the file they appear
in. They will affect next '--files-from' option, if there is any.

---------- Footnotes ----------

(1) Versions of GNU 'tar' up to 1.15.1 recognized only '-C' option in
file lists, and only if the option and its argument occupied two
consecutive lines.

6.3.1 'NUL'-Terminated File Names
---------------------------------

The '--null' option causes '--files-from=FILE-OF-NAMES' ('-T
FILE-OF-NAMES') to read file names terminated by a 'NUL' instead of a
newline, so files whose names contain newlines can be archived using
'--files-from'.

'--null'
Only consider 'NUL'-terminated file names, instead of files that
terminate in a newline.

'--no-null'
Undo the effect of any previous '--null' option.

The '--null' option is just like the one in GNU 'xargs' and 'cpio',
and is useful with the '-print0' predicate of GNU 'find'. In 'tar',
'--null' also disables special handling for file names that begin with
dash (similar to '--verbatim-files-from' option).

This example shows how to use 'find' to generate a list of files
larger than 800K in length and put that list into a file called
'long-files'. The '-print0' option to 'find' is just like '-print',
except that it separates files with a 'NUL' rather than with a newline.
You can then run 'tar' with both the '--null' and '-T' options to
specify that 'tar' gets the files from that file, 'long-files', to
create the archive 'big.tgz'. The '--null' option to 'tar' will cause
'tar' to recognize the 'NUL' separator between files.

$ find . -size +800 -print0 > long-files
$ tar -c -v --null --files-from=long-files --file=big.tar

The '--no-null' option can be used if you need to read both
'NUL'-terminated and newline-terminated files on the same command line.
For example, if 'flist' is a newline-terminated file, then the following
command can be used to combine it with the above command:

$ find . -size +800 -print0 |
tar -c -f big.tar --null -T - --no-null -T flist

This example uses short options for typographic reasons, to avoid
very long lines.

GNU 'tar' is tries to automatically detect 'NUL'-terminated file
lists, so in many cases it is safe to use them even without the '--null'
option. In this case 'tar' will print a warning and continue reading
such a file as if '--null' were actually given:

$ find . -size +800 -print0 | tar -c -f big.tar -T -
tar: -: file name read contains nul character

The null terminator, however, remains in effect only for this
particular file, any following '-T' options will assume newline
termination. Of course, the null autodetection applies to these
eventual surplus '-T' options as well.

6.4 Excluding Some Files
========================

To avoid operating on files whose names match a particular pattern, use
the '--exclude' or '--exclude-from' options.

'--exclude=PATTERN'
Causes 'tar' to ignore files that match the PATTERN.

The '--exclude=PATTERN' option prevents any file or member whose name
matches the shell wildcard (PATTERN) from being operated on. For
example, to create an archive with all the contents of the directory
'src' except for files whose names end in '.o', use the command 'tar -cf
src.tar --exclude='*.o' src'.

You may give multiple '--exclude' options.

'--exclude-from=FILE'
'-X FILE'
Causes 'tar' to ignore files that match the patterns listed in
FILE.

Use the '--exclude-from' option to read a list of patterns, one per
line, from FILE; 'tar' will ignore files matching those patterns. Thus
if 'tar' is called as 'tar -c -X foo .' and the file 'foo' contains a
single line '*.o', no files whose names end in '.o' will be added to the
archive.

Notice, that lines from FILE are read verbatim. One of the frequent
errors is leaving some extra whitespace after a file name, which is
difficult to catch using text editors.

However, empty lines are OK.

When archiving directories that are under some version control system
(VCS), it is often convenient to read exclusion patterns from this VCS'
ignore files (e.g. '.cvsignore', '.gitignore', etc.) The following
options provide such possibility:

'--exclude-vcs-ignores'
Before archiving a directory, see if it contains any of the
following files: 'cvsignore', '.gitignore', '.bzrignore', or
'.hgignore'. If so, read ignore patterns from these files.

The patterns are treated much as the corresponding VCS would treat
them, i.e.:

'.cvsignore'
Contains shell-style globbing patterns that apply only to the
directory where this file resides. No comments are allowed in
the file. Empty lines are ignored.

'.gitignore'
Contains shell-style globbing patterns. Applies to the
directory where '.gitfile' is located and all its
subdirectories.

Any line beginning with a '#' is a comment. Backslash escapes
the comment character.

'.bzrignore'
Contains shell globbing-patterns and regular expressions (if
prefixed with 'RE:'(1). Patterns affect the directory and all
its subdirectories.

Any line beginning with a '#' is a comment.

'.hgignore'
Contains posix regular expressions(2). The line 'syntax:
glob' switches to shell globbing patterns. The line 'syntax:
regexp' switches back. Comments begin with a '#'. Patterns
affect the directory and all its subdirectories.

'--exclude-ignore=FILE'
Before dumping a directory, 'tar' checks if it contains FILE. If
so, exclusion patterns are read from this file. The patterns
affect only the directory itself.

'--exclude-ignore-recursive=FILE'
Same as '--exclude-ignore', except that the patterns read affect
both the directory where FILE resides and all its subdirectories.

'--exclude-vcs'
Exclude files and directories used by following version control
systems: 'CVS', 'RCS', 'SCCS', 'SVN', 'Arch', 'Bazaar',
'Mercurial', and 'Darcs'.

As of version 1.29, the following files are excluded:

* 'CVS/', and everything under it
* 'RCS/', and everything under it
* 'SCCS/', and everything under it
* '.git/', and everything under it
* '.gitignore'
* '.gitmodules'
* '.gitattributes'
* '.cvsignore'
* '.svn/', and everything under it
* '.arch-ids/', and everything under it
* '{arch}/', and everything under it
* '=RELEASE-ID'
* '=meta-update'
* '=update'
* '.bzr'
* '.bzrignore'
* '.bzrtags'
* '.hg'
* '.hgignore'
* '.hgrags'
* '_darcs'

'--exclude-backups'
Exclude backup and lock files. This option causes exclusion of
files that match the following shell globbing patterns:

.#*
*~
#*#

When creating an archive, the '--exclude-caches' option family causes
'tar' to exclude all directories that contain a "cache directory tag".
A cache directory tag is a short file with the well-known name
'CACHEDIR.TAG' and having a standard header specified in
<http://www.brynosaurus.com/cachedir/spec.html>. Various applications
write cache directory tags into directories they use to hold
regenerable, non-precious data, so that such data can be more easily
excluded from backups.

There are three 'exclude-caches' options, each providing a different
exclusion semantics:

'--exclude-caches'
Do not archive the contents of the directory, but archive the
directory itself and the 'CACHEDIR.TAG' file.

'--exclude-caches-under'
Do not archive the contents of the directory, nor the
'CACHEDIR.TAG' file, archive only the directory itself.

'--exclude-caches-all'
Omit directories containing 'CACHEDIR.TAG' file entirely.

Another option family, '--exclude-tag', provides a generalization of
this concept. It takes a single argument, a file name to look for. Any
directory that contains this file will be excluded from the dump.
Similarly to 'exclude-caches', there are three options in this option
family:

'--exclude-tag=FILE'
Do not dump the contents of the directory, but dump the directory
itself and the FILE.

'--exclude-tag-under=FILE'
Do not dump the contents of the directory, nor the FILE, archive
only the directory itself.

'--exclude-tag-all=FILE'
Omit directories containing FILE file entirely.

Multiple '--exclude-tag*' options can be given.

For example, given this directory:

$ find dir
dir
dir/blues
dir/jazz
dir/folk
dir/folk/tagfile
dir/folk/sanjuan
dir/folk/trote

The '--exclude-tag' will produce the following:

$ tar -cf archive.tar --exclude-tag=tagfile -v dir
dir/
dir/blues
dir/jazz
dir/folk/
tar: dir/folk/: contains a cache directory tag tagfile;
contents not dumped
dir/folk/tagfile

Both the 'dir/folk' directory and its tagfile are preserved in the
archive, however the rest of files in this directory are not.

Now, using the '--exclude-tag-under' option will exclude 'tagfile'
from the dump, while still preserving the directory itself, as shown in
this example:

$ tar -cf archive.tar --exclude-tag-under=tagfile -v dir
dir/
dir/blues
dir/jazz
dir/folk/
./tar: dir/folk/: contains a cache directory tag tagfile;
contents not dumped

Finally, using '--exclude-tag-all' omits the 'dir/folk' directory
entirely:

$ tar -cf archive.tar --exclude-tag-all=tagfile -v dir
dir/
dir/blues
dir/jazz
./tar: dir/folk/: contains a cache directory tag tagfile;
directory not dumped

---------- Footnotes ----------

(1) According to the Bazaar docs, globbing-patterns are Korn-shell
style and regular expressions are perl-style. As of GNU 'tar' version
1.29, these are treated as shell-style globs and posix extended regexps.
This will be fixed in future releases.

(2) Support for perl-style regexps will appear in future releases.

Problems with Using the 'exclude' Options
-----------------------------------------

Some users find 'exclude' options confusing. Here are some common
pitfalls:

* The main operating mode of 'tar' does not act on a file name
explicitly listed on the command line, if one of its file name
components is excluded. In the example above, if you create an
archive and exclude files that end with '*.o', but explicitly name
the file 'dir.o/foo' after all the options have been listed,
'dir.o/foo' will be excluded from the archive.

* You can sometimes confuse the meanings of '--exclude' and
'--exclude-from'. Be careful: use '--exclude' when files to be
excluded are given as a pattern on the command line. Use
'--exclude-from' to introduce the name of a file which contains a
list of patterns, one per line; each of these patterns can exclude
zero, one, or many files.

* When you use '--exclude=PATTERN', be sure to quote the PATTERN
parameter, so GNU 'tar' sees wildcard characters like '*'. If you
do not do this, the shell might expand the '*' itself using files
at hand, so 'tar' might receive a list of files instead of one
pattern, or none at all, making the command somewhat illegal. This
might not correspond to what you want.

For example, write:

$ tar -c -f ARCHIVE.TAR --exclude '*.o' DIRECTORY

rather than:

# _Wrong!_
$ tar -c -f ARCHIVE.TAR --exclude *.o DIRECTORY

* You must use use shell syntax, or globbing, rather than 'regexp'
syntax, when using exclude options in 'tar'. If you try to use
'regexp' syntax to describe files to be excluded, your command
might fail.

*
In earlier versions of 'tar', what is now the '--exclude-from'
option was called '--exclude' instead. Now, '--exclude' applies to
patterns listed on the command line and '--exclude-from' applies to
patterns listed in a file.

6.5 Wildcards Patterns and Matching
===================================

"Globbing" is the operation by which "wildcard" characters, '*' or '?'
for example, are replaced and expanded into all existing files matching
the given pattern. GNU 'tar' can use wildcard patterns for matching (or
globbing) archive members when extracting from or listing an archive.
Wildcard patterns are also used for verifying volume labels of 'tar'
archives. This section has the purpose of explaining wildcard syntax
for 'tar'.

A PATTERN should be written according to shell syntax, using wildcard
characters to effect globbing. Most characters in the pattern stand for
themselves in the matched string, and case is significant: 'a' will
match only 'a', and not 'A'. The character '?' in the pattern matches
any single character in the matched string. The character '*' in the
pattern matches zero, one, or more single characters in the matched
string. The character '\' says to take the following character of the
pattern _literally_; it is useful when one needs to match the '?', '*',
'[' or '\' characters, themselves.

The character '[', up to the matching ']', introduces a character
class. A "character class" is a list of acceptable characters for the
next single character of the matched string. For example, '[abcde]'
would match any of the first five letters of the alphabet. Note that
within a character class, all of the "special characters" listed above
other than '\' lose their special meaning; for example, '[-\\[*?]]'
would match any of the characters, '-', '\', '[', '*', '?', or ']'.
(Due to parsing constraints, the characters '-' and ']' must either come
_first_ or _last_ in a character class.)

If the first character of the class after the opening '[' is '!' or
'^', then the meaning of the class is reversed. Rather than listing
character to match, it lists those characters which are _forbidden_ as
the next single character of the matched string.

Other characters of the class stand for themselves. The special
construction '[A-E]', using an hyphen between two letters, is meant to
represent all characters between A and E, inclusive.

Periods ('.') or forward slashes ('/') are not considered special for
wildcard matches. However, if a pattern completely matches a directory
prefix of a matched string, then it matches the full matched string:
thus, excluding a directory also excludes all the files beneath it.

Controlling Pattern-Matching
----------------------------

For the purposes of this section, we call "exclusion members" all member
names obtained while processing '--exclude' and '--exclude-from'
options, and "inclusion members" those member names that were given in
the command line or read from the file specified with '--files-from'
option.

These two pairs of member lists are used in the following operations:
'--diff', '--extract', '--list', '--update'.

There are no inclusion members in create mode ('--create' and
'--append'), since in this mode the names obtained from the command line
refer to _files_, not archive members.

By default, inclusion members are compared with archive members
literally (1) and exclusion members are treated as globbing patterns.
For example:

$ tar tf foo.tar
a.c
b.c
a.txt
[remarks]
# Member names are used verbatim:
$ tar -xf foo.tar -v '[remarks]'
[remarks]
# Exclude member names are globbed:
$ tar -xf foo.tar -v --exclude '*.c'
a.txt
[remarks]

This behavior can be altered by using the following options:

'--wildcards'
Treat all member names as wildcards.

'--no-wildcards'
Treat all member names as literal strings.

Thus, to extract files whose names end in '.c', you can use:

$ tar -xf foo.tar -v --wildcards '*.c'
a.c
b.c

Notice quoting of the pattern to prevent the shell from interpreting it.

The effect of '--wildcards' option is canceled by '--no-wildcards'.
This can be used to pass part of the command line arguments verbatim and
other part as globbing patterns. For example, the following invocation:

$ tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'

instructs 'tar' to extract from 'foo.tar' all files whose names end in
'.txt' and the file named '[remarks]'.

Normally, a pattern matches a name if an initial subsequence of the
name's components matches the pattern, where '*', '?', and '[...]' are
the usual shell wildcards, '\' escapes wildcards, and wildcards can
match '/'.

Other than optionally stripping leading '/' from names (*note
absolute::), patterns and names are used as-is. For example, trailing
'/' is not trimmed from a user-specified name before deciding whether to
exclude it.

However, this matching procedure can be altered by the options listed
below. These options accumulate. For example:

--ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'

ignores case when excluding 'makefile', but not when excluding 'readme'.

'--anchored'
'--no-anchored'
If anchored, a pattern must match an initial subsequence of the
name's components. Otherwise, the pattern can match any
subsequence. Default is '--no-anchored' for exclusion members and
'--anchored' inclusion members.

'--ignore-case'
'--no-ignore-case'
When ignoring case, upper-case patterns match lower-case names and
vice versa. When not ignoring case (the default), matching is
case-sensitive.

'--wildcards-match-slash'
'--no-wildcards-match-slash'
When wildcards match slash (the default for exclusion members), a
wildcard like '*' in the pattern can match a '/' in the name.
Otherwise, '/' is matched only by '/'.

The '--recursion' and '--no-recursion' options (*note recurse::) also
affect how member patterns are interpreted. If recursion is in effect,
a pattern matches a name if it matches any of the name's parent
directories.

The following table summarizes pattern-matching default values:

Members Default settings
--------------------------------------------------------------------------
Inclusion '--no-wildcards --anchored
--no-wildcards-match-slash'
Exclusion '--wildcards --no-anchored
--wildcards-match-slash'

---------- Footnotes ----------

(1) Notice that earlier GNU 'tar' versions used globbing for
inclusion members, which contradicted to UNIX98 specification and was
not documented. *Note Changes::, for more information on this and other
changes.

6.6 Quoting Member Names
========================

When displaying member names, 'tar' takes care to avoid ambiguities
caused by certain characters. This is called "name quoting". The
characters in question are:

* Non-printable control characters:
Character ASCII Character name
-------------------------------------------------------------------
\a 7 Audible bell
\b 8 Backspace
\f 12 Form feed
\n 10 New line
\r 13 Carriage return
\t 9 Horizontal tabulation
\v 11 Vertical tabulation

* Space (ASCII 32)

* Single and double quotes (''' and '"')

* Backslash ('\')

The exact way 'tar' uses to quote these characters depends on the
"quoting style". The default quoting style, called "escape" (see
below), uses backslash notation to represent control characters, space
and backslash. Using this quoting style, control characters are
represented as listed in column 'Character' in the above table, a space
is printed as '\ ' and a backslash as '\\'.

GNU 'tar' offers seven distinct quoting styles, which can be selected
using '--quoting-style' option:

'--quoting-style=STYLE'

Sets quoting style. Valid values for STYLE argument are: literal,
shell, shell-always, c, escape, locale, clocale.

These styles are described in detail below. To illustrate their
effect, we will use an imaginary tar archive 'arch.tar' containing the
following members:

# 1. Contains horizontal tabulation character.
a tab
# 2. Contains newline character
a
newline
# 3. Contains a space
a space
# 4. Contains double quotes
a"double"quote
# 5. Contains single quotes
a'single'quote
# 6. Contains a backslash character:
a\backslash

Here is how usual 'ls' command would have listed them, if they had
existed in the current working directory:

$ ls
a\ttab
a\nnewline
a\ space
a"double"quote
a'single'quote
a\\backslash

Quoting styles:

'literal'
No quoting, display each character as is:

$ tar tf arch.tar --quoting-style=literal
./
./a space
./a'single'quote
./a"double"quote
./a\backslash
./a tab
./a
newline

'shell'
Display characters the same way Bourne shell does: control
characters, except '\t' and '\n', are printed using backslash
escapes, '\t' and '\n' are printed as is, and a single quote is
printed as '\''. If a name contains any quoted characters, it is
enclosed in single quotes. In particular, if a name contains
single quotes, it is printed as several single-quoted strings:

$ tar tf arch.tar --quoting-style=shell
./
'./a space'
'./a'\''single'\''quote'
'./a"double"quote'
'./a\backslash'
'./a tab'
'./a
newline'

'shell-always'
Same as 'shell', but the names are always enclosed in single
quotes:

$ tar tf arch.tar --quoting-style=shell-always
'./'
'./a space'
'./a'\''single'\''quote'
'./a"double"quote'
'./a\backslash'
'./a tab'
'./a
newline'

'c'
Use the notation of the C programming language. All names are
enclosed in double quotes. Control characters are quoted using
backslash notations, double quotes are represented as '\"',
backslash characters are represented as '\\'. Single quotes and
spaces are not quoted:

$ tar tf arch.tar --quoting-style=c
"./"
"./a space"
"./a'single'quote"
"./a\"double\"quote"
"./a\\backslash"
"./a\ttab"
"./a\nnewline"

'escape'
Control characters are printed using backslash notation, a space is
printed as '\ ' and a backslash as '\\'. This is the default
quoting style, unless it was changed when configured the package.

$ tar tf arch.tar --quoting-style=escape
./
./a space
./a'single'quote
./a"double"quote
./a\\backslash
./a\ttab
./a\nnewline

'locale'
Control characters, single quote and backslash are printed using
backslash notation. All names are quoted using left and right
quotation marks, appropriate to the current locale. If it does not
define quotation marks, use ''' as left and as right quotation
marks. Any occurrences of the right quotation mark in a name are
escaped with '\', for example:

For example:

$ tar tf arch.tar --quoting-style=locale
'./'
'./a space'
'./a\'single\'quote'
'./a"double"quote'
'./a\\backslash'
'./a\ttab'
'./a\nnewline'

'clocale'
Same as 'locale', but '"' is used for both left and right quotation
marks, if not provided by the currently selected locale:

$ tar tf arch.tar --quoting-style=clocale
"./"
"./a space"
"./a'single'quote"
"./a\"double\"quote"
"./a\\backslash"
"./a\ttab"
"./a\nnewline"

You can specify which characters should be quoted in addition to
those implied by the current quoting style:

'--quote-chars=STRING'
Always quote characters from STRING, even if the selected quoting
style would not quote them.

For example, using 'escape' quoting (compare with the usual escape
listing above):

$ tar tf arch.tar --quoting-style=escape --quote-chars=' "'
./
./a\ space
./a'single'quote
./a\"double\"quote
./a\\backslash
./a\ttab
./a\nnewline

To disable quoting of such additional characters, use the following
option:

'--no-quote-chars=STRING'
Remove characters listed in STRING from the list of quoted
characters set by the previous '--quote-chars' option.

This option is particularly useful if you have added '--quote-chars'
to your 'TAR_OPTIONS' (*note TAR_OPTIONS::) and wish to disable it for
the current invocation.

Note, that '--no-quote-chars' does _not_ disable those characters
that are quoted by default in the selected quoting style.

6.7 Modifying File and Member Names
===================================

'Tar' archives contain detailed information about files stored in them
and full file names are part of that information. When storing a file
to an archive, its file name is recorded in it, along with the actual
file contents. When restoring from an archive, a file is created on
disk with exactly the same name as that stored in the archive. In the
majority of cases this is the desired behavior of a file archiver.
However, there are some cases when it is not.

First of all, it is often unsafe to extract archive members with
absolute file names or those that begin with a '../'. GNU 'tar' takes
special precautions when extracting such names and provides a special
option for handling them, which is described in *note absolute::.

Secondly, you may wish to extract file names without some leading
directory components, or with otherwise modified names. In other cases
it is desirable to store files under differing names in the archive.

GNU 'tar' provides several options for these needs.

'--strip-components=NUMBER'
Strip given NUMBER of leading components from file names before
extraction.

For example, suppose you have archived whole '/usr' hierarchy to a
tar archive named 'usr.tar'. Among other files, this archive contains
'usr/include/stdlib.h', which you wish to extract to the current working
directory. To do so, you type:

$ tar -xf usr.tar --strip=2 usr/include/stdlib.h

The option '--strip=2' instructs 'tar' to strip the two leading
components ('usr/' and 'include/') off the file name.

If you add the '--verbose' ('-v') option to the invocation above, you
will note that the verbose listing still contains the full file name,
with the two removed components still in place. This can be
inconvenient, so 'tar' provides a special option for altering this
behavior:

'--show-transformed-names'
Display file or member names with all requested transformations
applied.

For example:

$ tar -xf usr.tar -v --strip=2 usr/include/stdlib.h
usr/include/stdlib.h
$ tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h
stdlib.h

Notice that in both cases the file 'stdlib.h' is extracted to the
current working directory, '--show-transformed-names' affects only the
way its name is displayed.

This option is especially useful for verifying whether the invocation
will have the desired effect. Thus, before running

$ tar -x --strip=N

it is often advisable to run

$ tar -t -v --show-transformed --strip=N

to make sure the command will produce the intended results.

In case you need to apply more complex modifications to the file
name, GNU 'tar' provides a general-purpose transformation option:

'--transform=EXPRESSION'
'--xform=EXPRESSION'
Modify file names using supplied EXPRESSION.

The EXPRESSION is a 'sed'-like replace expression of the form:

s/REGEXP/REPLACE/[FLAGS]

where REGEXP is a "regular expression", REPLACE is a replacement for
each file name part that matches REGEXP. Both REGEXP and REPLACE are
described in detail in *note The "s" Command: (sed)The "s" Command.

Any delimiter can be used in lieu of '/', the only requirement being
that it be used consistently throughout the expression. For example,
the following two expressions are equivalent:

s/one/two/
s,one,two,

Changing delimiters is often useful when the REGEX contains slashes.
For example, it is more convenient to write 's,/,-,' than 's/\//-/'.

As in 'sed', you can give several replace expressions, separated by a
semicolon.

Supported FLAGS are:

'g'
Apply the replacement to _all_ matches to the REGEXP, not just the
first.

'i'
Use case-insensitive matching.

'x'
REGEXP is an "extended regular expression" (*note Extended regular
expressions: (sed)Extended regexps.).

'NUMBER'
Only replace the NUMBERth match of the REGEXP.

Note: the POSIX standard does not specify what should happen when
you mix the 'g' and NUMBER modifiers. GNU 'tar' follows the GNU
'sed' implementation in this regard, so the interaction is defined
to be: ignore matches before the NUMBERth, and then match and
replace all matches from the NUMBERth on.

In addition, several "transformation scope" flags are supported, that
control to what files transformations apply. These are:

'r'
Apply transformation to regular archive members.

'R'
Do not apply transformation to regular archive members.

's'
Apply transformation to symbolic link targets.

'S'
Do not apply transformation to symbolic link targets.

'h'
Apply transformation to hard link targets.

'H'
Do not apply transformation to hard link targets.

Default is 'rsh', which means to apply transformations to both
archive members and targets of symbolic and hard links.

Default scope flags can also be changed using 'flags=' statement in
the transform expression. The flags set this way remain in force until
next 'flags=' statement or end of expression, whichever occurs first.
For example:

--transform 'flags=S;s|^|/usr/local/|'

Here are several examples of '--transform' usage:

1. Extract 'usr/' hierarchy into 'usr/local/':

$ tar --transform='s,usr/,usr/local/,' -x -f arch.tar

2. Strip two leading directory components (equivalent to
'--strip-components=2'):

$ tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar

3. Convert each file name to lower case:

$ tar --transform 's/.*/\L&/' -x -f arch.tar

4. Prepend '/prefix/' to each file name:

$ tar --transform 's,^,/prefix/,' -x -f arch.tar

5. Archive the '/lib' directory, prepending '/usr/local' to each
archive member:

$ tar --transform 's,^,/usr/local/,S' -c -f arch.tar /lib

Notice the use of flags in the last example. The '/lib' directory
often contains many symbolic links to files within it. It may look, for
example, like this:

$ ls -l
drwxr-xr-x root/root 0 2008-07-08 16:20 /lib/
-rwxr-xr-x root/root 1250840 2008-05-25 07:44 /lib/libc-2.3.2.so
lrwxrwxrwx root/root 0 2008-06-24 17:12 /lib/libc.so.6 -> libc-2.3.2.so
...

Using the expression 's,^,/usr/local/,' would mean adding
'/usr/local' to both regular archive members and to link targets. In
this case, '/lib/libc.so.6' would become:

/usr/local/lib/libc.so.6 -> /usr/local/libc-2.3.2.so

This is definitely not desired. To avoid this, the 'S' flag is used,
which excludes symbolic link targets from filename transformations. The
result is:

$ tar --transform 's,^,/usr/local/,S', -c -v -f arch.tar \
--show-transformed /lib
drwxr-xr-x root/root 0 2008-07-08 16:20 /usr/local/lib/
-rwxr-xr-x root/root 1250840 2008-05-25 07:44 /usr/local/lib/libc-2.3.2.so
lrwxrwxrwx root/root 0 2008-06-24 17:12 /usr/local/lib/libc.so.6 \
-> libc-2.3.2.so

Unlike '--strip-components', '--transform' can be used in any GNU
'tar' operation mode. For example, the following command adds files to
the archive while replacing the leading 'usr/' component with 'var/':

$ tar -cf arch.tar --transform='s,^usr/,var/,' /

To test '--transform' effect we suggest using
'--show-transformed-names' option:

$ tar -cf arch.tar --transform='s,^usr/,var/,' \
--verbose --show-transformed-names /

If both '--strip-components' and '--transform' are used together,
then '--transform' is applied first, and the required number of
components is then stripped from its result.

You can use as many '--transform' options in a single command line as
you want. The specified expressions will then be applied in order of
their appearance. For example, the following two invocations are
equivalent:

$ tar -cf arch.tar --transform='s,/usr/var,/var/' \
--transform='s,/usr/local,/usr/,'
$ tar -cf arch.tar \
--transform='s,/usr/var,/var/;s,/usr/local,/usr/,'

6.8 Operating Only on New Files
===============================

The '--after-date=DATE' ('--newer=DATE', '-N DATE') option causes 'tar'
to only work on files whose data modification or status change times are
newer than the DATE given. If DATE starts with '/' or '.', it is taken
to be a file name; the data modification time of that file is used as
the date. If you use this option when creating or appending to an
archive, the archive will only include new files. If you use
'--after-date' when extracting an archive, 'tar' will only extract files
newer than the DATE you specify.

If you only want 'tar' to make the date comparison based on
modification of the file's data (rather than status changes), then use
the '--newer-mtime=DATE' option.

You may use these options with any operation. Note that these
options differ from the '--update' ('-u') operation in that they allow
you to specify a particular date against which 'tar' can compare when
deciding whether or not to archive the files.

'--after-date=DATE'
'--newer=DATE'
'-N DATE'
Only store files newer than DATE.

Acts on files only if their data modification or status change
times are later than DATE. Use in conjunction with any operation.

If DATE starts with '/' or '.', it is taken to be a file name; the
data modification time of that file is used as the date.

'--newer-mtime=DATE'
Acts like '--after-date', but only looks at data modification
times.

These options limit 'tar' to operate only on files which have been
modified after the date specified. A file's status is considered to
have changed if its contents have been modified, or if its owner,
permissions, and so forth, have been changed. (For more information on
how to specify a date, see *note Date input formats::; remember that the
entire date argument must be quoted if it contains any spaces.)

Gurus would say that '--after-date' tests both the data modification
time ('mtime', the time the contents of the file were last modified) and
the status change time ('ctime', the time the file's status was last
changed: owner, permissions, etc.) fields, while '--newer-mtime' tests
only the 'mtime' field.

To be precise, '--after-date' checks _both_ 'mtime' and 'ctime' and
processes the file if either one is more recent than DATE, while
'--newer-mtime' only checks 'mtime' and disregards 'ctime'. Neither
does it use 'atime' (the last time the contents of the file were looked
at).

Date specifiers can have embedded spaces. Because of this, you may
need to quote date arguments to keep the shell from parsing them as
separate arguments. For example, the following command will add to the
archive all the files modified less than two days ago:

$ tar -cf foo.tar --newer-mtime '2 days ago'

When any of these options is used with the option '--verbose' (*note
verbose tutorial::) GNU 'tar' will try to convert the specified date
back to its textual representation and compare that with the one given
with the option. If the two dates differ, 'tar' will print a warning
saying what date it will use. This is to help user ensure he is using
the right date. For example:

$ tar -c -f archive.tar --after-date='10 days ago' .
tar: Option --after-date: Treating date '10 days ago' as 2006-06-11
13:19:37.232434

*Please Note:* '--after-date' and '--newer-mtime' should not be
used for incremental backups. *Note Incremental Dumps::, for
proper way of creating incremental backups.

6.9 Descending into Directories
===============================

Usually, 'tar' will recursively explore all directories (either those
given on the command line or through the '--files-from' option) for the
various files they contain. However, you may not always want 'tar' to
act this way.

The '--no-recursion' option inhibits 'tar''s recursive descent into
specified directories. If you specify '--no-recursion', you can use the
'find' (*note find: (find)Top.) utility for hunting through levels of
directories to construct a list of file names which you could then pass
to 'tar'. 'find' allows you to be more selective when choosing which
files to archive; see *note files::, for more information on using
'find' with 'tar'.

'--no-recursion'
Prevents 'tar' from recursively descending directories.

'--recursion'
Requires 'tar' to recursively descend directories. This is the
default.

When you use '--no-recursion', GNU 'tar' grabs directory entries
themselves, but does not descend on them recursively. Many people use
'find' for locating files they want to back up, and since 'tar'
_usually_ recursively descends on directories, they have to use the
'-not -type d' test in their 'find' invocation (*note Type:
(find)Type.), as they usually do not want all the files in a directory.
They then use the '--files-from' option to archive the files located via
'find'.

The problem when restoring files archived in this manner is that the
directories themselves are not in the archive; so the
'--same-permissions' ('--preserve-permissions', '-p') option does not
affect them--while users might really like it to. Specifying
'--no-recursion' is a way to tell 'tar' to grab only the directory
entries given to it, adding no new files on its own. To summarize, if
you use 'find' to create a list of files to be stored in an archive, use
it as follows:

$ find DIR TESTS | \
tar -cf ARCHIVE -T - --no-recursion

The '--no-recursion' option also applies when extracting: it causes
'tar' to extract only the matched directory entries, not the files under
those directories.

The '--no-recursion' option also affects how globbing patterns are
interpreted (*note controlling pattern-matching::).

The '--no-recursion' and '--recursion' options apply to later options
and operands, and can be overridden by later occurrences of
'--no-recursion' and '--recursion'. For example:

$ tar -cf jams.tar --no-recursion grape --recursion grape/concord

creates an archive with one entry for 'grape', and the recursive
contents of 'grape/concord', but no entries under 'grape' other than
'grape/concord'.

6.10 Crossing File System Boundaries
====================================

'tar' will normally automatically cross file system boundaries in order
to archive files which are part of a directory tree. You can change
this behavior by running 'tar' and specifying '--one-file-system'. This
option only affects files that are archived because they are in a
directory that is being archived; 'tar' will still archive files
explicitly named on the command line or through '--files-from',
regardless of where they reside.

'--one-file-system'
Prevents 'tar' from crossing file system boundaries when archiving.
Use in conjunction with any write operation.

The '--one-file-system' option causes 'tar' to modify its normal
behavior in archiving the contents of directories. If a file in a
directory is not on the same file system as the directory itself, then
'tar' will not archive that file. If the file is a directory itself,
'tar' will not archive anything beneath it; in other words, 'tar' will
not cross mount points.

This option is useful for making full or incremental archival backups
of a file system. If this option is used in conjunction with
'--verbose' ('-v'), files that are excluded are mentioned by name on the
standard error.

6.10.1 Changing the Working Directory
-------------------------------------

To change the working directory in the middle of a list of file names,
either on the command line or in a file specified using '--files-from'
('-T'), use '--directory' ('-C'). This will change the working
directory to the specified directory after that point in the list.

'--directory=DIRECTORY'
'-C DIRECTORY'
Changes the working directory in the middle of a command line.

For example,

$ tar -c -f jams.tar grape prune -C food cherry

will place the files 'grape' and 'prune' from the current directory into
the archive 'jams.tar', followed by the file 'cherry' from the directory
'food'. This option is especially useful when you have several widely
separated files that you want to store in the same archive.

Note that the file 'cherry' is recorded in the archive under the
precise name 'cherry', _not_ 'food/cherry'. Thus, the archive will
contain three files that all appear to have come from the same
directory; if the archive is extracted with plain 'tar --extract', all
three files will be written in the current directory.

Contrast this with the command,

$ tar -c -f jams.tar grape prune -C food red/cherry

which records the third file in the archive under the name 'red/cherry'
so that, if the archive is extracted using 'tar --extract', the third
file will be written in a subdirectory named 'red'.

You can use the '--directory' option to make the archive independent
of the original name of the directory holding the files. The following
command places the files '/etc/passwd', '/etc/hosts', and '/lib/libc.a'
into the archive 'foo.tar':

$ tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a

However, the names of the archive members will be exactly what they were
on the command line: 'passwd', 'hosts', and 'libc.a'. They will not
appear to be related by file name to the original directories where
those files were located.

Note that '--directory' options are interpreted consecutively. If
'--directory' specifies a relative file name, it is interpreted relative
to the then current directory, which might not be the same as the
original current working directory of 'tar', due to a previous
'--directory' option.

When using '--files-from' (*note files::), you can put various 'tar'
options (including '-C') in the file list. Notice, however, that in
this case the option and its argument may not be separated by
whitespace. If you use short option, its argument must either follow
the option letter immediately, without any intervening whitespace, or
occupy the next line. Otherwise, if you use long option, separate its
argument by an equal sign.

For instance, the file list for the above example will be:

-C/etc
passwd
hosts
--directory=/lib
libc.a

To use it, you would invoke 'tar' as follows:

$ tar -c -f foo.tar --files-from list

The interpretation of options in file lists is disabled by
'--verbatim-files-from' and '--null' options.

6.10.2 Absolute File Names
--------------------------

By default, GNU 'tar' drops a leading '/' on input or output, and
complains about file names containing a '..' component. There is an
option that turns off this behavior:

'--absolute-names'
'-P'
Do not strip leading slashes from file names, and permit file names
containing a '..' file name component.

When 'tar' extracts archive members from an archive, it strips any
leading slashes ('/') from the member name. This causes absolute member
names in the archive to be treated as relative file names. This allows
you to have such members extracted wherever you want, instead of being
restricted to extracting the member in the exact directory named in the
archive. For example, if the archive member has the name '/etc/passwd',
'tar' will extract it as if the name were really 'etc/passwd'.

File names containing '..' can cause problems when extracting, so
'tar' normally warns you about such files when creating an archive, and
rejects attempts to extracts such files.

Other 'tar' programs do not do this. As a result, if you create an
archive whose member names start with a slash, they will be difficult
for other people with a non-GNU 'tar' program to use. Therefore, GNU
'tar' also strips leading slashes from member names when putting members
into the archive. For example, if you ask 'tar' to add the file
'/bin/ls' to an archive, it will do so, but the member name will be
'bin/ls'(1).

Symbolic links containing '..' or leading '/' can also cause problems
when extracting, so 'tar' normally extracts them last; it may create
empty files as placeholders during extraction.

If you use the '--absolute-names' ('-P') option, 'tar' will do none
of these transformations.

To archive or extract files relative to the root directory, specify
the '--absolute-names' ('-P') option.

Normally, 'tar' acts on files relative to the working
directory--ignoring superior directory names when archiving, and
ignoring leading slashes when extracting.

When you specify '--absolute-names' ('-P'), 'tar' stores file names
including all superior directory names, and preserves leading slashes.
If you only invoked 'tar' from the root directory you would never need
the '--absolute-names' option, but using this option may be more
convenient than switching to root.

'--absolute-names'
Preserves full file names (including superior directory names) when
archiving and extracting files.

'tar' prints out a message about removing the '/' from file names.
This message appears once per GNU 'tar' invocation. It represents
something which ought to be told; ignoring what it means can cause very
serious surprises, later.

Some people, nevertheless, do not want to see this message. Wanting
to play really dangerously, one may of course redirect 'tar' standard
error to the sink. For example, under 'sh':

$ tar -c -f archive.tar /home 2> /dev/null

Another solution, both nicer and simpler, would be to change to the '/'
directory first, and then avoid absolute notation. For example:

$ tar -c -f archive.tar -C / home

*Note Integrity::, for some of the security-related implications of
using this option.

---------- Footnotes ----------

(1) A side effect of this is that when '--create' is used with
'--verbose' the resulting output is not, generally speaking, the same as
the one you'd get running 'tar --list' command. This may be important
if you use some scripts for comparing both outputs. *Note listing
member and file names::, for the information on how to handle this case.

7 Date input formats
********************

First, a quote:

Our units of temporal measurement, from seconds on up to months,
are so complicated, asymmetrical and disjunctive so as to make
coherent mental reckoning in time all but impossible. Indeed, had
some tyrannical god contrived to enslave our minds to time, to make
it all but impossible for us to escape subjection to sodden
routines and unpleasant surprises, he could hardly have done better
than handing down our present system. It is like a set of
trapezoidal building blocks, with no vertical or horizontal
surfaces, like a language in which the simplest thought demands
ornate constructions, useless particles and lengthy
circumlocutions. Unlike the more successful patterns of language
and science, which enable us to face experience boldly or at least
level-headedly, our system of temporal calculation silently and
persistently encourages our terror of time.

... It is as though architects had to measure length in feet, width
in meters and height in ells; as though basic instruction manuals
demanded a knowledge of five different languages. It is no wonder
then that we often look into our own immediate past or future, last
Tuesday or a week from Sunday, with feelings of helpless confusion.
...

--Robert Grudin, 'Time and the Art of Living'.

This section describes the textual date representations that GNU
programs accept. These are the strings you, as a user, can supply as
arguments to the various programs. The C interface (via the
'parse_datetime' function) is not described here.

7.1 General date syntax
=======================

A "date" is a string, possibly empty, containing many items separated by
whitespace. The whitespace may be omitted when no ambiguity arises.
The empty string means the beginning of today (i.e., midnight). Order
of the items is immaterial. A date string may contain many flavors of
items:

* calendar date items
* time of day items
* time zone items
* combined date and time of day items
* day of the week items
* relative items
* pure numbers.

We describe each of these item types in turn, below.

A few ordinal numbers may be written out in words in some contexts.
This is most useful for specifying day of the week items or relative
items (see below). Among the most commonly used ordinal numbers, the
word 'last' stands for -1, 'this' stands for 0, and 'first' and 'next'
both stand for 1. Because the word 'second' stands for the unit of time
there is no way to write the ordinal number 2, but for convenience
'third' stands for 3, 'fourth' for 4, 'fifth' for 5, 'sixth' for 6,
'seventh' for 7, 'eighth' for 8, 'ninth' for 9, 'tenth' for 10,
'eleventh' for 11 and 'twelfth' for 12.

When a month is written this way, it is still considered to be
written numerically, instead of being "spelled in full"; this changes
the allowed strings.

In the current implementation, only English is supported for words
and abbreviations like 'AM', 'DST', 'EST', 'first', 'January', 'Sunday',
'tomorrow', and 'year'.

The output of the 'date' command is not always acceptable as a date
string, not only because of the language problem, but also because there
is no standard meaning for time zone items like 'IST'. When using
'date' to generate a date string intended to be parsed later, specify a
date format that is independent of language and that does not use time
zone items other than 'UTC' and 'Z'. Here are some ways to do this:

$ LC_ALL=C TZ=UTC0 date
Mon Mar 1 00:21:42 UTC 2004
$ TZ=UTC0 date +'%Y-%m-%d %H:%M:%SZ'
2004-03-01 00:21:42Z
$ date --rfc-3339=ns # --rfc-3339 is a GNU extension.
2004-02-29 16:21:42.692722128-08:00
$ date --rfc-2822 # a GNU extension
Sun, 29 Feb 2004 16:21:42 -0800
$ date +'%Y-%m-%d %H:%M:%S %z' # %z is a GNU extension.
2004-02-29 16:21:42 -0800
$ date +'@%s.%N' # %s and %N are GNU extensions.
@1078100502.692722128

Alphabetic case is completely ignored in dates. Comments may be
introduced between round parentheses, as long as included parentheses
are properly nested. Hyphens not followed by a digit are currently
ignored. Leading zeros on numbers are ignored.

Invalid dates like '2005-02-29' or times like '24:00' are rejected.
In the typical case of a host that does not support leap seconds, a time
like '23:59:60' is rejected even if it corresponds to a valid leap
second.

7.2 Calendar date items
=======================

A "calendar date item" specifies a day of the year. It is specified
differently, depending on whether the month is specified numerically or
literally. All these strings specify the same calendar date:

1972-09-24 # ISO 8601.
72-9-24 # Assume 19xx for 69 through 99,
# 20xx for 00 through 68.
72-09-24 # Leading zeros are ignored.
9/24/72 # Common U.S. writing.
24 September 1972
24 Sept 72 # September has a special abbreviation.
24 Sep 72 # Three-letter abbreviations always allowed.
Sep 24, 1972
24-sep-72
24sep72

The year can also be omitted. In this case, the last specified year
is used, or the current year if none. For example:

9/24
sep 24

Here are the rules.

For numeric months, the ISO 8601 format 'YEAR-MONTH-DAY' is allowed,
where YEAR is any positive number, MONTH is a number between 01 and 12,
and DAY is a number between 01 and 31. A leading zero must be present
if a number is less than ten. If YEAR is 68 or smaller, then 2000 is
added to it; otherwise, if YEAR is less than 100, then 1900 is added to
it. The construct 'MONTH/DAY/YEAR', popular in the United States, is
accepted. Also 'MONTH/DAY', omitting the year.

Literal months may be spelled out in full: 'January', 'February',
'March', 'April', 'May', 'June', 'July', 'August', 'September',
'October', 'November' or 'December'. Literal months may be abbreviated
to their first three letters, possibly followed by an abbreviating dot.
It is also permitted to write 'Sept' instead of 'September'.

When months are written literally, the calendar date may be given as
any of the following:

DAY MONTH YEAR
DAY MONTH
MONTH DAY YEAR
DAY-MONTH-YEAR

Or, omitting the year:

MONTH DAY

7.3 Time of day items
=====================

A "time of day item" in date strings specifies the time on a given day.
Here are some examples, all of which represent the same time:

20:02:00.000000
20:02
8:02pm
20:02-0500 # In EST (U.S. Eastern Standard Time).

More generally, the time of day may be given as 'HOUR:MINUTE:SECOND',
where HOUR is a number between 0 and 23, MINUTE is a number between 0
and 59, and SECOND is a number between 0 and 59 possibly followed by '.'
or ',' and a fraction containing one or more digits. Alternatively,
':SECOND' can be omitted, in which case it is taken to be zero. On the
rare hosts that support leap seconds, SECOND may be 60.

If the time is followed by 'am' or 'pm' (or 'a.m.' or 'p.m.'), HOUR
is restricted to run from 1 to 12, and ':MINUTE' may be omitted (taken
to be zero). 'am' indicates the first half of the day, 'pm' indicates
the second half of the day. In this notation, 12 is the predecessor of
1: midnight is '12am' while noon is '12pm'. (This is the zero-oriented
interpretation of '12am' and '12pm', as opposed to the old tradition
derived from Latin which uses '12m' for noon and '12pm' for midnight.)

The time may alternatively be followed by a time zone correction,
expressed as 'SHHMM', where S is '+' or '-', HH is a number of zone
hours and MM is a number of zone minutes. The zone minutes term, MM,
may be omitted, in which case the one- or two-digit correction is
interpreted as a number of hours. You can also separate HH from MM with
a colon. When a time zone correction is given this way, it forces
interpretation of the time relative to Coordinated Universal Time (UTC),
overriding any previous specification for the time zone or the local
time zone. For example, '+0530' and '+05:30' both stand for the time
zone 5.5 hours ahead of UTC (e.g., India). This is the best way to
specify a time zone correction by fractional parts of an hour. The
maximum zone correction is 24 hours.

Either 'am'/'pm' or a time zone correction may be specified, but not
both.

7.4 Time zone items
===================

A "time zone item" specifies an international time zone, indicated by a
small set of letters, e.g., 'UTC' or 'Z' for Coordinated Universal Time.
Any included periods are ignored. By following a non-daylight-saving
time zone by the string 'DST' in a separate word (that is, separated by
some white space), the corresponding daylight saving time zone may be
specified. Alternatively, a non-daylight-saving time zone can be
followed by a time zone correction, to add the two values. This is
normally done only for 'UTC'; for example, 'UTC+05:30' is equivalent to
'+05:30'.

Time zone items other than 'UTC' and 'Z' are obsolescent and are not
recommended, because they are ambiguous; for example, 'EST' has a
different meaning in Australia than in the United States. Instead, it's
better to use unambiguous numeric time zone corrections like '-0500', as
described in the previous section.

If neither a time zone item nor a time zone correction is supplied,
time stamps are interpreted using the rules of the default time zone
(*note Specifying time zone rules::).

7.5 Combined date and time of day items
=======================================

The ISO 8601 date and time of day extended format consists of an ISO
8601 date, a 'T' character separator, and an ISO 8601 time of day. This
format is also recognized if the 'T' is replaced by a space.

In this format, the time of day should use 24-hour notation.
Fractional seconds are allowed, with either comma or period preceding
the fraction. ISO 8601 fractional minutes and hours are not supported.
Typically, hosts support nanosecond timestamp resolution; excess
precision is silently discarded.

Here are some examples:

2012-09-24T20:02:00.052-0500
2012-12-31T23:59:59,999999999+1100
1970-01-01 00:00Z

7.6 Day of week items
=====================

The explicit mention of a day of the week will forward the date (only if
necessary) to reach that day of the week in the future.

Days of the week may be spelled out in full: 'Sunday', 'Monday',
'Tuesday', 'Wednesday', 'Thursday', 'Friday' or 'Saturday'. Days may be
abbreviated to their first three letters, optionally followed by a
period. The special abbreviations 'Tues' for 'Tuesday', 'Wednes' for
'Wednesday' and 'Thur' or 'Thurs' for 'Thursday' are also allowed.

A number may precede a day of the week item to move forward
supplementary weeks. It is best used in expression like 'third monday'.
In this context, 'last DAY' or 'next DAY' is also acceptable; they move
one week before or after the day that DAY by itself would represent.

A comma following a day of the week item is ignored.

7.7 Relative items in date strings
==================================

"Relative items" adjust a date (or the current date if none) forward or
backward. The effects of relative items accumulate. Here are some
examples:

1 year
1 year ago
3 years
2 days

The unit of time displacement may be selected by the string 'year' or
'month' for moving by whole years or months. These are fuzzy units, as
years and months are not all of equal duration. More precise units are
'fortnight' which is worth 14 days, 'week' worth 7 days, 'day' worth 24
hours, 'hour' worth 60 minutes, 'minute' or 'min' worth 60 seconds, and
'second' or 'sec' worth one second. An 's' suffix on these units is
accepted and ignored.

The unit of time may be preceded by a multiplier, given as an
optionally signed number. Unsigned numbers are taken as positively
signed. No number at all implies 1 for a multiplier. Following a
relative item by the string 'ago' is equivalent to preceding the unit by
a multiplier with value -1.

The string 'tomorrow' is worth one day in the future (equivalent to
'day'), the string 'yesterday' is worth one day in the past (equivalent
to 'day ago').

The strings 'now' or 'today' are relative items corresponding to
zero-valued time displacement, these strings come from the fact a
zero-valued time displacement represents the current time when not
otherwise changed by previous items. They may be used to stress other
items, like in '12:00 today'. The string 'this' also has the meaning of
a zero-valued time displacement, but is preferred in date strings like
'this thursday'.

When a relative item causes the resulting date to cross a boundary
where the clocks were adjusted, typically for daylight saving time, the
resulting date and time are adjusted accordingly.

The fuzz in units can cause problems with relative items. For
example, '2003-07-31 -1 month' might evaluate to 2003-07-01, because
2003-06-31 is an invalid date. To determine the previous month more
reliably, you can ask for the month before the 15th of the current
month. For example:

$ date -R
Thu, 31 Jul 2003 13:02:39 -0700
$ date --date='-1 month' +'Last month was %B?'
Last month was July?
$ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!'
Last month was June!

Also, take care when manipulating dates around clock changes such as
daylight saving leaps. In a few cases these have added or subtracted as
much as 24 hours from the clock, so it is often wise to adopt universal
time by setting the 'TZ' environment variable to 'UTC0' before embarking
on calendrical calculations.

7.8 Pure numbers in date strings
================================

The precise interpretation of a pure decimal number depends on the
context in the date string.

If the decimal number is of the form YYYYMMDD and no other calendar
date item (*note Calendar date items::) appears before it in the date
string, then YYYY is read as the year, MM as the month number and DD as
the day of the month, for the specified calendar date.

If the decimal number is of the form HHMM and no other time of day
item appears before it in the date string, then HH is read as the hour
of the day and MM as the minute of the hour, for the specified time of
day. MM can also be omitted.

If both a calendar date and a time of day appear to the left of a
number in the date string, but no relative item, then the number
overrides the year.

7.9 Seconds since the Epoch
===========================

If you precede a number with '@', it represents an internal time stamp
as a count of seconds. The number can contain an internal decimal point
(either '.' or ','); any excess precision not supported by the internal
representation is truncated toward minus infinity. Such a number cannot
be combined with any other date item, as it specifies a complete time
stamp.

Internally, computer times are represented as a count of seconds
since an epoch--a well-defined point of time. On GNU and POSIX systems,
the epoch is 1970-01-01 00:00:00 UTC, so '@0' represents this time, '@1'
represents 1970-01-01 00:00:01 UTC, and so forth. GNU and most other
POSIX-compliant systems support such times as an extension to POSIX,
using negative counts, so that '@-1' represents 1969-12-31 23:59:59 UTC.

Traditional Unix systems count seconds with 32-bit two's-complement
integers and can represent times from 1901-12-13 20:45:52 through
2038-01-19 03:14:07 UTC. More modern systems use 64-bit counts of
seconds with nanosecond subcounts, and can represent all the times in
the known lifetime of the universe to a resolution of 1 nanosecond.

On most hosts, these counts ignore the presence of leap seconds. For
example, on most hosts '@915148799' represents 1998-12-31 23:59:59 UTC,
'@915148800' represents 1999-01-01 00:00:00 UTC, and there is no way to
represent the intervening leap second 1998-12-31 23:59:60 UTC.

7.10 Specifying time zone rules
===============================

Normally, dates are interpreted using the rules of the current time
zone, which in turn are specified by the 'TZ' environment variable, or
by a system default if 'TZ' is not set. To specify a different set of
default time zone rules that apply just to one date, start the date with
a string of the form 'TZ="RULE"'. The two quote characters ('"') must
be present in the date, and any quotes or backslashes within RULE must
be escaped by a backslash.

For example, with the GNU 'date' command you can answer the question
"What time is it in New York when a Paris clock shows 6:30am on October
31, 2004?" by using a date beginning with 'TZ="Europe/Paris"' as shown
in the following shell transcript:

$ export TZ="America/New_York"
$ date --date='TZ="Europe/Paris" 2004-10-31 06:30'
Sun Oct 31 01:30:00 EDT 2004

In this example, the '--date' operand begins with its own 'TZ'
setting, so the rest of that operand is processed according to
'Europe/Paris' rules, treating the string '2004-10-31 06:30' as if it
were in Paris. However, since the output of the 'date' command is
processed according to the overall time zone rules, it uses New York
time. (Paris was normally six hours ahead of New York in 2004, but this
example refers to a brief Halloween period when the gap was five hours.)

A 'TZ' value is a rule that typically names a location in the 'tz'
database (http://www.twinsun.com/tz/tz-link.htm). A recent catalog of
location names appears in the TWiki Date and Time Gateway
(http://twiki.org/cgi-bin/xtra/tzdate). A few non-GNU hosts require a
colon before a location name in a 'TZ' setting, e.g.,
'TZ=":America/New_York"'.

The 'tz' database includes a wide variety of locations ranging from
'Arctic/Longyearbyen' to 'Antarctica/South_Pole', but if you are at sea
and have your own private time zone, or if you are using a non-GNU host
that does not support the 'tz' database, you may need to use a POSIX
rule instead. Simple POSIX rules like 'UTC0' specify a time zone
without daylight saving time; other rules can specify simple daylight
saving regimes. *Note Specifying the Time Zone with 'TZ': (libc)TZ
Variable.

7.11 Authors of 'parse_datetime'
================================

'parse_datetime' started life as 'getdate', as originally implemented by
Steven M. Bellovin (<smb@research.att.com>) while at the University of
North Carolina at Chapel Hill. The code was later tweaked by a couple
of people on Usenet, then completely overhauled by Rich $alz
(<rsalz@bbn.com>) and Jim Berets (<jberets@bbn.com>) in August, 1990.
Various revisions for the GNU system were made by David MacKenzie, Jim
Meyering, Paul Eggert and others, including renaming it to 'get_date' to
avoid a conflict with the alternative Posix function 'getdate', and a
later rename to 'parse_datetime'. The Posix function 'getdate' can
parse more locale-specific dates using 'strptime', but relies on an
environment variable and external file, and lacks the thread-safety of
'parse_datetime'.

This chapter was originally produced by Franc,ois Pinard
(<pinard@iro.umontreal.ca>) from the 'parse_datetime.y' source code, and
then edited by K. Berry (<kb@cs.umb.edu>).

8 Controlling the Archive Format
********************************

Due to historical reasons, there are several formats of tar archives.
All of them are based on the same principles, but have some subtle
differences that often make them incompatible with each other.

GNU tar is able to create and handle archives in a variety of
formats. The most frequently used formats are (in alphabetical order):

gnu
Format used by GNU 'tar' versions up to 1.13.25. This format
derived from an early POSIX standard, adding some improvements such
as sparse file handling and incremental archives. Unfortunately
these features were implemented in a way incompatible with other
archive formats.

Archives in 'gnu' format are able to hold file names of unlimited
length.

oldgnu
Format used by GNU 'tar' of versions prior to 1.12.

v7
Archive format, compatible with the V7 implementation of tar. This
format imposes a number of limitations. The most important of them
are:

1. The maximum length of a file name is limited to 99 characters.
2. The maximum length of a symbolic link is limited to 99
characters.
3. It is impossible to store special files (block and character
devices, fifos etc.)
4. Maximum value of user or group ID is limited to 2097151
(7777777 octal)
5. V7 archives do not contain symbolic ownership information
(user and group name of the file owner).

This format has traditionally been used by Automake when producing
Makefiles. This practice will change in the future, in the
meantime, however this means that projects containing file names
more than 99 characters long will not be able to use GNU 'tar' 1.29
and Automake prior to 1.9.

ustar
Archive format defined by POSIX.1-1988 specification. It stores
symbolic ownership information. It is also able to store special
files. However, it imposes several restrictions as well:

1. The maximum length of a file name is limited to 256
characters, provided that the file name can be split at a
directory separator in two parts, first of them being at most
155 bytes long. So, in most cases the maximum file name
length will be shorter than 256 characters.
2. The maximum length of a symbolic link name is limited to 100
characters.
3. Maximum size of a file the archive is able to accommodate is
8GB
4. Maximum value of UID/GID is 2097151.
5. Maximum number of bits in device major and minor numbers is
21.

star
Format used by Jo"rg Schilling 'star' implementation. GNU 'tar' is
able to read 'star' archives but currently does not produce them.

posix
Archive format defined by POSIX.1-2001 specification. This is the
most flexible and feature-rich format. It does not impose any
restrictions on file sizes or file name lengths. This format is
quite recent, so not all tar implementations are able to handle it
properly. However, this format is designed in such a way that any
tar implementation able to read 'ustar' archives will be able to
read most 'posix' archives as well, with the only exception that
any additional information (such as long file names etc.) will in
such case be extracted as plain text files along with the files it
refers to.

This archive format will be the default format for future versions
of GNU 'tar'.

The following table summarizes the limitations of each of these
formats:

Format UID File Size File Name Devn
--------------------------------------------------------------------
gnu 1.8e19 Unlimited Unlimited 63
oldgnu 1.8e19 Unlimited Unlimited 63
v7 2097151 8GB 99 n/a
ustar 2097151 8GB 256 21
posix Unlimited Unlimited Unlimited Unlimited

The default format for GNU 'tar' is defined at compilation time. You
may check it by running 'tar --help', and examining the last lines of
its output. Usually, GNU 'tar' is configured to create archives in
'gnu' format, however, future version will switch to 'posix'.

8.1 Using Less Space through Compression
========================================

8.1.1 Creating and Reading Compressed Archives
----------------------------------------------

GNU 'tar' is able to create and read compressed archives. It supports a
wide variety of compression programs, namely: 'gzip', 'bzip2', 'lzip',
'lzma', 'lzop', 'xz' and traditional 'compress'. The latter is
supported mostly for backward compatibility, and we recommend against
using it, because it is by far less effective than the other compression
programs(1).

Creating a compressed archive is simple: you just specify a
"compression option" along with the usual archive creation commands.
The compression option is '-z' ('--gzip') to create a 'gzip' compressed
archive, '-j' ('--bzip2') to create a 'bzip2' compressed archive,
'--lzip' to create an lzip compressed archive, '-J' ('--xz') to create
an XZ archive, '--lzma' to create an LZMA compressed archive, '--lzop'
to create an LSOP archive, and '-Z' ('--compress') to use 'compress'
program. For example:

$ tar czf archive.tar.gz .

You can also let GNU 'tar' select the compression program based on
the suffix of the archive file name. This is done using
'--auto-compress' ('-a') command line option. For example, the
following invocation will use 'bzip2' for compression:

$ tar caf archive.tar.bz2 .

whereas the following one will use 'lzma':

$ tar caf archive.tar.lzma .

For a complete list of file name suffixes recognized by GNU 'tar',
see *note auto-compress::.

Reading compressed archive is even simpler: you don't need to specify
any additional options as GNU 'tar' recognizes its format automatically.
Thus, the following commands will list and extract the archive created
in previous example:

# List the compressed archive
$ tar tf archive.tar.gz
# Extract the compressed archive
$ tar xf archive.tar.gz

The format recognition algorithm is based on "signatures", a special
byte sequences in the beginning of file, that are specific for certain
compression formats. If this approach fails, 'tar' falls back to using
archive name suffix to determine its format (*note auto-compress::, for
a list of recognized suffixes).

Some compression programs are able to handle different compression
formats. GNU 'tar' uses this, if the principal decompressor for the
given format is not available. For example, if 'compress' is not
installed, 'tar' will try to use 'gzip'. As of version 1.29 the
following alternatives are tried(2):

Format Main decompressor Alternatives
---------------------------------------------------------------------
compress compress gzip
lzma lzma xz
bzip2 bzip2 lbzip2

The only case when you have to specify a decompression option while
reading the archive is when reading from a pipe or from a tape drive
that does not support random access. However, in this case GNU 'tar'
will indicate which option you should use. For example:

$ cat archive.tar.gz | tar tf -
tar: Archive is compressed. Use -z option
tar: Error is not recoverable: exiting now

If you see such diagnostics, just add the suggested option to the
invocation of GNU 'tar':

$ cat archive.tar.gz | tar tzf -

Notice also, that there are several restrictions on operations on
compressed archives. First of all, compressed archives cannot be
modified, i.e., you cannot update ('--update', alias '-u') them or
delete ('--delete') members from them or add ('--append', alias '-r')
members to them. Likewise, you cannot append another 'tar' archive to a
compressed archive using '--concatenate' ('-A'). Secondly, multi-volume
archives cannot be compressed.

The following options allow to select a particular compressor
program:

'-z'
'--gzip'
'--ungzip'
Filter the archive through 'gzip'.

'-J'
'--xz'
Filter the archive through 'xz'.

'-j'
'--bzip2'
Filter the archive through 'bzip2'.

'--lzip'
Filter the archive through 'lzip'.

'--lzma'
Filter the archive through 'lzma'.

'--lzop'
Filter the archive through 'lzop'.

'-Z'
'--compress'
'--uncompress'
Filter the archive through 'compress'.

When any of these options is given, GNU 'tar' searches the compressor
binary in the current path and invokes it. The name of the compressor
program is specified at compilation time using a corresponding
'--with-COMPNAME' option to 'configure', e.g. '--with-bzip2' to select
a specific 'bzip2' binary. *Note lbzip2::, for a detailed discussion.

The output produced by 'tar --help' shows the actual compressor names
along with each of these options.

You can use any of these options on physical devices (tape drives,
etc.) and remote files as well as on normal files; data to or from such
devices or remote files is reblocked by another copy of the 'tar'
program to enforce the specified (or default) record size. The default
compression parameters are used. You can override them by using the
'-I' option (see below), e.g.:

$ tar -cf archive.tar.gz -I 'gzip -9 -n' subdir

A more traditional way to do this is to use a pipe:

$ tar cf - subdir | gzip -9 -n > archive.tar.gz

Compressed archives are easily corrupted, because compressed files
have little redundancy. The adaptive nature of the compression scheme
means that the compression tables are implicitly spread all over the
archive. If you lose a few blocks, the dynamic construction of the
compression tables becomes unsynchronized, and there is little chance
that you could recover later in the archive.

Other compression options provide better control over creating
compressed archives. These are:

'--auto-compress'
'-a'
Select a compression program to use by the archive file name
suffix. The following suffixes are recognized:

Suffix Compression program
-------------------------------------------------------------------
'.gz' 'gzip'
'.tgz' 'gzip'
'.taz' 'gzip'
'.Z' 'compress'
'.taZ' 'compress'
'.bz2' 'bzip2'
'.tz2' 'bzip2'
'.tbz2' 'bzip2'
'.tbz' 'bzip2'
'.lz' 'lzip'
'.lzma' 'lzma'
'.tlz' 'lzma'
'.lzo' 'lzop'
'.xz' 'xz'

'--use-compress-program=COMMAND'
'-I=COMMAND'
Use external compression program COMMAND. Use this option if you
want to specify options for the compression program, or if you are
not happy with the compression program associated with the suffix
at compile time, or if you have a compression program that GNU
'tar' does not support. The COMMAND argument is a valid command
invocation, as you would type it at the command line prompt, with
any additional options as needed. Enclose it in quotes if it
contains white space (*note Running External Commands: external.).

The COMMAND should follow two conventions:

First, when invoked without additional options, it should read data
from standard input, compress it and output it on standard output.

Secondly, if invoked with the additional '-d' option, it should do
exactly the opposite, i.e., read the compressed data from the
standard input and produce uncompressed data on the standard
output.

The latter requirement means that you must not use the '-d' option
as a part of the COMMAND itself.

The '--use-compress-program' option, in particular, lets you
implement your own filters, not necessarily dealing with
compression/decompression. For example, suppose you wish to implement
PGP encryption on top of compression, using 'gpg' (*note gpg:
(gpg)Top.). The following script does that:

#! /bin/sh
case $1 in
-d) gpg --decrypt - | gzip -d -c;;
'') gzip -c | gpg -s;;
*) echo "Unknown option $1">&2; exit 1;;
esac

Suppose you name it 'gpgz' and save it somewhere in your 'PATH'.
Then the following command will create a compressed archive signed with
your private key:

$ tar -cf foo.tar.gpgz -Igpgz .

Likewise, the command below will list its contents:

$ tar -tf foo.tar.gpgz -Igpgz .

---------- Footnotes ----------

(1) It also had patent problems in the past.

(2) To verbosely trace the decompressor selection, use the
'--warning=decompress-program' option (*note decompress-program:
warnings.).

8.1.1.1 Using lbzip2 with GNU 'tar'.
....................................

'Lbzip2' is a multithreaded utility for handling 'bzip2' compression,
written by Laszlo Ersek. It makes use of multiple processors to speed
up its operation and in general works considerably faster than 'bzip2'.
For a detailed description of 'lbzip2' see
<http://freshmeat.net/projects/lbzip2> and lbzip2: parallel bzip2
utility
(http://www.linuxinsight.com/lbzip2-parallel-bzip2-utility.html).

Recent versions of 'lbzip2' are mostly command line compatible with
'bzip2', which makes it possible to automatically invoke it via the
'--bzip2' GNU 'tar' command line option. To do so, GNU 'tar' must be
configured with the '--with-bzip2' command line option, like this:

$ ./configure --with-bzip2=lbzip2 [OTHER-OPTIONS]

Once configured and compiled this way, 'tar --help' will show the
following:

$ tar --help | grep -- --bzip2
-j, --bzip2 filter the archive through lbzip2

which means that running 'tar --bzip2' will invoke 'lbzip2'.

8.1.2 Archiving Sparse Files
----------------------------

Files in the file system occasionally have "holes". A "hole" in a file
is a section of the file's contents which was never written. The
contents of a hole reads as all zeros. On many operating systems,
actual disk storage is not allocated for holes, but they are counted in
the length of the file. If you archive such a file, 'tar' could create
an archive longer than the original. To have 'tar' attempt to recognize
the holes in a file, use '--sparse' ('-S'). When you use this option,
then, for any file using less disk space than would be expected from its
length, 'tar' searches the file for holes. It then records in the
archive for the file where the holes (consecutive stretches of zeros)
are, and only archives the "real contents" of the file. On extraction
(using '--sparse' is not needed on extraction) any such files have also
holes created wherever the holes were found. Thus, if you use
'--sparse', 'tar' archives won't take more space than the original.

GNU 'tar' uses two methods for detecting holes in sparse files.
These methods are described later in this subsection.

'-S'
'--sparse'
This option instructs 'tar' to test each file for sparseness before
attempting to archive it. If the file is found to be sparse it is
treated specially, thus allowing to decrease the amount of space
used by its image in the archive.

This option is meaningful only when creating or updating archives.
It has no effect on extraction.

Consider using '--sparse' when performing file system backups, to
avoid archiving the expanded forms of files stored sparsely in the
system.

Even if your system has no sparse files currently, some may be
created in the future. If you use '--sparse' while making file system
backups as a matter of course, you can be assured the archive will never
take more space on the media than the files take on disk (otherwise,
archiving a disk filled with sparse files might take hundreds of tapes).
*Note Incremental Dumps::.

However, be aware that '--sparse' option may present a serious
drawback. Namely, in order to determine the positions of holes in a
file 'tar' may have to read it before trying to archive it, so in total
the file may be read *twice*. This may happen when your OS or your FS
does not support "SEEK_HOLE/SEEK_DATA" feature in "lseek" (See
'--hole-detection', below).

When using 'POSIX' archive format, GNU 'tar' is able to store sparse
files using in three distinct ways, called "sparse formats". A sparse
format is identified by its "number", consisting, as usual of two
decimal numbers, delimited by a dot. By default, format '1.0' is used.
If, for some reason, you wish to use an earlier format, you can select
it using '--sparse-version' option.

'--sparse-version=VERSION'
Select the format to store sparse files in. Valid VERSION values
are: '0.0', '0.1' and '1.0'. *Note Sparse Formats::, for a
detailed description of each format.

Using '--sparse-format' option implies '--sparse'.

'--hole-detection=METHOD'
Enforce concrete hole detection method. Before the real contents
of sparse file are stored, 'tar' needs to gather knowledge about
file sparseness. This is because it needs to have the file's map
of holes stored into tar header before it starts archiving the file
contents. Currently, two methods of hole detection are
implemented:

* '--hole-detection=seek' Seeking the file for data and holes.
It uses enhancement of the 'lseek' system call ('SEEK_HOLE'
and 'SEEK_DATA') which is able to reuse file system knowledge
about sparse file contents - so the detection is usually very
fast. To use this feature, your file system and operating
system must support it. At the time of this writing (2015)
this feature, in spite of not being accepted by POSIX, is
fairly widely supported by different operating systems.

* '--hole-detection=raw' Reading byte-by-byte the whole sparse
file before the archiving. This method detects holes like
consecutive stretches of zeroes. Comparing to the previous
method, it is usually much slower, although more portable.

When no '--hole-detection' option is given, 'tar' uses the 'seek', if
supported by the operating system.

Using '--hole-detection' option implies '--sparse'.

8.2 Handling File Attributes
============================

When 'tar' reads files, it updates their access times. To avoid this,
use the '--atime-preserve[=METHOD]' option, which can either reset the
access time retroactively or avoid changing it in the first place.

'--atime-preserve'
'--atime-preserve=replace'
'--atime-preserve=system'
Preserve the access times of files that are read. This works only
for files that you own, unless you have superuser privileges.

'--atime-preserve=replace' works on most systems, but it also
restores the data modification time and updates the status change
time. Hence it doesn't interact with incremental dumps nicely
(*note Incremental Dumps::), and it can set access or data
modification times incorrectly if other programs access the file
while 'tar' is running.

'--atime-preserve=system' avoids changing the access time in the
first place, if the operating system supports this. Unfortunately,
this may or may not work on any given operating system or file
system. If 'tar' knows for sure it won't work, it complains right
away.

Currently '--atime-preserve' with no operand defaults to
'--atime-preserve=replace', but this is intended to change to
'--atime-preserve=system' when the latter is better-supported.

'-m'
'--touch'
Do not extract data modification time.

When this option is used, 'tar' leaves the data modification times
of the files it extracts as the times when the files were
extracted, instead of setting it to the times recorded in the
archive.

This option is meaningless with '--list' ('-t').

'--same-owner'
Create extracted files with the same ownership they have in the
archive.

This is the default behavior for the superuser, so this option is
meaningful only for non-root users, when 'tar' is executed on those
systems able to give files away. This is considered as a security
flaw by many people, at least because it makes quite difficult to
correctly account users for the disk space they occupy. Also, the
'suid' or 'sgid' attributes of files are easily and silently lost
when files are given away.

When writing an archive, 'tar' writes the user ID and user name
separately. If it can't find a user name (because the user ID is
not in '/etc/passwd'), then it does not write one. When restoring,
it tries to look the name (if one was written) up in '/etc/passwd'.
If it fails, then it uses the user ID stored in the archive
instead.

'--no-same-owner'
'-o'
Do not attempt to restore ownership when extracting. This is the
default behavior for ordinary users, so this option has an effect
only for the superuser.

'--numeric-owner'
The '--numeric-owner' option allows (ANSI) archives to be written
without user/group name information or such information to be
ignored when extracting. It effectively disables the generation
and/or use of user/group name information. This option forces
extraction using the numeric ids from the archive, ignoring the
names.

This is useful in certain circumstances, when restoring a backup
from an emergency floppy with different passwd/group files for
example. It is otherwise impossible to extract files with the
right ownerships if the password file in use during the extraction
does not match the one belonging to the file system(s) being
extracted. This occurs, for example, if you are restoring your
files after a major crash and had booted from an emergency floppy
with no password file or put your disk into another machine to do
the restore.

The numeric ids are _always_ saved into 'tar' archives. The
identifying names are added at create time when provided by the
system, unless '--format=oldgnu' is used. Numeric ids could be
used when moving archives between a collection of machines using a
centralized management for attribution of numeric ids to users and
groups. This is often made through using the NIS capabilities.

When making a 'tar' file for distribution to other sites, it is
sometimes cleaner to use a single owner for all files in the
distribution, and nicer to specify the write permission bits of the
files as stored in the archive independently of their actual value
on the file system. The way to prepare a clean distribution is
usually to have some Makefile rule creating a directory, copying
all needed files in that directory, then setting ownership and
permissions as wanted (there are a lot of possible schemes), and
only then making a 'tar' archive out of this directory, before
cleaning everything out. Of course, we could add a lot of options
to GNU 'tar' for fine tuning permissions and ownership. This is
not the good way, I think. GNU 'tar' is already crowded with
options and moreover, the approach just explained gives you a great
deal of control already.

'-p'
'--same-permissions'
'--preserve-permissions'
Extract all protection information.

This option causes 'tar' to set the modes (access permissions) of
extracted files exactly as recorded in the archive. If this option
is not used, the current 'umask' setting limits the permissions on
extracted files. This option is by default enabled when 'tar' is
executed by a superuser.

This option is meaningless with '--list' ('-t').

8.3 Making 'tar' Archives More Portable
=======================================

Creating a 'tar' archive on a particular system that is meant to be
useful later on many other machines and with other versions of 'tar' is
more challenging than you might think. 'tar' archive formats have been
evolving since the first versions of Unix. Many such formats are
around, and are not always compatible with each other. This section
discusses a few problems, and gives some advice about making 'tar'
archives more portable.

One golden rule is simplicity. For example, limit your 'tar'
archives to contain only regular files and directories, avoiding other
kind of special files. Do not attempt to save sparse files or
contiguous files as such. Let's discuss a few more problems, in turn.

8.3.1 Portable Names
--------------------

Use portable file and member names. A name is portable if it contains
only ASCII letters and digits, '/', '.', '_', and '-'; it cannot be
empty, start with '-' or '//', or contain '/-'. Avoid deep directory
nesting. For portability to old Unix hosts, limit your file name
components to 14 characters or less.

If you intend to have your 'tar' archives to be read under MSDOS, you
should not rely on case distinction for file names, and you might use
the GNU 'doschk' program for helping you further diagnosing illegal
MSDOS names, which are even more limited than System V's.

8.3.2 Symbolic Links
--------------------

Normally, when 'tar' archives a symbolic link, it writes a block to the
archive naming the target of the link. In that way, the 'tar' archive
is a faithful record of the file system contents. When '--dereference'
('-h') is used with '--create' ('-c'), 'tar' archives the files symbolic
links point to, instead of the links themselves.

When creating portable archives, use '--dereference' ('-h'): some
systems do not support symbolic links, and moreover, your distribution
might be unusable if it contains unresolved symbolic links.

When reading from an archive, the '--dereference' ('-h') option
causes 'tar' to follow an already-existing symbolic link when 'tar'
writes or reads a file named in the archive. Ordinarily, 'tar' does not
follow such a link, though it may remove the link before writing a new
file. *Note Dealing with Old Files::.

The '--dereference' option is unsafe if an untrusted user can modify
directories while 'tar' is running. *Note Security::.

8.3.3 Hard Links
----------------

Normally, when 'tar' archives a hard link, it writes a block to the
archive naming the target of the link (a '1' type block). In that way,
the actual file contents is stored in file only once. For example,
consider the following two files:

$ ls -l
-rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
-rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden

Here, 'jeden' is a link to 'one'. When archiving this directory with
a verbose level 2, you will get an output similar to the following:

$ tar cvvf ../archive.tar .
drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
-rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden

The last line shows that, instead of storing two copies of the file,
'tar' stored it only once, under the name 'jeden', and stored file 'one'
as a hard link to this file.

It may be important to know that all hard links to the given file are
stored in the archive. For example, this may be necessary for exact
reproduction of the file system. The following option does that:

'--check-links'
'-l'
Check the number of links dumped for each processed file. If this
number does not match the total number of hard links for the file,
print a warning message.

For example, trying to archive only file 'jeden' with this option
produces the following diagnostics:

$ tar -c -f ../archive.tar -l jeden
tar: Missing links to 'jeden'.

Although creating special records for hard links helps keep a
faithful record of the file system contents and makes archives more
compact, it may present some difficulties when extracting individual
members from the archive. For example, trying to extract file 'one'
from the archive created in previous examples produces, in the absence
of file 'jeden':

$ tar xf archive.tar ./one
tar: ./one: Cannot hard link to './jeden': No such file or directory
tar: Error exit delayed from previous errors

The reason for this behavior is that 'tar' cannot seek back in the
archive to the previous member (in this case, 'one'), to extract it(1).
If you wish to avoid such problems at the cost of a bigger archive, use
the following option:

'--hard-dereference'
Dereference hard links and store the files they refer to.

For example, trying this option on our two sample files, we get two
copies in the archive, each of which can then be extracted independently
of the other:

$ tar -c -vv -f ../archive.tar --hard-dereference .
drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
-rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
-rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one

---------- Footnotes ----------

(1) There are plans to fix this in future releases.

8.3.4 Old V7 Archives
---------------------

Certain old versions of 'tar' cannot handle additional information
recorded by newer 'tar' programs. To create an archive in V7 format
(not ANSI), which can be read by these old versions, specify the
'--format=v7' option in conjunction with the '--create' ('-c') ('tar'
also accepts '--portability' or '--old-archive' for this option). When
you specify it, 'tar' leaves out information about directories, pipes,
fifos, contiguous files, and device files, and specifies file ownership
by group and user IDs instead of group and user names.

When updating an archive, do not use '--format=v7' unless the archive
was created using this option.

In most cases, a _new_ format archive can be read by an _old_ 'tar'
program without serious trouble, so this option should seldom be needed.
On the other hand, most modern 'tar's are able to read old format
archives, so it might be safer for you to always use '--format=v7' for
your distributions. Notice, however, that 'ustar' format is a better
alternative, as it is free from many of 'v7''s drawbacks.

8.3.5 Ustar Archive Format
--------------------------

The archive format defined by the POSIX.1-1988 specification is called
'ustar'. Although it is more flexible than the V7 format, it still has
many restrictions (*note ustar: Formats, for the detailed description of
'ustar' format). Along with V7 format, 'ustar' format is a good choice
for archives intended to be read with other implementations of 'tar'.

To create an archive in 'ustar' format, use the '--format=ustar'
option in conjunction with '--create' ('-c').

8.3.6 GNU and old GNU 'tar' format
----------------------------------

GNU 'tar' was based on an early draft of the POSIX 1003.1 'ustar'
standard. GNU extensions to 'tar', such as the support for file names
longer than 100 characters, use portions of the 'tar' header record
which were specified in that POSIX draft as unused. Subsequent changes
in POSIX have allocated the same parts of the header record for other
purposes. As a result, GNU 'tar' format is incompatible with the
current POSIX specification, and with 'tar' programs that follow it.

In the majority of cases, 'tar' will be configured to create this
format by default. This will change in future releases, since we plan
to make 'POSIX' format the default.

To force creation a GNU 'tar' archive, use option '--format=gnu'.

8.3.7 GNU 'tar' and POSIX 'tar'
-------------------------------

Starting from version 1.14 GNU 'tar' features full support for
POSIX.1-2001 archives.

A POSIX conformant archive will be created if 'tar' was given
'--format=posix' ('--format=pax') option. No special option is required
to read and extract from a POSIX archive.

8.3.7.1 Controlling Extended Header Keywords
............................................

'--pax-option=KEYWORD-LIST'
Handle keywords in PAX extended headers. This option is equivalent
to '-o' option of the 'pax' utility.

KEYWORD-LIST is a comma-separated list of keyword options, each
keyword option taking one of the following forms:

'delete=PATTERN'
When used with one of archive-creation commands, this option
instructs 'tar' to omit from extended header records that it
produces any keywords matching the string PATTERN.

When used in extract or list mode, this option instructs tar to
ignore any keywords matching the given PATTERN in the extended
header records. In both cases, matching is performed using the
pattern matching notation described in POSIX 1003.2, 3.13 (*note
wildcards::). For example:

--pax-option delete=security.*

would suppress security-related information.

'exthdr.name=STRING'

This keyword allows user control over the name that is written into
the ustar header blocks for the extended headers. The name is
obtained from STRING after making the following substitutions:

Meta-character Replaced By
------------------------------------------------------------
%d The directory name of the file,
equivalent to the result of the
'dirname' utility on the translated
file name.
%f The name of the file with the
directory information stripped,
equivalent to the result of the
'basename' utility on the translated
file name.
%p The process ID of the 'tar' process.
%% A '%' character.

Any other '%' characters in STRING produce undefined results.

If no option 'exthdr.name=string' is specified, 'tar' will use the
following default value:

%d/PaxHeaders.%p/%f

'exthdr.mtime=VALUE'

This keyword defines the value of the 'mtime' field that is written
into the ustar header blocks for the extended headers. By default,
the 'mtime' field is set to the modification time of the archive
member described by that extended header (or to the value of the
'--mtime' option, if supplied).

'globexthdr.name=STRING'
This keyword allows user control over the name that is written into
the ustar header blocks for global extended header records. The
name is obtained from the contents of STRING, after making the
following substitutions:

Meta-character Replaced By
------------------------------------------------------------
%n An integer that represents the
sequence number of the global extended
header record in the archive, starting
at 1.
%p The process ID of the 'tar' process.
%% A '%' character.

Any other '%' characters in STRING produce undefined results.

If no option 'globexthdr.name=string' is specified, 'tar' will use
the following default value:

$TMPDIR/GlobalHead.%p.%n

where '$TMPDIR' represents the value of the TMPDIR environment
variable. If TMPDIR is not set, 'tar' uses '/tmp'.

'globexthdr.mtime=VALUE'

This keyword defines the value of the 'mtime' field that is written
into the ustar header blocks for the global extended headers. By
default, the 'mtime' field is set to the time when 'tar' was
invoked.

'KEYWORD=VALUE'
When used with one of archive-creation commands, these
keyword/value pairs will be included at the beginning of the
archive in a global extended header record. When used with one of
archive-reading commands, 'tar' will behave as if it has
encountered these keyword/value pairs at the beginning of the
archive in a global extended header record.

'KEYWORD:=VALUE'
When used with one of archive-creation commands, these
keyword/value pairs will be included as records at the beginning of
an extended header for each file. This is effectively equivalent
to KEYWORD=VALUE form except that it creates no global extended
header records.

When used with one of archive-reading commands, 'tar' will behave
as if these keyword/value pairs were included as records at the end
of each extended header; thus, they will override any global or
file-specific extended header record keywords of the same names.
For example, in the command:

tar --format=posix --create \
--file archive --pax-option gname:=user .

the group name will be forced to a new value for all files stored
in the archive.

In any of the forms described above, the VALUE may be a string
enclosed in curly braces. In that case, the string between the braces
is understood either as a textual time representation, as described in
*note Date input formats::, or a name of the existing file, starting
with '/' or '.'. In the latter case, the modification time of that file
is used.

For example, to set all modification times to the current date, you
use the following option:

--pax-option='mtime:={now}'

Note quoting of the option's argument.

As another example, here is the option that ensures that any two
archives created using it, will be binary equivalent if they have the
same contents:

--pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0

If you extract files from such an archive and recreate the archive from
them, you will also need to eliminate changes due to ctime, as shown in
examples below:

--pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0,ctime:=0

or

--pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0,delete=ctime

8.3.8 Checksumming Problems
---------------------------

SunOS and HP-UX 'tar' fail to accept archives created using GNU 'tar'
and containing non-ASCII file names, that is, file names having
characters with the eighth bit set, because they use signed checksums,
while GNU 'tar' uses unsigned checksums while creating archives, as per
POSIX standards. On reading, GNU 'tar' computes both checksums and
accepts either of them. It is somewhat worrying that a lot of people
may go around doing backup of their files using faulty (or at least
non-standard) software, not learning about it until it's time to restore
their missing files with an incompatible file extractor, or vice versa.

GNU 'tar' computes checksums both ways, and accepts either of them on
read, so GNU tar can read Sun tapes even with their wrong checksums.
GNU 'tar' produces the standard checksum, however, raising
incompatibilities with Sun. That is to say, GNU 'tar' has not been
modified to _produce_ incorrect archives to be read by buggy 'tar''s.
I've been told that more recent Sun 'tar' now read standard archives, so
maybe Sun did a similar patch, after all?

The story seems to be that when Sun first imported 'tar' sources on
their system, they recompiled it without realizing that the checksums
were computed differently, because of a change in the default signing of
'char''s in their compiler. So they started computing checksums
wrongly. When they later realized their mistake, they merely decided to
stay compatible with it, and with themselves afterwards. Presumably,
but I do not really know, HP-UX has chosen their 'tar' archives to be
compatible with Sun's. The current standards do not favor Sun 'tar'
format. In any case, it now falls on the shoulders of SunOS and HP-UX
users to get a 'tar' able to read the good archives they receive.

8.3.9 Large or Negative Values
------------------------------

_(This message will disappear, once this node revised.)_

The above sections suggest to use 'oldest possible' archive format if
in doubt. However, sometimes it is not possible. If you attempt to
archive a file whose metadata cannot be represented using required
format, GNU 'tar' will print error message and ignore such a file. You
will than have to switch to a format that is able to handle such values.
The format summary table (*note Formats::) will help you to do so.

In particular, when trying to archive files larger than 8GB or with
timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
12:56:31 UTC, you will have to chose between GNU and POSIX archive
formats. When considering which format to choose, bear in mind that the
GNU format uses two's-complement base-256 notation to store values that
do not fit into standard ustar range. Such archives can generally be
read only by a GNU 'tar' implementation. Moreover, they sometimes
cannot be correctly restored on another hosts even by GNU 'tar'. For
example, using two's complement representation for negative time stamps
that assumes a signed 32-bit 'time_t' generates archives that are not
portable to hosts with differing 'time_t' representations.

On the other hand, POSIX archives, generally speaking, can be
extracted by any tar implementation that understands older ustar format.
The only exception are files larger than 8GB.

8.3.10 How to Extract GNU-Specific Data Using Other 'tar' Implementations
-------------------------------------------------------------------------

In previous sections you became acquainted with various quirks necessary
to make your archives portable. Sometimes you may need to extract
archives containing GNU-specific members using some third-party 'tar'
implementation or an older version of GNU 'tar'. Of course your best
bet is to have GNU 'tar' installed, but if it is for some reason
impossible, this section will explain how to cope without it.

When we speak about "GNU-specific" members we mean two classes of
them: members split between the volumes of a multi-volume archive and
sparse members. You will be able to always recover such members if the
archive is in PAX format. In addition split members can be recovered
from archives in old GNU format. The following subsections describe the
required procedures in detail.

8.3.10.1 Extracting Members Split Between Volumes
.................................................

If a member is split between several volumes of an old GNU format
archive most third party 'tar' implementation will fail to extract it.
To extract it, use 'tarcat' program (*note Tarcat::). This program is
available from GNU 'tar' home page
(http://www.gnu.org/software/tar/utils/tarcat.html). It concatenates
several archive volumes into a single valid archive. For example, if
you have three volumes named from 'vol-1.tar' to 'vol-3.tar', you can do
the following to extract them using a third-party 'tar':

$ tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -

You could use this approach for most (although not all) PAX format
archives as well. However, extracting split members from a PAX archive
is a much easier task, because PAX volumes are constructed in such a way
that each part of a split member is extracted to a different file by
'tar' implementations that are not aware of GNU extensions. More
specifically, the very first part retains its original name, and all
subsequent parts are named using the pattern:

%d/GNUFileParts.%p/%f.%n

where symbols preceded by '%' are "macro characters" that have the
following meaning:

Meta-character Replaced By
------------------------------------------------------------
%d The directory name of the file,
equivalent to the result of the
'dirname' utility on its full name.
%f The file name of the file, equivalent
to the result of the 'basename'
utility on its full name.
%p The process ID of the 'tar' process
that created the archive.
%n Ordinal number of this particular
part.

For example, if the file 'var/longfile' was split during archive
creation between three volumes, and the creator 'tar' process had
process ID '27962', then the member names will be:

var/longfile
var/GNUFileParts.27962/longfile.1
var/GNUFileParts.27962/longfile.2

When you extract your archive using a third-party 'tar', these files
will be created on your disk, and the only thing you will need to do to
restore your file in its original form is concatenate them in the proper
order, for example:

$ cd var
$ cat GNUFileParts.27962/longfile.1 \
GNUFileParts.27962/longfile.2 >> longfile
$ rm -f GNUFileParts.27962

Notice, that if the 'tar' implementation you use supports PAX format
archives, it will probably emit warnings about unknown keywords during
extraction. They will look like this:

Tar file too small
Unknown extended header keyword 'GNU.volume.filename' ignored.
Unknown extended header keyword 'GNU.volume.size' ignored.
Unknown extended header keyword 'GNU.volume.offset' ignored.

You can safely ignore these warnings.

If your 'tar' implementation is not PAX-aware, you will get more
warnings and more files generated on your disk, e.g.:

$ tar xf vol-1.tar
var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
normal file
Unexpected EOF in archive
$ tar xf vol-2.tar
tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
'x', extracted as normal file

Ignore these warnings. The 'PaxHeaders.*' directories created will
contain files with "extended header keywords" describing the extracted
files. You can delete them, unless they describe sparse members. Read
further to learn more about them.

8.3.10.2 Extracting Sparse Members
..................................

Any 'tar' implementation will be able to extract sparse members from a
PAX archive. However, the extracted files will be "condensed", i.e.,
any zero blocks will be removed from them. When we restore such a
condensed file to its original form, by adding zero blocks (or "holes")
back to their original locations, we call this process "expanding" a
compressed sparse file.

To expand a file, you will need a simple auxiliary program called
'xsparse'. It is available in source form from GNU 'tar' home page
(http://www.gnu.org/software/tar/utils/xsparse.html).

Let's begin with archive members in "sparse format version 1.0"(1),
which are the easiest to expand. The condensed file will contain both
file map and file data, so no additional data will be needed to restore
it. If the original file name was 'DIR/NAME', then the condensed file
will be named 'DIR/GNUSparseFile.N/NAME', where N is a decimal
number(2).

To expand a version 1.0 file, run 'xsparse' as follows:

$ xsparse cond-file

where 'cond-file' is the name of the condensed file. The utility will
deduce the name for the resulting expanded file using the following
algorithm:

1. If 'cond-file' does not contain any directories, '../cond-file'
will be used;

2. If 'cond-file' has the form 'DIR/T/NAME', where both T and NAME are
simple names, with no '/' characters in them, the output file name
will be 'DIR/NAME'.

3. Otherwise, if 'cond-file' has the form 'DIR/NAME', the output file
name will be 'NAME'.

In the unlikely case when this algorithm does not suit your needs,
you can explicitly specify output file name as a second argument to the
command:

$ xsparse cond-file out-file

It is often a good idea to run 'xsparse' in "dry run" mode first. In
this mode, the command does not actually expand the file, but verbosely
lists all actions it would be taking to do so. The dry run mode is
enabled by '-n' command line argument:

$ xsparse -n /home/gray/GNUSparseFile.6058/sparsefile
Reading v.1.0 sparse map
Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
'/home/gray/sparsefile'
Finished dry run

To actually expand the file, you would run:

$ xsparse /home/gray/GNUSparseFile.6058/sparsefile

The program behaves the same way all UNIX utilities do: it will keep
quiet unless it has something important to tell you (e.g. an error
condition or something). If you wish it to produce verbose output,
similar to that from the dry run mode, use '-v' option:

$ xsparse -v /home/gray/GNUSparseFile.6058/sparsefile
Reading v.1.0 sparse map
Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
'/home/gray/sparsefile'
Done

Additionally, if your 'tar' implementation has extracted the
"extended headers" for this file, you can instruct 'xstar' to use them
in order to verify the integrity of the expanded file. The option '-x'
sets the name of the extended header file to use. Continuing our
example:

$ xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
/home/gray/GNUSparseFile.6058/sparsefile
Reading extended header file
Found variable GNU.sparse.major = 1
Found variable GNU.sparse.minor = 0
Found variable GNU.sparse.name = sparsefile
Found variable GNU.sparse.realsize = 217481216
Reading v.1.0 sparse map
Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
'/home/gray/sparsefile'
Done

An "extended header" is a special 'tar' archive header that precedes
an archive member and contains a set of "variables", describing the
member properties that cannot be stored in the standard 'ustar' header.
While optional for expanding sparse version 1.0 members, the use of
extended headers is mandatory when expanding sparse members in older
sparse formats: v.0.0 and v.0.1 (The sparse formats are described in
detail in *note Sparse Formats::.) So, for these formats, the question
is: how to obtain extended headers from the archive?

If you use a 'tar' implementation that does not support PAX format,
extended headers for each member will be extracted as a separate file.
If we represent the member name as 'DIR/NAME', then the extended header
file will be named 'DIR/PaxHeaders.N/NAME', where N is an integer
number.

Things become more difficult if your 'tar' implementation does
support PAX headers, because in this case you will have to manually
extract the headers. We recommend the following algorithm:

1. Consult the documentation of your 'tar' implementation for an
option that prints "block numbers" along with the archive listing
(analogous to GNU 'tar''s '-R' option). For example, 'star' has
'-block-number'.

2. Obtain verbose listing using the 'block number' option, and find
block numbers of the sparse member in question and the member
immediately following it. For example, running 'star' on our
archive we obtain:

$ star -t -v -block-number -f arc.tar
...
star: Unknown extended header keyword 'GNU.sparse.size' ignored.
star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored.
star: Unknown extended header keyword 'GNU.sparse.name' ignored.
star: Unknown extended header keyword 'GNU.sparse.map' ignored.
block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile
block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README
...

(as usual, ignore the warnings about unknown keywords.)

3. Let SIZE be the size of the sparse member, BS be its block number
and BN be the block number of the next member. Compute:

N = BS - BN - SIZE/512 - 2

This number gives the size of the extended header part in tar
"blocks". In our example, this formula gives: '897 - 56 - 425984 /
512 - 2 = 7'.

4. Use 'dd' to extract the headers:

dd if=ARCHIVE of=HNAME bs=512 skip=BS count=N

where ARCHIVE is the archive name, HNAME is a name of the file to
store the extended header in, BS and N are computed in previous
steps.

In our example, this command will be

$ dd if=arc.tar of=xhdr bs=512 skip=56 count=7

Finally, you can expand the condensed file, using the obtained
header:

$ xsparse -v -x xhdr GNUSparseFile.6058/sparsefile
Reading extended header file
Found variable GNU.sparse.size = 217481216
Found variable GNU.sparse.numblocks = 208
Found variable GNU.sparse.name = sparsefile
Found variable GNU.sparse.map = 0,2048,1050624,2048,...
Expanding file 'GNUSparseFile.28124/sparsefile' to 'sparsefile'
Done

---------- Footnotes ----------

(1) *Note PAX 1::.

(2) Technically speaking, N is a "process ID" of the 'tar' process
which created the archive (*note PAX keywords::).

8.4 Comparison of 'tar' and 'cpio'
==================================

_(This message will disappear, once this node revised.)_

The 'cpio' archive formats, like 'tar', do have maximum file name
lengths. The binary and old ASCII formats have a maximum file length of
256, and the new ASCII and CRC ASCII formats have a max file length of
1024. GNU 'cpio' can read and write archives with arbitrary file name
lengths, but other 'cpio' implementations may crash unexplainedly trying
to read them.

'tar' handles symbolic links in the form in which it comes in BSD;
'cpio' doesn't handle symbolic links in the form in which it comes in
System V prior to SVR4, and some vendors may have added symlinks to
their system without enhancing 'cpio' to know about them. Others may
have enhanced it in a way other than the way I did it at Sun, and which
was adopted by AT&T (and which is, I think, also present in the 'cpio'
that Berkeley picked up from AT&T and put into a later BSD release--I
think I gave them my changes).

(SVR4 does some funny stuff with 'tar'; basically, its 'cpio' can
handle 'tar' format input, and write it on output, and it probably
handles symbolic links. They may not have bothered doing anything to
enhance 'tar' as a result.)

'cpio' handles special files; traditional 'tar' doesn't.

'tar' comes with V7, System III, System V, and BSD source; 'cpio'
comes only with System III, System V, and later BSD (4.3-tahoe and
later).

'tar''s way of handling multiple hard links to a file can handle file
systems that support 32-bit i-numbers (e.g., the BSD file system);
'cpio's way requires you to play some games (in its "binary" format,
i-numbers are only 16 bits, and in its "portable ASCII" format, they're
18 bits--it would have to play games with the "file system ID" field of
the header to make sure that the file system ID/i-number pairs of
different files were always different), and I don't know which 'cpio's,
if any, play those games. Those that don't might get confused and think
two files are the same file when they're not, and make hard links
between them.

'tar's way of handling multiple hard links to a file places only one
copy of the link on the tape, but the name attached to that copy is the
_only_ one you can use to retrieve the file; 'cpio's way puts one copy
for every link, but you can retrieve it using any of the names.

What type of check sum (if any) is used, and how is this
calculated.

See the attached manual pages for 'tar' and 'cpio' format. 'tar'
uses a checksum which is the sum of all the bytes in the 'tar' header
for a file; 'cpio' uses no checksum.

If anyone knows why 'cpio' was made when 'tar' was present at the
unix scene,

It wasn't. 'cpio' first showed up in PWB/UNIX 1.0; no
generally-available version of UNIX had 'tar' at the time. I don't know
whether any version that was generally available _within AT&T_ had
'tar', or, if so, whether the people within AT&T who did 'cpio' knew
about it.

On restore, if there is a corruption on a tape 'tar' will stop at
that point, while 'cpio' will skip over it and try to restore the rest
of the files.

The main difference is just in the command syntax and header format.

'tar' is a little more tape-oriented in that everything is blocked to
start on a record boundary.

Is there any differences between the ability to recover crashed
archives between the two of them. (Is there any chance of
recovering crashed archives at all.)

Theoretically it should be easier under 'tar' since the blocking lets
you find a header with some variation of 'dd skip=NN'. However, modern
'cpio''s and variations have an option to just search for the next file
header after an error with a reasonable chance of resyncing. However,
lots of tape driver software won't allow you to continue past a media
error which should be the only reason for getting out of sync unless a
file changed sizes while you were writing the archive.

If anyone knows why 'cpio' was made when 'tar' was present at the
unix scene, please tell me about this too.

Probably because it is more media efficient (by not blocking
everything and using only the space needed for the headers where 'tar'
always uses 512 bytes per file header) and it knows how to archive
special files.

You might want to look at the freely available alternatives. The
major ones are 'afio', GNU 'tar', and 'pax', each of which have their
own extensions with some backwards compatibility.

Sparse files were 'tar'red as sparse files (which you can easily
test, because the resulting archive gets smaller, and GNU 'cpio' can no
longer read it).

9 Tapes and Other Archive Media
*******************************

_(This message will disappear, once this node revised.)_

A few special cases about tape handling warrant more detailed
description. These special cases are discussed below.

Many complexities surround the use of 'tar' on tape drives. Since
the creation and manipulation of archives located on magnetic tape was
the original purpose of 'tar', it contains many features making such
manipulation easier.

Archives are usually written on dismountable media--tape cartridges,
mag tapes, or floppy disks.

The amount of data a tape or disk holds depends not only on its size,
but also on how it is formatted. A 2400 foot long reel of mag tape
holds 40 megabytes of data when formatted at 1600 bits per inch. The
physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.

Magnetic media are re-usable--once the archive on a tape is no longer
needed, the archive can be erased and the tape or disk used over. Media
quality does deteriorate with use, however. Most tapes or disks should
be discarded when they begin to produce data errors. EXABYTE tape
cartridges should be discarded when they generate an "error count"
(number of non-usable bits) of more than 10k.

Magnetic media are written and erased using magnetic fields, and
should be protected from such fields to avoid damage to stored data.
Sticking a floppy disk to a filing cabinet using a magnet is probably
not a good idea.

9.1 Device Selection and Switching
==================================

_(This message will disappear, once this node revised.)_

'-f [HOSTNAME:]FILE'
'--file=[HOSTNAME:]FILE'
Use archive file or device FILE on HOSTNAME.

This option is used to specify the file name of the archive 'tar'
works on.

If the file name is '-', 'tar' reads the archive from standard input
(when listing or extracting), or writes it to standard output (when
creating). If the '-' file name is given when updating an archive,
'tar' will read the original archive from its standard input, and will
write the entire new archive to its standard output.

If the file name contains a ':', it is interpreted as 'hostname:file
name'. If the HOSTNAME contains an "at" sign ('@'), it is treated as
'user@hostname:file name'. In either case, 'tar' will invoke the
command 'rsh' (or 'remsh') to start up an '/usr/libexec/rmt' on the
remote machine. If you give an alternate login name, it will be given
to the 'rsh'. Naturally, the remote machine must have an executable
'/usr/libexec/rmt'. This program is free software from the University
of California, and a copy of the source code can be found with the
sources for 'tar'; it's compiled and installed by default. The exact
path to this utility is determined when configuring the package. It is
'PREFIX/libexec/rmt', where PREFIX stands for your installation prefix.
This location may also be overridden at runtime by using the
'--rmt-command=COMMAND' option (*Note --rmt-command: Option Summary, for
detailed description of this option. *Note Remote Tape Server::, for
the description of 'rmt' command).

If this option is not given, but the environment variable 'TAPE' is
set, its value is used; otherwise, old versions of 'tar' used a default
archive name (which was picked when 'tar' was compiled). The default is
normally set up to be the "first" tape drive or other transportable I/O
medium on the system.

Starting with version 1.11.5, GNU 'tar' uses standard input and
standard output as the default device, and I will not try anymore
supporting automatic device detection at installation time. This was
failing really in too many cases, it was hopeless. This is now
completely left to the installer to override standard input and standard
output for default device, if this seems preferable. Further, I think
_most_ actual usages of 'tar' are done with pipes or disks, not really
tapes, cartridges or diskettes.

Some users think that using standard input and output is running
after trouble. This could lead to a nasty surprise on your screen if
you forget to specify an output file name--especially if you are going
through a network or terminal server capable of buffering large amounts
of output. We had so many bug reports in that area of configuring
default tapes automatically, and so many contradicting requests, that we
finally consider the problem to be portably intractable. We could of
course use something like '/dev/tape' as a default, but this is _also_
running after various kind of trouble, going from hung processes to
accidental destruction of real tapes. After having seen all this mess,
using standard input and output as a default really sounds like the only
clean choice left, and a very useful one too.

GNU 'tar' reads and writes archive in records, I suspect this is the
main reason why block devices are preferred over character devices.
Most probably, block devices are more efficient too. The installer
could also check for 'DEFTAPE' in '<sys/mtio.h>'.

'--force-local'
Archive file is local even if it contains a colon.

'--rsh-command=COMMAND'
Use remote COMMAND instead of 'rsh'. This option exists so that
people who use something other than the standard 'rsh' (e.g., a
Kerberized 'rsh') can access a remote device.

When this command is not used, the shell command found when the
'tar' program was installed is used instead. This is the first
found of '/usr/ucb/rsh', '/usr/bin/remsh', '/usr/bin/rsh',
'/usr/bsd/rsh' or '/usr/bin/nsh'. The installer may have
overridden this by defining the environment variable 'RSH' _at
installation time_.

'-[0-7][lmh]'
Specify drive and density.

'-M'
'--multi-volume'
Create/list/extract multi-volume archive.

This option causes 'tar' to write a "multi-volume" archive--one
that may be larger than will fit on the medium used to hold it.
*Note Multi-Volume Archives::.

'-L NUM'
'--tape-length=SIZE[SUF]'
Change tape after writing SIZE units of data. Unless SUF is given,
SIZE is treated as kilobytes, i.e. 'SIZE x 1024' bytes. The
following suffixes alter this behavior:

Suffix Units Byte Equivalent
-------------------------------------------------------------
b Blocks SIZE x 512
B Kilobytes SIZE x 1024
c Bytes SIZE
G Gigabytes SIZE x 1024^3
K Kilobytes SIZE x 1024
k Kilobytes SIZE x 1024
M Megabytes SIZE x 1024^2
P Petabytes SIZE x 1024^5
T Terabytes SIZE x 1024^4
w Words SIZE x 2

Table 9.1: Size Suffixes

This option might be useful when your tape drivers do not properly
detect end of physical tapes. By being slightly conservative on
the maximum tape length, you might avoid the problem entirely.

'-F COMMAND'
'--info-script=COMMAND'
'--new-volume-script=COMMAND'
Execute COMMAND at end of each tape. This implies '--multi-volume'
('-M'). *Note info-script::, for a detailed description of this
option.

9.2 Remote Tape Server
======================

In order to access the tape drive on a remote machine, 'tar' uses the
remote tape server written at the University of California at Berkeley.
The remote tape server must be installed as 'PREFIX/libexec/rmt' on any
machine whose tape drive you want to use. 'tar' calls 'rmt' by running
an 'rsh' or 'remsh' to the remote machine, optionally using a different
login name if one is supplied.

A copy of the source for the remote tape server is provided. Its
source code can be freely distributed. It is compiled and installed by
default.

Unless you use the '--absolute-names' ('-P') option, GNU 'tar' will
not allow you to create an archive that contains absolute file names (a
file name beginning with '/'). If you try, 'tar' will automatically
remove the leading '/' from the file names it stores in the archive. It
will also type a warning message telling you what it is doing.

When reading an archive that was created with a different 'tar'
program, GNU 'tar' automatically extracts entries in the archive which
have absolute file names as if the file names were not absolute. This
is an important feature. A visitor here once gave a 'tar' tape to an
operator to restore; the operator used Sun 'tar' instead of GNU 'tar',
and the result was that it replaced large portions of our '/bin' and
friends with versions from the tape; needless to say, we were unhappy
about having to recover the file system from backup tapes.

For example, if the archive contained a file '/usr/bin/computoy', GNU
'tar' would extract the file to 'usr/bin/computoy', relative to the
current directory. If you want to extract the files in an archive to
the same absolute names that they had when the archive was created, you
should do a 'cd /' before extracting the files from the archive, or you
should either use the '--absolute-names' option, or use the command 'tar
-C / ...'.

Some versions of Unix (Ultrix 3.1 is known to have this problem), can
claim that a short write near the end of a tape succeeded, when it
actually failed. This will result in the -M option not working
correctly. The best workaround at the moment is to use a significantly
larger blocking factor than the default 20.

In order to update an archive, 'tar' must be able to backspace the
archive in order to reread or rewrite a record that was just read (or
written). This is currently possible only on two kinds of files: normal
disk files (or any other file that can be backspaced with 'lseek'), and
industry-standard 9-track magnetic tape (or any other kind of tape that
can be backspaced with the 'MTIOCTOP' 'ioctl').

This means that the '--append', '--concatenate', and '--delete'
commands will not work on any other kind of file. Some media simply
cannot be backspaced, which means these commands and options will never
be able to work on them. These non-backspacing media include pipes and
cartridge tape drives.

Some other media can be backspaced, and 'tar' will work on them once
'tar' is modified to do so.

Archives created with the '--multi-volume', '--label', and
'--incremental' ('-G') options may not be readable by other version of
'tar'. In particular, restoring a file that was split over a volume
boundary will require some careful work with 'dd', if it can be done at
all. Other versions of 'tar' may also create an empty file whose name
is that of the volume header. Some versions of 'tar' may create normal
files instead of directories archived with the '--incremental' ('-G')
option.

9.3 Some Common Problems and their Solutions
============================================

errors from system:
permission denied
no such file or directory
not owner

errors from 'tar':
directory checksum error
header format error

errors from media/system:
i/o error
device busy

9.4 Blocking
============

"Block" and "record" terminology is rather confused, and it is also
confusing to the expert reader. On the other hand, readers who are new
to the field have a fresh mind, and they may safely skip the next two
paragraphs, as the remainder of this manual uses those two terms in a
quite consistent way.

John Gilmore, the writer of the public domain 'tar' from which GNU
'tar' was originally derived, wrote (June 1995):

The nomenclature of tape drives comes from IBM, where I believe
they were invented for the IBM 650 or so. On IBM mainframes, what
is recorded on tape are tape blocks. The logical organization of
data is into records. There are various ways of putting records
into blocks, including 'F' (fixed sized records), 'V' (variable
sized records), 'FB' (fixed blocked: fixed size records, N to a
block), 'VB' (variable size records, N to a block), 'VSB' (variable
spanned blocked: variable sized records that can occupy more than
one block), etc. The 'JCL' 'DD RECFORM=' parameter specified this
to the operating system.

The Unix man page on 'tar' was totally confused about this. When I
wrote 'PD TAR', I used the historically correct terminology ('tar'
writes data records, which are grouped into blocks). It appears
that the bogus terminology made it into POSIX (no surprise here),
and now Franc,ois has migrated that terminology back into the
source code too.

The term "physical block" means the basic transfer chunk from or to a
device, after which reading or writing may stop without anything being
lost. In this manual, the term "block" usually refers to a disk
physical block, _assuming_ that each disk block is 512 bytes in length.
It is true that some disk devices have different physical blocks, but
'tar' ignore these differences in its own format, which is meant to be
portable, so a 'tar' block is always 512 bytes in length, and "block"
always mean a 'tar' block. The term "logical block" often represents
the basic chunk of allocation of many disk blocks as a single entity,
which the operating system treats somewhat atomically; this concept is
only barely used in GNU 'tar'.

The term "physical record" is another way to speak of a physical
block, those two terms are somewhat interchangeable. In this manual,
the term "record" usually refers to a tape physical block, _assuming_
that the 'tar' archive is kept on magnetic tape. It is true that
archives may be put on disk or used with pipes, but nevertheless, 'tar'
tries to read and write the archive one "record" at a time, whatever the
medium in use. One record is made up of an integral number of blocks,
and this operation of putting many disk blocks into a single tape block
is called "reblocking", or more simply, "blocking". The term "logical
record" refers to the logical organization of many characters into
something meaningful to the application. The term "unit record"
describes a small set of characters which are transmitted whole to or by
the application, and often refers to a line of text. Those two last
terms are unrelated to what we call a "record" in GNU 'tar'.

When writing to tapes, 'tar' writes the contents of the archive in
chunks known as "records". To change the default blocking factor, use
the '--blocking-factor=512-SIZE' ('-b 512-SIZE') option. Each record
will then be composed of 512-SIZE blocks. (Each 'tar' block is 512
bytes. *Note Standard::.) Each file written to the archive uses at
least one full record. As a result, using a larger record size can
result in more wasted space for small files. On the other hand, a
larger record size can often be read and written much more efficiently.

Further complicating the problem is that some tape drives ignore the
blocking entirely. For these, a larger record size can still improve
performance (because the software layers above the tape drive still
honor the blocking), but not as dramatically as on tape drives that
honor blocking.

When reading an archive, 'tar' can usually figure out the record size
on itself. When this is the case, and a non-standard record size was
used when the archive was created, 'tar' will print a message about a
non-standard blocking factor, and then operate normally(1). On some
tape devices, however, 'tar' cannot figure out the record size itself.
On most of those, you can specify a blocking factor (with
'--blocking-factor') larger than the actual blocking factor, and then
use the '--read-full-records' ('-B') option. (If you specify a blocking
factor with '--blocking-factor' and don't use the '--read-full-records'
option, then 'tar' will not attempt to figure out the recording size
itself.) On some devices, you must always specify the record size
exactly with '--blocking-factor' when reading, because 'tar' cannot
figure it out. In any case, use '--list' ('-t') before doing any
extractions to see whether 'tar' is reading the archive correctly.

'tar' blocks are all fixed size (512 bytes), and its scheme for
putting them into records is to put a whole number of them (one or more)
into each record. 'tar' records are all the same size; at the end of
the file there's a block containing all zeros, which is how you tell
that the remainder of the last record(s) are garbage.

In a standard 'tar' file (no options), the block size is 512 and the
record size is 10240, for a blocking factor of 20. What the
'--blocking-factor' option does is sets the blocking factor, changing
the record size while leaving the block size at 512 bytes. 20 was fine
for ancient 800 or 1600 bpi reel-to-reel tape drives; most tape drives
these days prefer much bigger records in order to stream and not waste
tape. When writing tapes for myself, some tend to use a factor of the
order of 2048, say, giving a record size of around one megabyte.

If you use a blocking factor larger than 20, older 'tar' programs
might not be able to read the archive, so we recommend this as a limit
to use in practice. GNU 'tar', however, will support arbitrarily large
record sizes, limited only by the amount of virtual memory or the
physical characteristics of the tape device.

---------- Footnotes ----------

(1) If this message is not needed, you can turn it off using the
'--warning=no-record-size' option.

9.4.1 Format Variations
-----------------------

_(This message will disappear, once this node revised.)_

Format parameters specify how an archive is written on the archive
media. The best choice of format parameters will vary depending on the
type and number of files being archived, and on the media used to store
the archive.

To specify format parameters when accessing or creating an archive,
you can use the options described in the following sections. If you do
not specify any format parameters, 'tar' uses default parameters. You
cannot modify a compressed archive. If you create an archive with the
'--blocking-factor' option specified (*note Blocking Factor::), you must
specify that blocking-factor when operating on the archive. *Note
Formats::, for other examples of format parameter considerations.

9.4.2 The Blocking Factor of an Archive
---------------------------------------

_(This message will disappear, once this node revised.)_

The data in an archive is grouped into blocks, which are 512 bytes.
Blocks are read and written in whole number multiples called "records".
The number of blocks in a record (i.e., the size of a record in units of
512 bytes) is called the "blocking factor". The
'--blocking-factor=512-SIZE' ('-b 512-SIZE') option specifies the
blocking factor of an archive. The default blocking factor is typically
20 (i.e., 10240 bytes), but can be specified at installation. To find
out the blocking factor of an existing archive, use 'tar --list
--file=ARCHIVE-NAME'. This may not work on some devices.

Records are separated by gaps, which waste space on the archive
media. If you are archiving on magnetic tape, using a larger blocking
factor (and therefore larger records) provides faster throughput and
allows you to fit more data on a tape (because there are fewer gaps).
If you are archiving on cartridge, a very large blocking factor (say 126
or more) greatly increases performance. A smaller blocking factor, on
the other hand, may be useful when archiving small files, to avoid
archiving lots of nulls as 'tar' fills out the archive to the end of the
record. In general, the ideal record size depends on the size of the
inter-record gaps on the tape you are using, and the average size of the
files you are archiving. *Note create::, for information on writing
archives.

Archives with blocking factors larger than 20 cannot be read by very
old versions of 'tar', or by some newer versions of 'tar' running on old
machines with small address spaces. With GNU 'tar', the blocking factor
of an archive is limited only by the maximum record size of the device
containing the archive, or by the amount of available virtual memory.

Also, on some systems, not using adequate blocking factors, as
sometimes imposed by the device drivers, may yield unexpected
diagnostics. For example, this has been reported:

Cannot write to /dev/dlt: Invalid argument

In such cases, it sometimes happen that the 'tar' bundled by the system
is aware of block size idiosyncrasies, while GNU 'tar' requires an
explicit specification for the block size, which it cannot guess. This
yields some people to consider GNU 'tar' is misbehaving, because by
comparison, 'the bundle 'tar' works OK'. Adding '-b 256', for example,
might resolve the problem.

If you use a non-default blocking factor when you create an archive,
you must specify the same blocking factor when you modify that archive.
Some archive devices will also require you to specify the blocking
factor when reading that archive, however this is not typically the
case. Usually, you can use '--list' ('-t') without specifying a
blocking factor--'tar' reports a non-default record size and then lists
the archive members as it would normally. To extract files from an
archive with a non-standard blocking factor (particularly if you're not
sure what the blocking factor is), you can usually use the
'--read-full-records' ('-B') option while specifying a blocking factor
larger then the blocking factor of the archive (i.e., 'tar --extract
--read-full-records --blocking-factor=300'). *Note list::, for more
information on the '--list' ('-t') operation. *Note Reading::, for a
more detailed explanation of that option.

'--blocking-factor=NUMBER'
'-b NUMBER'
Specifies the blocking factor of an archive. Can be used with any
operation, but is usually not necessary with '--list' ('-t').

Device blocking

'-b BLOCKS'
'--blocking-factor=BLOCKS'
Set record size to BLOCKS*512 bytes.

This option is used to specify a "blocking factor" for the archive.
When reading or writing the archive, 'tar', will do reads and
writes of the archive in records of BLOCK*512 bytes. This is true
even when the archive is compressed. Some devices requires that
all write operations be a multiple of a certain size, and so, 'tar'
pads the archive out to the next record boundary.

The default blocking factor is set when 'tar' is compiled, and is
typically 20. Blocking factors larger than 20 cannot be read by
very old versions of 'tar', or by some newer versions of 'tar'
running on old machines with small address spaces.

With a magnetic tape, larger records give faster throughput and fit
more data on a tape (because there are fewer inter-record gaps).
If the archive is in a disk file or a pipe, you may want to specify
a smaller blocking factor, since a large one will result in a large
number of null bytes at the end of the archive.

When writing cartridge or other streaming tapes, a much larger
blocking factor (say 126 or more) will greatly increase
performance. However, you must specify the same blocking factor
when reading or updating the archive.

Apparently, Exabyte drives have a physical block size of 8K bytes.
If we choose our blocksize as a multiple of 8k bytes, then the
problem seems to disappear. Id est, we are using block size of 112
right now, and we haven't had the problem since we switched...

With GNU 'tar' the blocking factor is limited only by the maximum
record size of the device containing the archive, or by the amount
of available virtual memory.

However, deblocking or reblocking is virtually avoided in a special
case which often occurs in practice, but which requires all the
following conditions to be simultaneously true:
* the archive is subject to a compression option,
* the archive is not handled through standard input or output,
nor redirected nor piped,
* the archive is directly handled to a local disk, instead of
any special device,
* '--blocking-factor' is not explicitly specified on the 'tar'
invocation.

If the output goes directly to a local disk, and not through
stdout, then the last write is not extended to a full record size.
Otherwise, reblocking occurs. Here are a few other remarks on this
topic:

* 'gzip' will complain about trailing garbage if asked to
uncompress a compressed archive on tape, there is an option to
turn the message off, but it breaks the regularity of simply
having to use 'PROG -d' for decompression. It would be nice
if gzip was silently ignoring any number of trailing zeros.
I'll ask Jean-loup Gailly, by sending a copy of this message
to him.

* 'compress' does not show this problem, but as Jean-loup
pointed out to Michael, 'compress -d' silently adds garbage
after the result of decompression, which tar ignores because
it already recognized its end-of-file indicator. So this bug
may be safely ignored.

* 'gzip -d -q' will be silent about the trailing zeros indeed,
but will still return an exit status of 2 which tar reports in
turn. 'tar' might ignore the exit status returned, but I hate
doing that, as it weakens the protection 'tar' offers users
against other possible problems at decompression time. If
'gzip' was silently skipping trailing zeros _and_ also
avoiding setting the exit status in this innocuous case, that
would solve this situation.

* 'tar' should become more solid at not stopping to read a pipe
at the first null block encountered. This inelegantly breaks
the pipe. 'tar' should rather drain the pipe out before
exiting itself.

'-i'
'--ignore-zeros'
Ignore blocks of zeros in archive (means EOF).

The '--ignore-zeros' ('-i') option causes 'tar' to ignore blocks of
zeros in the archive. Normally a block of zeros indicates the end
of the archive, but when reading a damaged archive, or one which
was created by concatenating several archives together, this option
allows 'tar' to read the entire archive. This option is not on by
default because many versions of 'tar' write garbage after the
zeroed blocks.

Note that this option causes 'tar' to read to the end of the
archive file, which may sometimes avoid problems when multiple
files are stored on a single physical tape.

'-B'
'--read-full-records'
Reblock as we read (for reading 4.2BSD pipes).

If '--read-full-records' is used, 'tar' will not panic if an
attempt to read a record from the archive does not return a full
record. Instead, 'tar' will keep reading until it has obtained a
full record.

This option is turned on by default when 'tar' is reading an
archive from standard input, or from a remote machine. This is
because on BSD Unix systems, a read of a pipe will return however
much happens to be in the pipe, even if it is less than 'tar'
requested. If this option was not used, 'tar' would fail as soon
as it read an incomplete record from the pipe.

This option is also useful with the commands for updating an
archive.

Tape blocking

When handling various tapes or cartridges, you have to take care of
selecting a proper blocking, that is, the number of disk blocks you put
together as a single tape block on the tape, without intervening tape
gaps. A "tape gap" is a small landing area on the tape with no
information on it, used for decelerating the tape to a full stop, and
for later regaining the reading or writing speed. When the tape driver
starts reading a record, the record has to be read whole without
stopping, as a tape gap is needed to stop the tape motion without losing
information.

Using higher blocking (putting more disk blocks per tape block) will
use the tape more efficiently as there will be less tape gaps. But
reading such tapes may be more difficult for the system, as more memory
will be required to receive at once the whole record. Further, if there
is a reading error on a huge record, this is less likely that the system
will succeed in recovering the information. So, blocking should not be
too low, nor it should be too high. 'tar' uses by default a blocking of
20 for historical reasons, and it does not really matter when reading or
writing to disk. Current tape technology would easily accommodate
higher blockings. Sun recommends a blocking of 126 for Exabytes and 96
for DATs. We were told that for some DLT drives, the blocking should be
a multiple of 4Kb, preferably 64Kb ('-b 128') or 256 for decent
performance. Other manufacturers may use different recommendations for
the same tapes. This might also depends of the buffering techniques
used inside modern tape controllers. Some imposes a minimum blocking,
or a maximum blocking. Others request blocking to be some exponent of
two.

So, there is no fixed rule for blocking. But blocking at read time
should ideally be the same as blocking used at write time. At one place
I know, with a wide variety of equipment, they found it best to use a
blocking of 32 to guarantee that their tapes are fully interchangeable.

I was also told that, for recycled tapes, prior erasure (by the same
drive unit that will be used to create the archives) sometimes lowers
the error rates observed at rewriting time.

I might also use '--number-blocks' instead of '--block-number', so
'--block' will then expand to '--blocking-factor' unambiguously.

9.5 Many Archives on One Tape
=============================

Most tape devices have two entries in the '/dev' directory, or entries
that come in pairs, which differ only in the minor number for this
device. Let's take for example '/dev/tape', which often points to the
only or usual tape device of a given system. There might be a
corresponding '/dev/nrtape' or '/dev/ntape'. The simpler name is the
_rewinding_ version of the device, while the name having 'nr' in it is
the _no rewinding_ version of the same device.

A rewinding tape device will bring back the tape to its beginning
point automatically when this device is opened or closed. Since 'tar'
opens the archive file before using it and closes it afterwards, this
means that a simple:

$ tar cf /dev/tape DIRECTORY

will reposition the tape to its beginning both prior and after saving
DIRECTORY contents to it, thus erasing prior tape contents and making it
so that any subsequent write operation will destroy what has just been
saved.

So, a rewinding device is normally meant to hold one and only one
file. If you want to put more than one 'tar' archive on a given tape,
you will need to avoid using the rewinding version of the tape device.
You will also have to pay special attention to tape positioning. Errors
in positioning may overwrite the valuable data already on your tape.
Many people, burnt by past experiences, will only use rewinding devices
and limit themselves to one file per tape, precisely to avoid the risk
of such errors. Be fully aware that writing at the wrong position on a
tape loses all information past this point and most probably until the
end of the tape, and this destroyed information _cannot_ be recovered.

To save DIRECTORY-1 as a first archive at the beginning of a tape,
and leave that tape ready for a second archive, you should use:

$ mt -f /dev/nrtape rewind
$ tar cf /dev/nrtape DIRECTORY-1

"Tape marks" are special magnetic patterns written on the tape media,
which are later recognizable by the reading hardware. These marks are
used after each file, when there are many on a single tape. An empty
file (that is to say, two tape marks in a row) signal the logical end of
the tape, after which no file exist. Usually, non-rewinding tape device
drivers will react to the close request issued by 'tar' by first writing
two tape marks after your archive, and by backspacing over one of these.
So, if you remove the tape at that time from the tape drive, it is
properly terminated. But if you write another file at the current
position, the second tape mark will be erased by the new information,
leaving only one tape mark between files.

So, you may now save DIRECTORY-2 as a second archive after the first
on the same tape by issuing the command:

$ tar cf /dev/nrtape DIRECTORY-2

and so on for all the archives you want to put on the same tape.

Another usual case is that you do not write all the archives the same
day, and you need to remove and store the tape between two archive
sessions. In general, you must remember how many files are already
saved on your tape. Suppose your tape already has 16 files on it, and
that you are ready to write the 17th. You have to take care of skipping
the first 16 tape marks before saving DIRECTORY-17, say, by using these
commands:

$ mt -f /dev/nrtape rewind
$ mt -f /dev/nrtape fsf 16
$ tar cf /dev/nrtape DIRECTORY-17

In all the previous examples, we put aside blocking considerations,
but you should do the proper things for that as well. *Note Blocking::.

9.5.1 Tape Positions and Tape Marks
-----------------------------------

_(This message will disappear, once this node revised.)_

Just as archives can store more than one file from the file system,
tapes can store more than one archive file. To keep track of where
archive files (or any other type of file stored on tape) begin and end,
tape archive devices write magnetic "tape marks" on the archive media.
Tape drives write one tape mark between files, two at the end of all the
file entries.

If you think of data as a series of records "rrrr"'s, and tape marks
as "*"'s, a tape might look like the following:

rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------

Tape devices read and write tapes using a read/write "tape head"--a
physical part of the device which can only access one point on the tape
at a time. When you use 'tar' to read or write archive data from a tape
device, the device will begin reading or writing from wherever on the
tape the tape head happens to be, regardless of which archive or what
part of the archive the tape head is on. Before writing an archive, you
should make sure that no data on the tape will be overwritten (unless it
is no longer needed). Before reading an archive, you should make sure
the tape head is at the beginning of the archive you want to read. You
can do it manually via 'mt' utility (*note mt::). The 'restore' script
does that automatically (*note Scripted Restoration::).

If you want to add new archive file entries to a tape, you should
advance the tape to the end of the existing file entries, backspace over
the last tape mark, and write the new archive file. If you were to add
two archives to the example above, the tape might look like the
following:

rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------

9.5.2 The 'mt' Utility
----------------------

_(This message will disappear, once this node revised.)_

*Note Blocking Factor::.

You can use the 'mt' utility to advance or rewind a tape past a
specified number of archive files on the tape. This will allow you to
move to the beginning of an archive before extracting or reading it, or
to the end of all the archives before writing a new one.

The syntax of the 'mt' command is:

mt [-f TAPENAME] OPERATION [NUMBER]

where TAPENAME is the name of the tape device, NUMBER is the number
of times an operation is performed (with a default of one), and
OPERATION is one of the following:

'eof'
'weof'
Writes NUMBER tape marks at the current position on the tape.

'fsf'
Moves tape position forward NUMBER files.

'bsf'
Moves tape position back NUMBER files.

'rewind'
Rewinds the tape. (Ignores NUMBER.)

'offline'
'rewoff1'
Rewinds the tape and takes the tape device off-line. (Ignores
NUMBER.)

'status'
Prints status information about the tape unit.

If you don't specify a TAPENAME, 'mt' uses the environment variable
'TAPE'; if 'TAPE' is not set, 'mt' will use the default device specified
in your 'sys/mtio.h' file ('DEFTAPE' variable). If this is not defined,
the program will display a descriptive error message and exit with code
1.

'mt' returns a 0 exit status when the operation(s) were successful, 1
if the command was unrecognized, and 2 if an operation failed.

9.6 Using Multiple Tapes
========================

Often you might want to write a large archive, one larger than will fit
on the actual tape you are using. In such a case, you can run multiple
'tar' commands, but this can be inconvenient, particularly if you are
using options like '--exclude=PATTERN' or dumping entire file systems.
Therefore, 'tar' provides a special mode for creating multi-volume
archives.

"Multi-volume" archive is a single 'tar' archive, stored on several
media volumes of fixed size. Although in this section we will often
call 'volume' a "tape", there is absolutely no requirement for
multi-volume archives to be stored on tapes. Instead, they can use
whatever media type the user finds convenient, they can even be located
on files.

When creating a multi-volume archive, GNU 'tar' continues to fill
current volume until it runs out of space, then it switches to next
volume (usually the operator is queried to replace the tape on this
point), and continues working on the new volume. This operation
continues until all requested files are dumped. If GNU 'tar' detects
end of media while dumping a file, such a file is archived in split
form. Some very big files can even be split across several volumes.

Each volume is itself a valid GNU 'tar' archive, so it can be read
without any special options. Consequently any file member residing
entirely on one volume can be extracted or otherwise operated upon
without needing the other volume. Sure enough, to extract a split
member you would need all volumes its parts reside on.

Multi-volume archives suffer from several limitations. In
particular, they cannot be compressed.

GNU 'tar' is able to create multi-volume archives of two formats
(*note Formats::): 'GNU' and 'POSIX'.

9.6.1 Archives Longer than One Tape or Disk
-------------------------------------------

To create an archive that is larger than will fit on a single unit of
the media, use the '--multi-volume' ('-M') option in conjunction with
the '--create' option (*note create::). A "multi-volume" archive can be
manipulated like any other archive (provided the '--multi-volume' option
is specified), but is stored on more than one tape or file.

When you specify '--multi-volume', 'tar' does not report an error
when it comes to the end of an archive volume (when reading), or the end
of the media (when writing). Instead, it prompts you to load a new
storage volume. If the archive is on a magnetic tape, you should change
tapes when you see the prompt; if the archive is on a floppy disk, you
should change disks; etc.

'--multi-volume'
'-M'
Creates a multi-volume archive, when used in conjunction with
'--create' ('-c'). To perform any other operation on a
multi-volume archive, specify '--multi-volume' in conjunction with
that operation. For example:

$ tar --create --multi-volume --file=/dev/tape FILES

The method 'tar' uses to detect end of tape is not perfect, and fails
on some operating systems or on some devices. If 'tar' cannot detect
the end of the tape itself, you can use '--tape-length' option to inform
it about the capacity of the tape:

'--tape-length=SIZE[SUF]'
'-L SIZE[SUF]'
Set maximum length of a volume. The SUF, if given, specifies units
in which SIZE is expressed, e.g. '2M' mean 2 megabytes (*note
Table 9.1: size-suffixes, for a list of allowed size suffixes).
Without SUF, units of 1024 bytes (kilobyte) are assumed.

This option selects '--multi-volume' automatically. For example:

$ tar --create --tape-length=41943040 --file=/dev/tape FILES

or, which is equivalent:

$ tar --create --tape-length=4G --file=/dev/tape FILES

When GNU 'tar' comes to the end of a storage media, it asks you to
change the volume. The built-in prompt for POSIX locale is(1):

Prepare volume #N for 'ARCHIVE' and hit return:

where N is the ordinal number of the volume to be created and ARCHIVE is
archive file or device name.

When prompting for a new tape, 'tar' accepts any of the following
responses:

'?'
Request 'tar' to explain possible responses.
'q'
Request 'tar' to exit immediately.
'n FILE-NAME'
Request 'tar' to write the next volume on the file FILE-NAME.
'!'
Request 'tar' to run a subshell. This option can be disabled by
giving '--restrict' command line option to 'tar'(2).
'y'
Request 'tar' to begin writing the next volume.

(You should only type 'y' after you have changed the tape; otherwise
'tar' will write over the volume it just finished.)

The volume number used by 'tar' in its tape-changing prompt can be
changed; if you give the '--volno-file=FILE-OF-NUMBER' option, then
FILE-OF-NUMBER should be an non-existing file to be created, or else, a
file already containing a decimal number. That number will be used as
the volume number of the first volume written. When 'tar' is finished,
it will rewrite the file with the now-current volume number. (This does
not change the volume number written on a tape label, as per *note
label::, it _only_ affects the number used in the prompt.)

If you want more elaborate behavior than this, you can write a
special "new volume script", that will be responsible for changing the
volume, and instruct 'tar' to use it instead of its normal prompting
procedure:

'--info-script=COMMAND'
'--new-volume-script=COMMAND'
'-F COMMAND'
Specify the command to invoke when switching volumes. The COMMAND
can be used to eject cassettes, or to broadcast messages such as
'Someone please come change my tape' when performing unattended
backups.

The COMMAND can contain additional options, if such are needed.
*Note Running External Commands: external, for a detailed discussion of
the way GNU 'tar' runs external commands. It inherits 'tar''s shell
environment. Additional data is passed to it via the following
environment variables:

'TAR_VERSION'
GNU 'tar' version number.

'TAR_ARCHIVE'
The name of the archive 'tar' is processing.

'TAR_BLOCKING_FACTOR'
Current blocking factor (*note Blocking::).

'TAR_VOLUME'
Ordinal number of the volume 'tar' is about to start.

'TAR_SUBCOMMAND'
A short option describing the operation 'tar' is executing. *Note
Operations::, for a complete list of subcommand options.

'TAR_FORMAT'
Format of the archive being processed. *Note Formats::, for a
complete list of archive format names.

'TAR_FD'
File descriptor which can be used to communicate the new volume
name to 'tar'.

These variables can be used in the COMMAND itself, provided that they
are properly quoted to prevent them from being expanded by the shell
that invokes 'tar'.

The volume script can instruct 'tar' to use new archive name, by
writing in to file descriptor '$TAR_FD' (see below for an example).

If the info script fails, 'tar' exits; otherwise, it begins writing
the next volume.

If you want 'tar' to cycle through a series of files or tape drives,
there are three approaches to choose from. First of all, you can give
'tar' multiple '--file' options. In this case the specified files will
be used, in sequence, as the successive volumes of the archive. Only
when the first one in the sequence needs to be used again will 'tar'
prompt for a tape change (or run the info script). For example, suppose
someone has two tape drives on a system named '/dev/tape0' and
'/dev/tape1'. For having GNU 'tar' to switch to the second drive when
it needs to write the second tape, and then back to the first tape,
etc., just do either of:

$ tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 FILES
$ tar -cM -f /dev/tape0 -f /dev/tape1 FILES

The second method is to use the 'n' response to the tape-change
prompt.

Finally, the most flexible approach is to use a volume script, that
writes new archive name to the file descriptor '$TAR_FD'. For example,
the following volume script will create a series of archive files, named
'ARCHIVE-VOL', where ARCHIVE is the name of the archive being created
(as given by '--file' option) and VOL is the ordinal number of the
archive being created:

#! /bin/bash
# For this script it's advisable to use a shell, such as Bash,
# that supports a TAR_FD value greater than 9.

echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.

name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
case $TAR_SUBCOMMAND in
-c) ;;
-d|-x|-t) test -r ${name:-$TAR_ARCHIVE}-$TAR_VOLUME || exit 1
;;
*) exit 1
esac

echo ${name:-$TAR_ARCHIVE}-$TAR_VOLUME >&$TAR_FD

The same script can be used while listing, comparing or extracting
from the created archive. For example:

# Create a multi-volume archive:
$ tar -c -L1024 -f archive.tar -F new-volume .
# Extract from the created archive:
$ tar -x -f archive.tar -F new-volume .

Notice, that the first command had to use '-L' option, since otherwise
GNU 'tar' will end up writing everything to file 'archive.tar'.

You can read each individual volume of a multi-volume archive as if
it were an archive by itself. For example, to list the contents of one
volume, use '--list', without '--multi-volume' specified. To extract an
archive member from one volume (assuming it is described that volume),
use '--extract', again without '--multi-volume'.

If an archive member is split across volumes (i.e., its entry begins
on one volume of the media and ends on another), you need to specify
'--multi-volume' to extract it successfully. In this case, you should
load the volume where the archive member starts, and use 'tar --extract
--multi-volume'--'tar' will prompt for later volumes as it needs them.
*Note extracting archives::, for more information about extracting
archives.

Multi-volume archives can be modified like any other archive. To add
files to a multi-volume archive, you need to only mount the last volume
of the archive media (and new volumes, if needed). For all other
operations, you need to use the entire archive.

If a multi-volume archive was labeled using '--label=ARCHIVE-LABEL'
(*note label::) when it was created, 'tar' will not automatically label
volumes which are added later. To label subsequent volumes, specify
'--label=ARCHIVE-LABEL' again in conjunction with the '--append',
'--update' or '--concatenate' operation.

Notice that multi-volume support is a GNU extension and the archives
created in this mode should be read only using GNU 'tar'. If you
absolutely have to process such archives using a third-party 'tar'
implementation, read *note Split Recovery::.

---------- Footnotes ----------

(1) If you run GNU 'tar' under a different locale, the translation to
the locale's language will be used.

(2) *Note --restrict::, for more information about this option.

9.6.2 Tape Files
----------------

_(This message will disappear, once this node revised.)_

To give the archive a name which will be recorded in it, use the
'--label=VOLUME-LABEL' ('-V VOLUME-LABEL') option. This will write a
special block identifying VOLUME-LABEL as the name of the archive to the
front of the archive which will be displayed when the archive is listed
with '--list'. If you are creating a multi-volume archive with
'--multi-volume' (*note Using Multiple Tapes::), then the volume label
will have 'Volume NNN' appended to the name you give, where NNN is the
number of the volume of the archive. If you use the
'--label=VOLUME-LABEL' option when reading an archive, it checks to make
sure the label on the tape matches the one you gave. *Note label::.

When 'tar' writes an archive to tape, it creates a single tape file.
If multiple archives are written to the same tape, one after the other,
they each get written as separate tape files. When extracting, it is
necessary to position the tape at the right place before running 'tar'.
To do this, use the 'mt' command. For more information on the 'mt'
command and on the organization of tapes into a sequence of tape files,
see *note mt::.

People seem to often do:

--label="SOME-PREFIX `date +SOME-FORMAT`"

or such, for pushing a common date in all volumes or an archive set.

9.6.3 Concatenate Volumes into a Single Archive
-----------------------------------------------

Sometimes it is necessary to convert existing GNU 'tar' multi-volume
archive to a single 'tar' archive. Simply concatenating all volumes
into one will not work, since each volume carries an additional
information at the beginning. GNU 'tar' is shipped with the shell
script 'tarcat' designed for this purpose.

The script takes a list of files comprising a multi-volume archive
and creates the resulting archive at the standard output. For example:

tarcat vol.1 vol.2 vol.3 | tar tf -

The script implements a simple heuristics to determine the format of
the first volume file and to decide how to process the rest of the
files. However, it makes no attempt to verify whether the files are
given in order or even if they are valid 'tar' archives. It uses 'dd'
and does not filter its standard error, so you will usually see lots of
spurious messages.

9.7 Including a Label in the Archive
====================================

To avoid problems caused by misplaced paper labels on the archive media,
you can include a "label" entry -- an archive member which contains the
name of the archive -- in the archive itself. Use the
'--label=ARCHIVE-LABEL' ('-V ARCHIVE-LABEL') option(1) in conjunction
with the '--create' operation to include a label entry in the archive as
it is being created.

'--label=ARCHIVE-LABEL'
'-V ARCHIVE-LABEL'
Includes an "archive-label" at the beginning of the archive when
the archive is being created, when used in conjunction with the
'--create' operation. Checks to make sure the archive label
matches the one specified (when used in conjunction with any other
operation).

If you create an archive using both '--label=ARCHIVE-LABEL' ('-V
ARCHIVE-LABEL') and '--multi-volume' ('-M'), each volume of the archive
will have an archive label of the form 'ARCHIVE-LABEL Volume N', where N
is 1 for the first volume, 2 for the next, and so on. *Note Using
Multiple Tapes::, for information on creating multiple volume archives.

The volume label will be displayed by '--list' along with the file
contents. If verbose display is requested, it will also be explicitly
marked as in the example below:

$ tar --verbose --list --file=iamanarchive
V--------- 0/0 0 1992-03-07 12:01 iamalabel--Volume Header--
-rw-r--r-- ringo/user 40 1990-05-21 13:30 iamafilename

However, '--list' option will cause listing entire contents of the
archive, which may be undesirable (for example, if the archive is stored
on a tape). You can request checking only the volume label by
specifying '--test-label' option. This option reads only the first
block of an archive, so it can be used with slow storage devices. For
example:

$ tar --test-label --file=iamanarchive
iamalabel

If '--test-label' is used with one or more command line arguments,
'tar' compares the volume label with each argument. It exits with code
0 if a match is found, and with code 1 otherwise(2). No output is
displayed, unless you also used the '--verbose' option. For example:

$ tar --test-label --file=iamanarchive 'iamalabel'
=> 0
$ tar --test-label --file=iamanarchive 'alabel'
=> 1

When used with the '--verbose' option, 'tar' prints the actual volume
label (if any), and a verbose diagnostics in case of a mismatch:

$ tar --test-label --verbose --file=iamanarchive 'iamalabel'
iamalabel
=> 0
$ tar --test-label --verbose --file=iamanarchive 'alabel'
iamalabel
tar: Archive label mismatch
=> 1

If you request any operation, other than '--create', along with using
'--label' option, 'tar' will first check if the archive label matches
the one specified and will refuse to proceed if it does not. Use this
as a safety precaution to avoid accidentally overwriting existing
archives. For example, if you wish to add files to 'archive',
presumably labeled with string 'My volume', you will get:

$ tar -rf archive --label 'My volume' .
tar: Archive not labeled to match 'My volume'

in case its label does not match. This will work even if 'archive' is
not labeled at all.

Similarly, 'tar' will refuse to list or extract the archive if its
label doesn't match the ARCHIVE-LABEL specified. In those cases,
ARCHIVE-LABEL argument is interpreted as a globbing-style pattern which
must match the actual magnetic volume label. *Note exclude::, for a
precise description of how match is attempted(3). If the switch
'--multi-volume' ('-M') is being used, the volume label matcher will
also suffix ARCHIVE-LABEL by ' Volume [1-9]*' if the initial match
fails, before giving up. Since the volume numbering is automatically
added in labels at creation time, it sounded logical to equally help the
user taking care of it when the archive is being read.

You can also use '--label' to get a common information on all tapes
of a series. For having this information different in each series
created through a single script used on a regular basis, just manage to
get some date string as part of the label. For example:

$ tar -cM -f /dev/tape -V "Daily backup for `date +%Y-%m-%d`"
$ tar --create --file=/dev/tape --multi-volume \
--label="Daily backup for `date +%Y-%m-%d`"

Some more notes about volume labels:

* Each label has its own date and time, which corresponds to the time
when GNU 'tar' initially attempted to write it, often soon after
the operator launches 'tar' or types the carriage return telling
that the next tape is ready.

* Comparing date labels to get an idea of tape throughput is
unreliable. It gives correct results only if the delays for
rewinding tapes and the operator switching them were negligible,
which is usually not the case.

---------- Footnotes ----------

(1) Until version 1.10, that option was called '--volume', but is not
available under that name anymore.

(2) Note that GNU 'tar' versions up to 1.23 indicated mismatch with
an exit code 2 and printed a spurious diagnostics on stderr.

(3) Previous versions of 'tar' used full regular expression matching,
or before that, only exact string matching, instead of wildcard
matchers. We decided for the sake of simplicity to use a uniform
matching device through 'tar'.

9.8 Verifying Data as It is Stored
==================================

'-W'
'--verify'
Attempt to verify the archive after writing.

This option causes 'tar' to verify the archive after writing it.
Each volume is checked after it is written, and any discrepancies are
recorded on the standard error output.

Verification requires that the archive be on a back-space-able
medium. This means pipes, some cartridge tape drives, and some other
devices cannot be verified.

You can insure the accuracy of an archive by comparing files in the
system with archive members. 'tar' can compare an archive to the file
system as the archive is being written, to verify a write operation, or
can compare a previously written archive, to insure that it is up to
date.

To check for discrepancies in an archive immediately after it is
written, use the '--verify' ('-W') option in conjunction with the
'--create' operation. When this option is specified, 'tar' checks
archive members against their counterparts in the file system, and
reports discrepancies on the standard error.

To verify an archive, you must be able to read it from before the end
of the last written entry. This option is useful for detecting data
errors on some tapes. Archives written to pipes, some cartridge tape
drives, and some other devices cannot be verified.

One can explicitly compare an already made archive with the file
system by using the '--compare' ('--diff', '-d') option, instead of
using the more automatic '--verify' option. *Note compare::.

Note that these two options have a slightly different intent. The
'--compare' option checks how identical are the logical contents of some
archive with what is on your disks, while the '--verify' option is
really for checking if the physical contents agree and if the recording
media itself is of dependable quality. So, for the '--verify'
operation, 'tar' tries to defeat all in-memory cache pertaining to the
archive, while it lets the speed optimization undisturbed for the
'--compare' option. If you nevertheless use '--compare' for media
verification, you may have to defeat the in-memory cache yourself, maybe
by opening and reclosing the door latch of your recording unit, forcing
some doubt in your operating system about the fact this is really the
same volume as the one just written or read.

The '--verify' option would not be necessary if drivers were indeed
able to detect dependably all write failures. This sometimes require
many magnetic heads, some able to read after the writes occurred. One
would not say that drivers unable to detect all cases are necessarily
flawed, as long as programming is concerned.

The '--verify' ('-W') option will not work in conjunction with the
'--multi-volume' ('-M') option or the '--append' ('-r'), '--update'
('-u') and '--delete' operations. *Note Operations::, for more
information on these operations.

Also, since 'tar' normally strips leading '/' from file names (*note
absolute::), a command like 'tar --verify -cf /tmp/foo.tar /etc' will
work as desired only if the working directory is '/', as 'tar' uses the
archive's relative member names (e.g., 'etc/motd') when verifying the
archive.

9.9 Write Protection
====================

Almost all tapes and diskettes, and in a few rare cases, even disks can
be "write protected", to protect data on them from being changed. Once
an archive is written, you should write protect the media to prevent the
archive from being accidentally overwritten or deleted. (This will
protect the archive from being changed with a tape or floppy drive--it
will not protect it from magnet fields or other physical hazards.)

The write protection device itself is usually an integral part of the
physical media, and can be a two position (write enabled/write disabled)
switch, a notch which can be popped out or covered, a ring which can be
removed from the center of a tape reel, or some other changeable
feature.

10 Reliability and Security
***************************

The 'tar' command reads and writes files as any other application does,
and is subject to the usual caveats about reliability and security.
This section contains some commonsense advice on the topic.

10.1 Reliability
================

Ideally, when 'tar' is creating an archive, it reads from a file system
that is not being modified, and encounters no errors or inconsistencies
while reading and writing. If this is the case, the archive should
faithfully reflect what was read. Similarly, when extracting from an
archive, ideally 'tar' ideally encounters no errors and the extracted
files faithfully reflect what was in the archive.

However, when reading or writing real-world file systems, several
things can go wrong; these include permissions problems, corruption of
data, and race conditions.

10.1.1 Permissions Problems
---------------------------

If 'tar' encounters errors while reading or writing files, it normally
reports an error and exits with nonzero status. The work it does may
therefore be incomplete. For example, when creating an archive, if
'tar' cannot read a file then it cannot copy the file into the archive.

10.1.2 Data Corruption and Repair
---------------------------------

If an archive becomes corrupted by an I/O error, this may corrupt the
data in an extracted file. Worse, it may corrupt the file's metadata,
which may cause later parts of the archive to become misinterpreted. An
tar-format archive contains a checksum that most likely will detect
errors in the metadata, but it will not detect errors in the data.

If data corruption is a concern, you can compute and check your own
checksums of an archive by using other programs, such as 'cksum'.

When attempting to recover from a read error or data corruption in an
archive, you may need to skip past the questionable data and read the
rest of the archive. This requires some expertise in the archive format
and in other software tools.

10.1.3 Race conditions
----------------------

If some other process is modifying the file system while 'tar' is
reading or writing files, the result may well be inconsistent due to
race conditions. For example, if another process creates some files in
a directory while 'tar' is creating an archive containing the
directory's files, 'tar' may see some of the files but not others, or it
may see a file that is in the process of being created. The resulting
archive may not be a snapshot of the file system at any point in time.
If an application such as a database system depends on an accurate
snapshot, restoring from the 'tar' archive of a live file system may
therefore break that consistency and may break the application. The
simplest way to avoid the consistency issues is to avoid making other
changes to the file system while tar is reading it or writing it.

When creating an archive, several options are available to avoid race
conditions. Some hosts have a way of snapshotting a file system, or of
temporarily suspending all changes to a file system, by (say) suspending
the only virtual machine that can modify a file system; if you use these
facilities and have 'tar -c' read from a snapshot when creating an
archive, you can avoid inconsistency problems. More drastically, before
starting 'tar' you could suspend or shut down all processes other than
'tar' that have access to the file system, or you could unmount the file
system and then mount it read-only.

When extracting from an archive, one approach to avoid race
conditions is to create a directory that no other process can write to,
and extract into that.

10.2 Security
=============

In some cases 'tar' may be used in an adversarial situation, where an
untrusted user is attempting to gain information about or modify
otherwise-inaccessible files. Dealing with untrusted data (that is,
data generated by an untrusted user) typically requires extra care,
because even the smallest mistake in the use of 'tar' is more likely to
be exploited by an adversary than by a race condition.

10.2.1 Privacy
--------------

Standard privacy concerns apply when using 'tar'. For example, suppose
you are archiving your home directory into a file '/archive/myhome.tar'.
Any secret information in your home directory, such as your SSH secret
keys, are copied faithfully into the archive. Therefore, if your home
directory contains any file that should not be read by some other user,
the archive itself should be not be readable by that user. And even if
the archive's data are inaccessible to untrusted users, its metadata
(such as size or last-modified date) may reveal some information about
your home directory; if the metadata are intended to be private, the
archive's parent directory should also be inaccessible to untrusted
users.

One precaution is to create '/archive' so that it is not accessible
to any user, unless that user also has permission to access all the
files in your home directory.

Similarly, when extracting from an archive, take care that the
permissions of the extracted files are not more generous than what you
want. Even if the archive itself is readable only to you, files
extracted from it have their own permissions that may differ.

10.2.2 Integrity
----------------

When creating archives, take care that they are not writable by a
untrusted user; otherwise, that user could modify the archive, and when
you later extract from the archive you will get incorrect data.

When 'tar' extracts from an archive, by default it writes into files
relative to the working directory. If the archive was generated by an
untrusted user, that user therefore can write into any file under the
working directory. If the working directory contains a symbolic link to
another directory, the untrusted user can also write into any file under
the referenced directory. When extracting from an untrusted archive, it
is therefore good practice to create an empty directory and run 'tar' in
that directory.

When extracting from two or more untrusted archives, each one should
be extracted independently, into different empty directories.
Otherwise, the first archive could create a symbolic link into an area
outside the working directory, and the second one could follow the link
and overwrite data that is not under the working directory. For
example, when restoring from a series of incremental dumps, the archives
should have been created by a trusted process, as otherwise the
incremental restores might alter data outside the working directory.

If you use the '--absolute-names' ('-P') option when extracting,
'tar' respects any file names in the archive, even file names that begin
with '/' or contain '..'. As this lets the archive overwrite any file
in your system that you can write, the '--absolute-names' ('-P') option
should be used only for trusted archives.

Conversely, with the '--keep-old-files' ('-k') and '--skip-old-files'
options, 'tar' refuses to replace existing files when extracting. The
difference between the two options is that the former treats existing
files as errors whereas the latter just silently ignores them.

Finally, with the '--no-overwrite-dir' option, 'tar' refuses to
replace the permissions or ownership of already-existing directories.
These options may help when extracting from untrusted archives.

10.2.3 Dealing with Live Untrusted Data
---------------------------------------

Extra care is required when creating from or extracting into a file
system that is accessible to untrusted users. For example, superusers
who invoke 'tar' must be wary about its actions being hijacked by an
adversary who is reading or writing the file system at the same time
that 'tar' is operating.

When creating an archive from a live file system, 'tar' is vulnerable
to denial-of-service attacks. For example, an adversarial user could
create the illusion of an indefinitely-deep directory hierarchy
'd/e/f/g/...' by creating directories one step ahead of 'tar', or the
illusion of an indefinitely-long file by creating a sparse file but
arranging for blocks to be allocated just before 'tar' reads them.
There is no easy way for 'tar' to distinguish these scenarios from
legitimate uses, so you may need to monitor 'tar', just as you'd need to
monitor any other system service, to detect such attacks.

While a superuser is extracting from an archive into a live file
system, an untrusted user might replace a directory with a symbolic
link, in hopes that 'tar' will follow the symbolic link and extract data
into files that the untrusted user does not have access to. Even if the
archive was generated by the superuser, it may contain a file such as
'd/etc/passwd' that the untrusted user earlier created in order to break
in; if the untrusted user replaces the directory 'd/etc' with a symbolic
link to '/etc' while 'tar' is running, 'tar' will overwrite
'/etc/passwd'. This attack can be prevented by extracting into a
directory that is inaccessible to untrusted users.

Similar attacks via symbolic links are also possible when creating an
archive, if the untrusted user can modify an ancestor of a top-level
argument of 'tar'. For example, an untrusted user that can modify
'/home/eve' can hijack a running instance of 'tar -cf -
/home/eve/Documents/yesterday' by replacing '/home/eve/Documents' with a
symbolic link to some other location. Attacks like these can be
prevented by making sure that untrusted users cannot modify any files
that are top-level arguments to 'tar', or any ancestor directories of
these files.

10.2.4 Security Rules of Thumb
------------------------------

This section briefly summarizes rules of thumb for avoiding security
pitfalls.

* Protect archives at least as much as you protect any of the files
being archived.

* Extract from an untrusted archive only into an otherwise-empty
directory. This directory and its parent should be accessible only
to trusted users. For example:

$ chmod go-rwx .
$ mkdir -m go-rwx dir
$ cd dir
$ tar -xvf /archives/got-it-off-the-net.tar.gz

As a corollary, do not do an incremental restore from an untrusted
archive.

* Do not let untrusted users access files extracted from untrusted
archives without checking first for problems such as setuid
programs.

* Do not let untrusted users modify directories that are ancestors of
top-level arguments of 'tar'. For example, while you are executing
'tar -cf /archive/u-home.tar /u/home', do not let an untrusted user
modify '/', '/archive', or '/u'.

* Pay attention to the diagnostics and exit status of 'tar'.

* When archiving live file systems, monitor running instances of
'tar' to detect denial-of-service attacks.

* Avoid unusual options such as '--absolute-names' ('-P'),
'--dereference' ('-h'), '--overwrite', '--recursive-unlink', and
'--remove-files' unless you understand their security implications.

Appendix A Changes
******************

This appendix lists some important user-visible changes between version
GNU 'tar' 1.29 and previous versions. An up-to-date version of this
document is available at the GNU 'tar' documentation page
(http://www.gnu.org/software/tar/manual/changes.html).

Use of globbing patterns when listing and extracting.

Previous versions of GNU tar assumed shell-style globbing when
extracting from or listing an archive. For example:

$ tar xf foo.tar '*.c'

would extract all files whose names end in '.c'. This behavior was
not documented and was incompatible with traditional tar
implementations. Therefore, starting from version 1.15.91, GNU tar
no longer uses globbing by default. For example, the above
invocation is now interpreted as a request to extract from the
archive the file named '*.c'.

To facilitate transition to the new behavior for those users who
got used to the previous incorrect one, 'tar' will print a warning
if it finds out that a requested member was not found in the
archive and its name looks like a globbing pattern. For example:

$ tar xf foo.tar '*.c'
tar: Pattern matching characters used in file names. Please,
tar: use --wildcards to enable pattern matching, or --no-wildcards to
tar: suppress this warning.
tar: *.c: Not found in archive
tar: Error exit delayed from previous errors

To treat member names as globbing patterns, use the '--wildcards'
option. If you want to tar to mimic the behavior of versions prior
to 1.15.91, add this option to your 'TAR_OPTIONS' variable.

*Note wildcards::, for the detailed discussion of the use of
globbing patterns by GNU 'tar'.

Use of short option '-o'.

Earlier versions of GNU 'tar' understood '-o' command line option
as a synonym for '--old-archive'.

GNU 'tar' starting from version 1.13.90 understands this option as
a synonym for '--no-same-owner'. This is compatible with UNIX98
'tar' implementations.

However, to facilitate transition, '-o' option retains its old
semantics when it is used with one of archive-creation commands.
Users are encouraged to use '--format=oldgnu' instead.

It is especially important, since versions of GNU Automake up to
and including 1.8.4 invoke tar with this option to produce
distribution tarballs. *Note v7: Formats, for the detailed
discussion of this issue and its implications.

*Note tar-formats: (automake)Options, for a description on how to
use various archive formats with 'automake'.

Future versions of GNU 'tar' will understand '-o' only as a synonym
for '--no-same-owner'.

Use of short option '-l'

Earlier versions of GNU 'tar' understood '-l' option as a synonym
for '--one-file-system'. Since such usage contradicted to UNIX98
specification and harmed compatibility with other implementations,
it was declared deprecated in version 1.14. However, to facilitate
transition to its new semantics, it was supported by versions 1.15
and 1.15.90. The present use of '-l' as a short variant of
'--check-links' was introduced in version 1.15.91.

Use of options '--portability' and '--old-archive'

These options are deprecated. Please use '--format=v7' instead.

Use of option '--posix'

This option is deprecated. Please use '--format=posix' instead.

Appendix B Configuring Help Summary
***********************************

Running 'tar --help' displays the short 'tar' option summary (*note
help::). This summary is organized by "groups" of semantically close
options. The options within each group are printed in the following
order: a short option, eventually followed by a list of corresponding
long option names, followed by a short description of the option. For
example, here is an excerpt from the actual 'tar --help' output:

Main operation mode:

-A, --catenate, --concatenate append tar files to an archive
-c, --create create a new archive
-d, --diff, --compare find differences between archive and
file system
--delete delete from the archive

The exact visual representation of the help output is configurable
via 'ARGP_HELP_FMT' environment variable. The value of this variable is
a comma-separated list of "format variable" assignments. There are two
kinds of format variables. An "offset variable" keeps the offset of
some part of help output text from the leftmost column on the screen. A
"boolean" variable is a flag that toggles some output feature on or off.
Depending on the type of the corresponding variable, there are two kinds
of assignments:

Offset assignment

The assignment to an offset variable has the following syntax:

VARIABLE=VALUE

where VARIABLE is the variable name, and VALUE is a numeric value
to be assigned to the variable.

Boolean assignment

To assign 'true' value to a variable, simply put this variable
name. To assign 'false' value, prefix the variable name with
'no-'. For example:

# Assign true value:
dup-args
# Assign false value:
no-dup-args

Following variables are declared:

-- Help Output: boolean dup-args
If true, arguments for an option are shown with both short and long
options, even when a given option has both forms, for example:

-f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE

If false, then if an option has both short and long forms, the
argument is only shown with the long one, for example:

-f, --file=ARCHIVE use archive file or device ARCHIVE

and a message indicating that the argument is applicable to both
forms is printed below the options. This message can be disabled
using 'dup-args-note' (see below).

The default is false.

-- Help Output: boolean dup-args-note
If this variable is true, which is the default, the following
notice is displayed at the end of the help output:

Mandatory or optional arguments to long options are also
mandatory or optional for any corresponding short options.

Setting 'no-dup-args-note' inhibits this message. Normally, only
one of variables 'dup-args' or 'dup-args-note' should be set.

-- Help Output: offset short-opt-col
Column in which short options start. Default is 2.

$ tar --help|grep ARCHIVE
-f, --file=ARCHIVE use archive file or device ARCHIVE
$ ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE
-f, --file=ARCHIVE use archive file or device ARCHIVE

-- Help Output: offset long-opt-col
Column in which long options start. Default is 6. For example:

$ tar --help|grep ARCHIVE
-f, --file=ARCHIVE use archive file or device ARCHIVE
$ ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE
-f, --file=ARCHIVE use archive file or device ARCHIVE

-- Help Output: offset doc-opt-col
Column in which "doc options" start. A doc option isn't actually
an option, but rather an arbitrary piece of documentation that is
displayed in much the same manner as the options. For example, in
the description of '--format' option:

-H, --format=FORMAT create archive of the given format.

FORMAT is one of the following:

gnu GNU tar 1.13.x format
oldgnu GNU format as per tar <= 1.12
pax POSIX 1003.1-2001 (pax) format
posix same as pax
ustar POSIX 1003.1-1988 (ustar) format
v7 old V7 tar format

the format names are doc options. Thus, if you set
'ARGP_HELP_FMT=doc-opt-col=6' the above part of the help output
will look as follows:

-H, --format=FORMAT create archive of the given format.

FORMAT is one of the following:

gnu GNU tar 1.13.x format
oldgnu GNU format as per tar <= 1.12
pax POSIX 1003.1-2001 (pax) format
posix same as pax
ustar POSIX 1003.1-1988 (ustar) format
v7 old V7 tar format

-- Help Output: offset opt-doc-col
Column in which option description starts. Default is 29.

$ tar --help|grep ARCHIVE
-f, --file=ARCHIVE use archive file or device ARCHIVE
$ ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE
-f, --file=ARCHIVE use archive file or device ARCHIVE
$ ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE
-f, --file=ARCHIVE
use archive file or device ARCHIVE

Notice, that the description starts on a separate line if
'opt-doc-col' value is too small.

-- Help Output: offset header-col
Column in which "group headers" are printed. A group header is a
descriptive text preceding an option group. For example, in the
following text:

Main operation mode:

-A, --catenate, --concatenate append tar files to
an archive
-c, --create create a new archive
'Main operation mode:' is the group header.

The default value is 1.

-- Help Output: offset usage-indent
Indentation of wrapped usage lines. Affects '--usage' output.
Default is 12.

-- Help Output: offset rmargin
Right margin of the text output. Used for wrapping.

Appendix C Fixing Snapshot Files
********************************

Various situations can cause device numbers to change: upgrading your
kernel version, reconfiguring your hardware, loading kernel modules in a
different order, using virtual volumes that are assembled dynamically
(such as with LVM or RAID), hot-plugging drives (e.g. external USB or
Firewire drives), etc. In the majority of cases this change is
unnoticed by the users. However, it influences 'tar' incremental
backups: the device number is stored in tar snapshot files (*note
Snapshot Files::) and is used to determine whether the file has changed
since the last backup. If the device numbers change for some reason, by
default the next backup you run will be a full backup.

To minimize the impact in these cases, GNU 'tar' comes with the
'tar-snapshot-edit' utility for inspecting and updating device numbers
in snapshot files. (The utility, written by Dustin J. Mitchell, is also
available from the GNU 'tar' home page
(http://www.gnu.org/software/tar/utils/tar-snapshot-edit.html).)

To obtain a summary of the device numbers found in the snapshot file,
run

$ tar-snapshot-edit SNAPFILE

where SNAPFILE is the name of the snapshot file (you can supply as many
files as you wish in a single command line). You can then compare the
numbers across snapshot files, or against those currently in use on the
live filesystem (using 'ls -l' or 'stat').

Assuming the device numbers have indeed changed, it's often possible
to simply tell GNU 'tar' to ignore the device number when processing the
incremental snapshot files for these backups, using the
'--no-check-device' option (*note device numbers::).

Alternatively, you can use the 'tar-edit-snapshot' script's '-r'
option to update all occurrences of the given device number in the
snapshot file(s). It takes a single argument of the form
'OLDDEV-NEWDEV', where OLDDEV is the device number used in the snapshot
file, and NEWDEV is the corresponding new device number. Both numbers
may be specified in hex (e.g., '0xfe01'), decimal (e.g., '65025'), or as
a major:minor number pair (e.g., '254:1'). To change several device
numbers at once, specify them in a single comma-separated list, as in
'-r 0x3060-0x4500,0x307-0x4600'.

Before updating the snapshot file, it is a good idea to create a
backup copy of it. This is accomplished by '-b' option. The name of
the backup file is obtained by appending '~' to the original file name.

An example session:
$ tar-snapshot-edit root_snap.0 boot_snap.0
File: root_snap.0
Detected snapshot file version: 2

Device 0x0000 occurs 1 times.
Device 0x0003 occurs 1 times.
Device 0x0005 occurs 1 times.
Device 0x0013 occurs 1 times.
Device 0x6801 occurs 1 times.
Device 0x6803 occurs 6626 times.
Device 0xfb00 occurs 1 times.

File: boot_snap.0
Detected snapshot file version: 2

Device 0x6801 occurs 3 times.
$ tar-snapshot-edit -b -r 0x6801-0x6901,0x6803-0x6903 root_snap.0 boot_snap.0
File: root_snap.0
Detected snapshot file version: 2

Updated 6627 records.

File: boot_snap.0
Detected snapshot file version: 2

Updated 3 records.

Appendix D Tar Internals
************************

Basic Tar Format
================

_(This message will disappear, once this node revised.)_

While an archive may contain many files, the archive itself is a
single ordinary file. Like any other file, an archive file can be
written to a storage device such as a tape or disk, sent through a pipe
or over a network, saved on the active file system, or even stored in
another archive. An archive file is not easy to read or manipulate
without using the 'tar' utility or Tar mode in GNU Emacs.

Physically, an archive consists of a series of file entries
terminated by an end-of-archive entry, which consists of two 512 blocks
of zero bytes. A file entry usually describes one of the files in the
archive (an "archive member"), and consists of a file header and the
contents of the file. File headers contain file names and statistics,
checksum information which 'tar' uses to detect file corruption, and
information about file types.

Archives are permitted to have more than one member with the same
member name. One way this situation can occur is if more than one
version of a file has been stored in the archive. For information about
adding new versions of a file to an archive, see *note update::.

In addition to entries describing archive members, an archive may
contain entries which 'tar' itself uses to store information. *Note
label::, for an example of such an archive entry.

A 'tar' archive file contains a series of blocks. Each block
contains 'BLOCKSIZE' bytes. Although this format may be thought of as
being on magnetic tape, other media are often used.

Each file archived is represented by a header block which describes
the file, followed by zero or more blocks which give the contents of the
file. At the end of the archive file there are two 512-byte blocks
filled with binary zeros as an end-of-file marker. A reasonable system
should write such end-of-file marker at the end of an archive, but must
not assume that such a block exists when reading an archive. In
particular GNU 'tar' always issues a warning if it does not encounter
it.

The blocks may be "blocked" for physical I/O operations. Each record
of N blocks (where N is set by the '--blocking-factor=512-SIZE' ('-b
512-SIZE') option to 'tar') is written with a single 'write ()'
operation. On magnetic tapes, the result of such a write is a single
record. When writing an archive, the last record of blocks should be
written at the full size, with blocks after the zero block containing
all zeros. When reading an archive, a reasonable system should properly
handle an archive whose last record is shorter than the rest, or which
contains garbage records after a zero block.

The header block is defined in C as follows. In the GNU 'tar'
distribution, this is part of file 'src/tar.h':


/* tar Header Block, from POSIX 1003.1-1990. */

/* POSIX header. */

struct posix_header
{ /* byte offset */
char name[100]; /* 0 */
char mode[8]; /* 100 */
char uid[8]; /* 108 */
char gid[8]; /* 116 */
char size[12]; /* 124 */
char mtime[12]; /* 136 */
char chksum[8]; /* 148 */
char typeflag; /* 156 */
char linkname[100]; /* 157 */
char magic[6]; /* 257 */
char version[2]; /* 263 */
char uname[32]; /* 265 */
char gname[32]; /* 297 */
char devmajor[8]; /* 329 */
char devminor[8]; /* 337 */
char prefix[155]; /* 345 */
/* 500 */
};

#define TMAGIC "ustar" /* ustar and a null */
#define TMAGLEN 6
#define TVERSION "00" /* 00 and no null */
#define TVERSLEN 2

/* Values used in typeflag field. */
#define REGTYPE '0' /* regular file */
#define AREGTYPE '\0' /* regular file */
#define LNKTYPE '1' /* link */
#define SYMTYPE '2' /* reserved */
#define CHRTYPE '3' /* character special */
#define BLKTYPE '4' /* block special */
#define DIRTYPE '5' /* directory */
#define FIFOTYPE '6' /* FIFO special */
#define CONTTYPE '7' /* reserved */

#define XHDTYPE 'x' /* Extended header referring to the
next file in the archive */
#define XGLTYPE 'g' /* Global extended header */

/* Bits used in the mode field, values in octal. */
#define TSUID 04000 /* set UID on execution */
#define TSGID 02000 /* set GID on execution */
#define TSVTX 01000 /* reserved */
/* file permissions */
#define TUREAD 00400 /* read by owner */
#define TUWRITE 00200 /* write by owner */
#define TUEXEC 00100 /* execute/search by owner */
#define TGREAD 00040 /* read by group */
#define TGWRITE 00020 /* write by group */
#define TGEXEC 00010 /* execute/search by group */
#define TOREAD 00004 /* read by other */
#define TOWRITE 00002 /* write by other */
#define TOEXEC 00001 /* execute/search by other */

/* tar Header Block, GNU extensions. */

/* In GNU tar, SYMTYPE is for to symbolic links, and CONTTYPE is for
contiguous files, so maybe disobeying the "reserved" comment in POSIX
header description. I suspect these were meant to be used this way, and
should not have really been "reserved" in the published standards. */

/* *BEWARE* *BEWARE* *BEWARE* that the following information is still
boiling, and may change. Even if the OLDGNU format description should be
accurate, the so-called GNU format is not yet fully decided. It is
surely meant to use only extensions allowed by POSIX, but the sketch
below repeats some ugliness from the OLDGNU format, which should rather
go away. Sparse files should be saved in such a way that they do *not*
require two passes at archive creation time. Huge files get some POSIX
fields to overflow, alternate solutions have to be sought for this. */

/* Descriptor for a single file hole. */

struct sparse
{ /* byte offset */
char offset[12]; /* 0 */
char numbytes[12]; /* 12 */
/* 24 */
};

/* Sparse files are not supported in POSIX ustar format. For sparse files
with a POSIX header, a GNU extra header is provided which holds overall
sparse information and a few sparse descriptors. When an old GNU header
replaces both the POSIX header and the GNU extra header, it holds some
sparse descriptors too. Whether POSIX or not, if more sparse descriptors
are still needed, they are put into as many successive sparse headers as
necessary. The following constants tell how many sparse descriptors fit
in each kind of header able to hold them. */

#define SPARSES_IN_EXTRA_HEADER 16
#define SPARSES_IN_OLDGNU_HEADER 4
#define SPARSES_IN_SPARSE_HEADER 21

/* Extension header for sparse files, used immediately after the GNU extra
header, and used only if all sparse information cannot fit into that
extra header. There might even be many such extension headers, one after
the other, until all sparse information has been recorded. */

struct sparse_header
{ /* byte offset */
struct sparse sp[SPARSES_IN_SPARSE_HEADER];
/* 0 */
char isextended; /* 504 */
/* 505 */
};

/* The old GNU format header conflicts with POSIX format in such a way that
POSIX archives may fool old GNU tar's, and POSIX tar's might well be
fooled by old GNU tar archives. An old GNU format header uses the space
used by the prefix field in a POSIX header, and cumulates information
normally found in a GNU extra header. With an old GNU tar header, we
never see any POSIX header nor GNU extra header. Supplementary sparse
headers are allowed, however. */

struct oldgnu_header
{ /* byte offset */
char unused_pad1[345]; /* 0 */
char atime[12]; /* 345 Incr. archive: atime of the file */
char ctime[12]; /* 357 Incr. archive: ctime of the file */
char offset[12]; /* 369 Multivolume archive: the offset of
the start of this volume */
char longnames[4]; /* 381 Not used */
char unused_pad2; /* 385 */
struct sparse sp[SPARSES_IN_OLDGNU_HEADER];
/* 386 */
char isextended; /* 482 Sparse file: Extension sparse header
follows */
char realsize[12]; /* 483 Sparse file: Real size*/
/* 495 */
};

/* OLDGNU_MAGIC uses both magic and version fields, which are contiguous.
Found in an archive, it indicates an old GNU header format, which will be
hopefully become obsolescent. With OLDGNU_MAGIC, uname and gname are
valid, though the header is not truly POSIX conforming. */
#define OLDGNU_MAGIC "ustar " /* 7 chars and a null */

/* The standards committee allows only capital A through capital Z for
user-defined expansion. Other letters in use include:

'A' Solaris Access Control List
'E' Solaris Extended Attribute File
'I' Inode only, as in 'star'
'N' Obsolete GNU tar, for file names that do not fit into the main header.
'X' POSIX 1003.1-2001 eXtended (VU version) */

/* This is a dir entry that contains the names of files that were in the
dir at the time the dump was made. */
#define GNUTYPE_DUMPDIR 'D'

/* Identifies the *next* file on the tape as having a long linkname. */
#define GNUTYPE_LONGLINK 'K'

/* Identifies the *next* file on the tape as having a long name. */
#define GNUTYPE_LONGNAME 'L'

/* This is the continuation of a file that began on another volume. */
#define GNUTYPE_MULTIVOL 'M'

/* This is for sparse files. */
#define GNUTYPE_SPARSE 'S'

/* This file is a tape/volume header. Ignore it on extraction. */
#define GNUTYPE_VOLHDR 'V'

/* Solaris extended header */
#define SOLARIS_XHDTYPE 'X'

/* Jo"rg Schilling star header */

struct star_header
{ /* byte offset */
char name[100]; /* 0 */
char mode[8]; /* 100 */
char uid[8]; /* 108 */
char gid[8]; /* 116 */
char size[12]; /* 124 */
char mtime[12]; /* 136 */
char chksum[8]; /* 148 */
char typeflag; /* 156 */
char linkname[100]; /* 157 */
char magic[6]; /* 257 */
char version[2]; /* 263 */
char uname[32]; /* 265 */
char gname[32]; /* 297 */
char devmajor[8]; /* 329 */
char devminor[8]; /* 337 */
char prefix[131]; /* 345 */
char atime[12]; /* 476 */
char ctime[12]; /* 488 */
/* 500 */
};

#define SPARSES_IN_STAR_HEADER 4
#define SPARSES_IN_STAR_EXT_HEADER 21

struct star_in_header
{
char fill[345]; /* 0 Everything that is before t_prefix */
char prefix[1]; /* 345 t_name prefix */
char fill2; /* 346 */
char fill3[8]; /* 347 */
char isextended; /* 355 */
struct sparse sp[SPARSES_IN_STAR_HEADER]; /* 356 */
char realsize[12]; /* 452 Actual size of the file */
char offset[12]; /* 464 Offset of multivolume contents */
char atime[12]; /* 476 */
char ctime[12]; /* 488 */
char mfill[8]; /* 500 */
char xmagic[4]; /* 508 "tar" */
};

struct star_ext_header
{
struct sparse sp[SPARSES_IN_STAR_EXT_HEADER];
char isextended;
};


All characters in header blocks are represented by using 8-bit
characters in the local variant of ASCII. Each field within the
structure is contiguous; that is, there is no padding used within the
structure. Each character on the archive medium is stored contiguously.

Bytes representing the contents of files (after the header block of
each file) are not translated in any way and are not constrained to
represent characters in any character set. The 'tar' format does not
distinguish text files from binary files, and no translation of file
contents is performed.

The 'name', 'linkname', 'magic', 'uname', and 'gname' are
null-terminated character strings. All other fields are zero-filled
octal numbers in ASCII. Each numeric field of width W contains W minus 1
digits, and a null.

The 'name' field is the file name of the file, with directory names
(if any) preceding the file name, separated by slashes.

The 'mode' field provides nine bits specifying file permissions and
three bits to specify the Set UID, Set GID, and Save Text ("sticky")
modes. Values for these bits are defined above. When special
permissions are required to create a file with a given mode, and the
user restoring files from the archive does not hold such permissions,
the mode bit(s) specifying those special permissions are ignored. Modes
which are not supported by the operating system restoring files from the
archive will be ignored. Unsupported modes should be faked up when
creating or updating an archive; e.g., the group permission could be
copied from the _other_ permission.

The 'uid' and 'gid' fields are the numeric user and group ID of the
file owners, respectively. If the operating system does not support
numeric user or group IDs, these fields should be ignored.

The 'size' field is the size of the file in bytes; linked files are
archived with this field specified as zero.

The 'mtime' field is the data modification time of the file at the
time it was archived. It is the ASCII representation of the octal value
of the last time the file's contents were modified, represented as an
integer number of seconds since January 1, 1970, 00:00 Coordinated
Universal Time.

The 'chksum' field is the ASCII representation of the octal value of
the simple sum of all bytes in the header block. Each 8-bit byte in the
header is added to an unsigned integer, initialized to zero, the
precision of which shall be no less than seventeen bits. When
calculating the checksum, the 'chksum' field is treated as if it were
all blanks.

The 'typeflag' field specifies the type of file archived. If a
particular implementation does not recognize or permit the specified
type, the file will be extracted as if it were a regular file. As this
action occurs, 'tar' issues a warning to the standard error.

The 'atime' and 'ctime' fields are used in making incremental
backups; they store, respectively, the particular file's access and
status change times.

The 'offset' is used by the '--multi-volume' ('-M') option, when
making a multi-volume archive. The offset is number of bytes into the
file that we need to restart at to continue the file on the next tape,
i.e., where we store the location that a continued file is continued at.

The following fields were added to deal with sparse files. A file is
"sparse" if it takes in unallocated blocks which end up being
represented as zeros, i.e., no useful data. A test to see if a file is
sparse is to look at the number blocks allocated for it versus the
number of characters in the file; if there are fewer blocks allocated
for the file than would normally be allocated for a file of that size,
then the file is sparse. This is the method 'tar' uses to detect a
sparse file, and once such a file is detected, it is treated differently
from non-sparse files.

Sparse files are often 'dbm' files, or other database-type files
which have data at some points and emptiness in the greater part of the
file. Such files can appear to be very large when an 'ls -l' is done on
them, when in truth, there may be a very small amount of important data
contained in the file. It is thus undesirable to have 'tar' think that
it must back up this entire file, as great quantities of room are wasted
on empty blocks, which can lead to running out of room on a tape far
earlier than is necessary. Thus, sparse files are dealt with so that
these empty blocks are not written to the tape. Instead, what is
written to the tape is a description, of sorts, of the sparse file:
where the holes are, how big the holes are, and how much data is found
at the end of the hole. This way, the file takes up potentially far
less room on the tape, and when the file is extracted later on, it will
look exactly the way it looked beforehand. The following is a
description of the fields used to handle a sparse file:

The 'sp' is an array of 'struct sparse'. Each 'struct sparse'
contains two 12-character strings which represent an offset into the
file and a number of bytes to be written at that offset. The offset is
absolute, and not relative to the offset in preceding array element.

The header can hold four of these 'struct sparse' at the moment; if
more are needed, they are not stored in the header.

The 'isextended' flag is set when an 'extended_header' is needed to
deal with a file. Note that this means that this flag can only be set
when dealing with a sparse file, and it is only set in the event that
the description of the file will not fit in the allotted room for sparse
structures in the header. In other words, an extended_header is needed.

The 'extended_header' structure is used for sparse files which need
more sparse structures than can fit in the header. The header can fit 4
such structures; if more are needed, the flag 'isextended' gets set and
the next block is an 'extended_header'.

Each 'extended_header' structure contains an array of 21 sparse
structures, along with a similar 'isextended' flag that the header had.
There can be an indeterminate number of such 'extended_header's to
describe a sparse file.

'REGTYPE'
'AREGTYPE'
These flags represent a regular file. In order to be compatible
with older versions of 'tar', a 'typeflag' value of 'AREGTYPE'
should be silently recognized as a regular file. New archives
should be created using 'REGTYPE'. Also, for backward
compatibility, 'tar' treats a regular file whose name ends with a
slash as a directory.

'LNKTYPE'
This flag represents a file linked to another file, of any type,
previously archived. Such files are identified in Unix by each
file having the same device and inode number. The linked-to name
is specified in the 'linkname' field with a trailing null.

'SYMTYPE'
This represents a symbolic link to another file. The linked-to
name is specified in the 'linkname' field with a trailing null.

'CHRTYPE'
'BLKTYPE'
These represent character special files and block special files
respectively. In this case the 'devmajor' and 'devminor' fields
will contain the major and minor device numbers respectively.
Operating systems may map the device specifications to their own
local specification, or may ignore the entry.

'DIRTYPE'
This flag specifies a directory or sub-directory. The directory
name in the 'name' field should end with a slash. On systems where
disk allocation is performed on a directory basis, the 'size' field
will contain the maximum number of bytes (which may be rounded to
the nearest disk block allocation unit) which the directory may
hold. A 'size' field of zero indicates no such limiting. Systems
which do not support limiting in this manner should ignore the
'size' field.

'FIFOTYPE'
This specifies a FIFO special file. Note that the archiving of a
FIFO file archives the existence of this file and not its contents.

'CONTTYPE'
This specifies a contiguous file, which is the same as a normal
file except that, in operating systems which support it, all its
space is allocated contiguously on the disk. Operating systems
which do not allow contiguous allocation should silently treat this
type as a normal file.

'A' ... 'Z'
These are reserved for custom implementations. Some of these are
used in the GNU modified format, as described below.

Other values are reserved for specification in future revisions of
the P1003 standard, and should not be used by any 'tar' program.

The 'magic' field indicates that this archive was output in the P1003
archive format. If this field contains 'TMAGIC', the 'uname' and
'gname' fields will contain the ASCII representation of the owner and
group of the file respectively. If found, the user and group IDs are
used rather than the values in the 'uid' and 'gid' fields.

For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990,
pages 169-173 (section 10.1) for 'Archive/Interchange File Format'; and
IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940
(section E.4.48) for 'pax - Portable archive interchange'.

GNU Extensions to the Archive Format
====================================

_(This message will disappear, once this node revised.)_

The GNU format uses additional file types to describe new types of
files in an archive. These are listed below.

'GNUTYPE_DUMPDIR'
''D''
This represents a directory and a list of files created by the
'--incremental' ('-G') option. The 'size' field gives the total
size of the associated list of files. Each file name is preceded
by either a 'Y' (the file should be in this archive) or an 'N'.
(The file is a directory, or is not stored in the archive.) Each
file name is terminated by a null. There is an additional null
after the last file name.

'GNUTYPE_MULTIVOL'
''M''
This represents a file continued from another volume of a
multi-volume archive created with the '--multi-volume' ('-M')
option. The original type of the file is not given here. The
'size' field gives the maximum size of this piece of the file
(assuming the volume does not end before the file is written out).
The 'offset' field gives the offset from the beginning of the file
where this part of the file begins. Thus 'size' plus 'offset'
should equal the original size of the file.

'GNUTYPE_SPARSE'
''S''
This flag indicates that we are dealing with a sparse file. Note
that archiving a sparse file requires special operations to find
holes in the file, which mark the positions of these holes, along
with the number of bytes of data to be found after the hole.

'GNUTYPE_VOLHDR'
''V''
This file type is used to mark the volume header that was given
with the '--label=ARCHIVE-LABEL' ('-V ARCHIVE-LABEL') option when
the archive was created. The 'name' field contains the 'name'
given after the '--label=ARCHIVE-LABEL' ('-V ARCHIVE-LABEL')
option. The 'size' field is zero. Only the first file in each
volume of an archive should have this type.

You may have trouble reading a GNU format archive on a non-GNU system
if the options '--incremental' ('-G'), '--multi-volume' ('-M'),
'--sparse' ('-S'), or '--label=ARCHIVE-LABEL' ('-V ARCHIVE-LABEL') were
used when writing the archive. In general, if 'tar' does not use the
GNU-added fields of the header, other versions of 'tar' should be able
to read the archive. Otherwise, the 'tar' program will give an error,
the most likely one being a checksum error.

Storing Sparse Files
====================

The notion of sparse file, and the ways of handling it from the point of
view of GNU 'tar' user have been described in detail in *note sparse::.
This chapter describes the internal format GNU 'tar' uses to store such
files.

The support for sparse files in GNU 'tar' has a long history. The
earliest version featuring this support that I was able to find was
1.09, released in November, 1990. The format introduced back then is
called "old GNU" sparse format and in spite of the fact that its design
contained many flaws, it was the only format GNU 'tar' supported until
version 1.14 (May, 2004), which introduced initial support for sparse
archives in PAX archives (*note posix::). This format was not free from
design flaws, either and it was subsequently improved in versions 1.15.2
(November, 2005) and 1.15.92 (June, 2006).

In addition to GNU sparse format, GNU 'tar' is able to read and
extract sparse files archived by 'star'.

The following subsections describe each format in detail.

Old GNU Format
--------------

The format introduced in November 1990 (v. 1.09) was designed on top of
standard 'ustar' headers in such an unfortunate way that some of its
fields overwrote fields required by POSIX.

An old GNU sparse header is designated by type 'S' ('GNUTYPE_SPARSE')
and has the following layout:

Offset Size Name Data type Contents
----------------------------------------------------------------------------
0 345 N/A Not used.
345 12 atime Number 'atime' of the file.
357 12 ctime Number 'ctime' of the file .
369 12 offset Number For multivolume archives:
the offset of the start of
this volume.
381 4 N/A Not used.
385 1 N/A Not used.
386 96 sp 'sparse_header'(4 entries) File map.
482 1 isextended Bool '1' if an extension sparse
header follows, '0'
otherwise.
483 12 realsize Number Real size of the file.

Each of 'sparse_header' object at offset 386 describes a single data
chunk. It has the following structure:

Offset Size Data type Contents
---------------------------------------------------------------------------
0 12 Number Offset of the beginning of the chunk.
12 12 Number Size of the chunk.

If the member contains more than four chunks, the 'isextended' field
of the header has the value '1' and the main header is followed by one
or more "extension headers". Each such header has the following
structure:

Offset Size Name Data type Contents
----------------------------------------------------------------------------
0 21 sp 'sparse_header'(21 entries) File map.
504 1 isextended Bool '1' if an extension sparse
header follows, or '0'
otherwise.

A header with 'isextended=0' ends the map.

PAX Format, Versions 0.0 and 0.1
--------------------------------

There are two formats available in this branch. The version '0.0' is
the initial version of sparse format used by 'tar' versions 1.14-1.15.1.
The sparse file map is kept in extended ('x') PAX header variables:

'GNU.sparse.size'
Real size of the stored file;

'GNU.sparse.numblocks'
Number of blocks in the sparse map;

'GNU.sparse.offset'
Offset of the data block;

'GNU.sparse.numbytes'
Size of the data block.

The latter two variables repeat for each data block, so the overall
structure is like this:

GNU.sparse.size=SIZE
GNU.sparse.numblocks=NUMBLOCKS
repeat NUMBLOCKS times
GNU.sparse.offset=OFFSET
GNU.sparse.numbytes=NUMBYTES
end repeat

This format presented the following two problems:

1. Whereas the POSIX specification allows a variable to appear
multiple times in a header, it requires that only the last
occurrence be meaningful. Thus, multiple occurrences of
'GNU.sparse.offset' and 'GNU.sparse.numbytes' are conflicting with
the POSIX specs.

2. Attempting to extract such archives using a third-party's 'tar'
results in extraction of sparse files in _condensed form_. If the
'tar' implementation in question does not support POSIX format, it
will also extract a file containing extension header attributes.
This file can be used to expand the file to its original state.
However, posix-aware 'tar's will usually ignore the unknown
variables, which makes restoring the file more difficult. *Note
Extraction of sparse members in v.0.0 format: extracting sparse
v.0.x, for the detailed description of how to restore such members
using non-GNU 'tar's.

GNU 'tar' 1.15.2 introduced sparse format version '0.1', which
attempted to solve these problems. As its predecessor, this format
stores sparse map in the extended POSIX header. It retains
'GNU.sparse.size' and 'GNU.sparse.numblocks' variables, but instead of
'GNU.sparse.offset'/'GNU.sparse.numbytes' pairs it uses a single
variable:

'GNU.sparse.map'
Map of non-null data chunks. It is a string consisting of
comma-separated values "OFFSET,SIZE[,OFFSET-1,SIZE-1...]"

To address the 2nd problem, the 'name' field in 'ustar' is replaced
with a special name, constructed using the following pattern:

%d/GNUSparseFile.%p/%f

The real name of the sparse file is stored in the variable
'GNU.sparse.name'. Thus, those 'tar' implementations that are not aware
of GNU extensions will at least extract the files into separate
directories, giving the user a possibility to expand it afterwards.
*Note Extraction of sparse members in v.0.1 format: extracting sparse
v.0.x, for the detailed description of how to restore such members using
non-GNU 'tar's.

The resulting 'GNU.sparse.map' string can be _very_ long. Although
POSIX does not impose any limit on the length of a 'x' header variable,
this possibly can confuse some 'tar's.

PAX Format, Version 1.0
-----------------------

The version '1.0' of sparse format was introduced with GNU 'tar'
1.15.92. Its main objective was to make the resulting file extractable
with little effort even by non-posix aware 'tar' implementations.
Starting from this version, the extended header preceding a sparse
member always contains the following variables that identify the format
being used:

'GNU.sparse.major'
Major version

'GNU.sparse.minor'
Minor version

The 'name' field in 'ustar' header contains a special name,
constructed using the following pattern:

%d/GNUSparseFile.%p/%f

The real name of the sparse file is stored in the variable
'GNU.sparse.name'. The real size of the file is stored in the variable
'GNU.sparse.realsize'.

The sparse map itself is stored in the file data block, preceding the
actual file data. It consists of a series of octal numbers of arbitrary
length, delimited by newlines. The map is padded with nulls to the
nearest block boundary.

The first number gives the number of entries in the map. Following
are map entries, each one consisting of two numbers giving the offset
and size of the data block it describes.

The format is designed in such a way that non-posix aware 'tar's and
'tar's not supporting 'GNU.sparse.*' keywords will extract each sparse
file in its condensed form with the file map prepended and will place it
into a separate directory. Then, using a simple program it would be
possible to expand the file to its original form even without GNU 'tar'.
*Note Sparse Recovery::, for the detailed information on how to extract
sparse members without GNU 'tar'.

Format of the Incremental Snapshot Files
========================================

A "snapshot file" (or "directory file") is created during incremental
backups (*note Incremental Dumps::). It contains the status of the file
system at the time of the dump and is used to determine which files were
modified since the last backup.

GNU 'tar' version 1.29 supports three snapshot file formats. The
first format, called "format 0", is the one used by GNU 'tar' versions
up to and including 1.15.1. The second format, called "format 1" is an
extended version of this format, that contains more metadata and allows
for further extensions. It was used by alpha release version 1.15.90.
For alpha version 1.15.91 and stable releases version 1.16 up through
1.29, the "format 2" is used.

GNU 'tar' is able to read all three formats, but will create
snapshots only in format 2.

This appendix describes all three formats in detail.

0. 'Format 0' snapshot file begins with a line containing a decimal
number that represents a UNIX timestamp of the beginning of the
last archivation. This line is followed by directory metadata
descriptions, one per line. Each description has the following
format:

[NFS]DEV INODE NAME

where:

NFS
A single plus character ('+'), if this directory is located on
an NFS-mounted partition, otherwise empty.

(That is, for non-NFS directories, the first character on the
description line contains the start of the DEV field.)

DEV
Device number of the directory;

INODE
I-node number of the directory;

NAME
Name of the directory. Any special characters (white-space,
backslashes, etc.) are quoted.

1. 'Format 1' snapshot file begins with a line specifying the format
of the file. This line has the following structure:

'GNU tar-'TAR-VERSION'-'INCR-FORMAT-VERSION

where TAR-VERSION is the version number of GNU 'tar' implementation
that created this snapshot, and INCR-FORMAT-VERSION is the version
number of the snapshot format (in this case '1').

Next line contains two decimal numbers, representing the time of
the last backup. First number is the number of seconds, the second
one is the number of nanoseconds, since the beginning of the epoch.

Lines that follow contain directory metadata, one line per
directory. Each line is formatted as follows:

[NFS]MTIME-SEC MTIME-NSEC DEV INODE NAME

where MTIME-SEC and MTIME-NSEC represent last modification time of
this directory with nanosecond precision; NFS, DEV, INODE and NAME
have the same meaning as with 'format 0'.

2. 'Format 2' snapshot file begins with a format identifier, as
described for version 1, e.g.:

GNU tar-1.29-2

This line is followed by newline. Rest of file consists of
records, separated by null (ASCII 0) characters. Thus, in contrast
to the previous formats, format 2 snapshot is a binary file.

First two records are decimal integers, representing the time of
the last backup. First number is the number of seconds, the second
one is the number of nanoseconds, since the beginning of the epoch.
These are followed by arbitrary number of directory records.

Each "directory record" contains a set of metadata describing a
particular directory. Parts of a directory record are delimited
with ASCII 0 characters. The following table describes each part.
The "Number" type in this table stands for a decimal integer in
ASCII notation. (Negative values are preceded with a "-"
character, while positive values have no leading punctuation.)

Field Type Description
---------------------------------------------------------------------------
nfs Character '1' if the directory is located on an
NFS-mounted partition, or '0' otherwise;
timestamp_sec Number Modification time, seconds;
timestamp_nsec Number Modification time, nanoseconds;
dev Number Device number;
ino Number I-node number;
name String Directory name; in contrast to the
previous versions it is not quoted;
contents Dumpdir Contents of the directory; *Note
Dumpdir::, for a description of its
format.

Dumpdirs stored in snapshot files contain only records of types
'Y', 'N' and 'D'.

The specific range of values allowed in each of the "Number" fields
depends on the underlying C datatypes as determined when 'tar' is
compiled. To see the specific ranges allowed for a particular
'tar' binary, you can use the '--show-snapshot-field-ranges'
option:

$ tar --show-shapshot-field-ranges
This tar's snapshot file field ranges are
(field name => [ min, max ]):

nfs => [ 0, 1 ],
timestamp_sec => [ -9223372036854775808, 9223372036854775807 ],
timestamp_nsec => [ 0, 999999999 ],
dev => [ 0, 18446744073709551615 ],
ino => [ 0, 18446744073709551615 ],

(This example is from a GNU/Linux x86_64 system.)

Dumpdir
=======

Incremental archives keep information about contents of each dumped
directory in special data blocks called "dumpdirs".

Dumpdir is a sequence of entries of the following form:

C FILENAME \0

where C is one of the "control codes" described below, FILENAME is the
name of the file C operates upon, and '\0' represents a nul character
(ASCII 0). The white space characters were added for readability, real
dumpdirs do not contain them.

Each dumpdir ends with a single nul character.

The following table describes control codes and their meanings:

'Y'
FILENAME is contained in the archive.

'N'
FILENAME was present in the directory at the time the archive was
made, yet it was not dumped to the archive, because it had not
changed since the last backup.

'D'
FILENAME is a directory.

'R'
This code requests renaming of the FILENAME to the name specified
with the 'T' command, that immediately follows it.

'T'
Specify target file name for 'R' command (see below).

'X'
Specify "temporary directory" name for a rename operation (see
below).

Codes 'Y', 'N' and 'D' require FILENAME argument to be a relative
file name to the directory this dumpdir describes, whereas codes 'R',
'T' and 'X' require their argument to be an absolute file name.

The three codes 'R', 'T' and 'X' specify a "renaming operation". In
the simplest case it is:

Rsource\0Tdest\0

which means "rename file 'source' to file 'dest'".

However, there are cases that require using a "temporary directory".
For example, consider the following scenario:

1. Previous run dumped a directory 'foo' which contained the following
three directories:

a
b
c

2. They were renamed _cyclically_, so that:

a became b
b became c
c became a

3. New incremental dump was made.

This case cannot be handled by three successive renames, since
renaming 'a' to 'b' will destroy the existing directory. To correctly
process it, GNU 'tar' needs a temporary directory, so it creates the
following dumpdir (newlines have been added for readability):

Xfoo\0
Rfoo/a\0T\0
Rfoo/b\0Tfoo/c\0
Rfoo/c\0Tfoo/a\0
R\0Tfoo/a\0

The first command, 'Xfoo\0', instructs the extractor to create a
temporary directory in the directory 'foo'. Second command,
'Rfoo/aT\0', says "rename file 'foo/a' to the temporary directory that
has just been created" (empty file name after a command means use
temporary directory). Third and fourth commands work as usual, and,
finally, the last command, 'R\0Tfoo/a\0' tells tar to rename the
temporary directory to 'foo/a'.

The exact placement of a dumpdir in the archive depends on the
archive format (*note Formats::):

* PAX archives

In PAX archives, dumpdir is stored in the extended header of the
corresponding directory, in variable 'GNU.dumpdir'.

* GNU and old GNU archives

These formats implement special header type 'D', which is similar
to ustar header '5' (directory), except that it precedes a data
block containing the dumpdir.

Appendix E Genfile
******************

This appendix describes 'genfile', an auxiliary program used in the GNU
tar testsuite. If you are not interested in developing GNU tar, skip
this appendix.

Initially, 'genfile' was used to generate data files for the
testsuite, hence its name. However, new operation modes were being
implemented as the testsuite grew more sophisticated, and now 'genfile'
is a multi-purpose instrument.

There are three basic operation modes:

File Generation
This is the default mode. In this mode, 'genfile' generates data
files.

File Status
In this mode 'genfile' displays status of specified files.

Synchronous Execution.
In this mode 'genfile' executes the given program with
'--checkpoint' option and executes a set of actions when specified
checkpoints are reached.

E.1 Generate Mode
=================

In this mode 'genfile' creates a data file for the test suite. The size
of the file is given with the '--length' ('-l') option. By default the
file contents is written to the standard output, this can be changed
using '--file' ('-f') command line option. Thus, the following two
commands are equivalent:

genfile --length 100 > outfile
genfile --length 100 --file outfile

If '--length' is not given, 'genfile' will generate an empty
(zero-length) file.

The command line option '--seek=N' istructs 'genfile' to skip the
given number of bytes (N) in the output file before writing to it. It
is similar to the 'seek=N' of the 'dd' utility.

You can instruct 'genfile' to create several files at one go, by
giving it '--files-from' ('-T') option followed by a name of file
containing a list of file names. Using dash ('-') instead of the file
name causes 'genfile' to read file list from the standard input. For
example:

# Read file names from file file.list
genfile --files-from file.list
# Read file names from standard input
genfile --files-from -

The list file is supposed to contain one file name per line. To use
file lists separated by ASCII NUL character, use '--null' ('-0') command
line option:

genfile --null --files-from file.list

The default data pattern for filling the generated file consists of
first 256 letters of ASCII code, repeated enough times to fill the
entire file. This behavior can be changed with '--pattern' option.
This option takes a mandatory argument, specifying pattern name to use.
Currently two patterns are implemented:

'--pattern=default'
The default pattern as described above.

'--pattern=zero'
Fills the file with zeroes.

If no file name was given, the program exits with the code '0'.
Otherwise, it exits with '0' only if it was able to create a file of the
specified length.

Special option '--sparse' ('-s') instructs 'genfile' to create a
sparse file. Sparse files consist of "data fragments", separated by
"holes" or blocks of zeros. On many operating systems, actual disk
storage is not allocated for holes, but they are counted in the length
of the file. To create a sparse file, 'genfile' should know where to
put data fragments, and what data to use to fill them. So, when
'--sparse' is given the rest of the command line specifies a so-called
"file map".

The file map consists of any number of "fragment descriptors". Each
descriptor is composed of two values: a number, specifying fragment
offset from the end of the previous fragment or, for the very first
fragment, from the beginning of the file, and "contents string", that
specifies the pattern to fill the fragment with. File offset can be
suffixed with the following quantifiers:

'k'
'K'
The number is expressed in kilobytes.
'm'
'M'
The number is expressed in megabytes.
'g'
'G'
The number is expressed in gigabytes.

Contents string can be either a fragment size or a pattern. Fragment
size is a decimal number, prefixed with an equals sign. It can be
suffixed with a quantifier, as discussed above. If fragment size is
given, the fragment of that size will be filled with the currently
selected pattern (*note -pattern: Generate Mode.) and written to the
file.

A pattern is a string of arbitrary ASCII characters. For each of
them, 'genfile' will generate a "block" of data, filled with that
character and will write it to the fragment. The size of block is given
by '--block-size' option. It defaults to 512. Thus, if pattern
consists of N characters, the resulting file fragment will contain
'N*BLOCK-SIZE' bytes of data.

The last fragment descriptor can have only file offset part. In this
case 'genfile' will create a hole at the end of the file up to the given
offset.

A dash appearing as a fragment descriptor instructs 'genfile' to read
file map from the standard input. Each line of input should consist of
fragment offset and contents string, separated by any amount of
whitespace.

For example, consider the following invocation:

genfile --sparse --file sparsefile 0 ABCD 1M EFGHI 2000K

It will create 3101184-bytes long file of the following structure:

Offset Length Contents
0 4*512=2048 Four 512-byte blocks, filled
with letters 'A', 'B', 'C' and
'D'.
2048 1046528 Zero bytes
1050624 5*512=2560 Five blocks, filled with
letters 'E', 'F', 'G', 'H',
'I'.
1053184 2048000 Zero bytes

The exit code of 'genfile --sparse' command is '0' only if created
file is actually sparse. If it is not, the appropriate error message is
displayed and the command exists with code '1'. The '--quite' ('-q')
option suppresses this behavior. If '--quite' is given, 'genfile
--sparse' exits with code '0' if it was able to create the file, whether
the resulting file is sparse or not.

E.2 Status Mode
===============

In status mode, 'genfile' prints file system status for each file
specified in the command line. This mode is toggled by '--stat' ('-S')
command line option. An optional argument to this option specifies
output "format": a comma-separated list of 'struct stat' fields to be
displayed. This list can contain following identifiers:

name
The file name.

dev
st_dev
Device number in decimal.

ino
st_ino
Inode number.

mode[.NUMBER]
st_mode[.NUMBER]

File mode in octal. Optional NUMBER specifies octal mask to be
applied to the mode before outputting. For example, '--stat
mode.777' will preserve lower nine bits of it. Notice, that you
can use any punctuation character in place of '.'.

nlink
st_nlink
Number of hard links.

uid
st_uid
User ID of owner.

gid
st_gid
Group ID of owner.

size
st_size
File size in decimal.

blksize
st_blksize
The size in bytes of each file block.

blocks
st_blocks
Number of blocks allocated.

atime
st_atime
Time of last access.

mtime
st_mtime
Time of last modification

ctime
st_ctime
Time of last status change

sparse
A boolean value indicating whether the file is 'sparse'.

Modification times are displayed in UTC as UNIX timestamps, unless
suffixed with 'H' (for "human-readable"), as in 'ctimeH', in which case
usual 'tar tv' output format is used.

The default output format is: 'name,dev,ino,mode,
nlink,uid,gid,size,blksize,blocks,atime,mtime,ctime'.

For example, the following command will display file names and
corresponding times of last access for each file in the current working
directory:

genfile --stat=name,atime *

E.3 Exec Mode
=============

This mode is designed for testing the behavior of 'paxutils' commands
when some of the files change during archiving. It is an experimental
mode.

The 'Exec Mode' is toggled by '--run' command line option (or its
alias '-r'). The non-optional arguments to 'getopt' give the command
line to be executed. Normally, it should contain at least the
'--checkpoint' option.

A set of options is provided for defining checkpoint values and
actions to be executed upon reaching them. Checkpoint values are
introduced with the '--checkpoint' command line option. Argument to
this option is the number of checkpoint in decimal.

Any number of "actions" may be specified after a checkpoint.
Available actions are

'--cut FILE'
'--truncate FILE'
Truncate FILE to the size specified by previous '--length' option
(or 0, if it is not given).

'--append FILE'
Append data to FILE. The size of data and its pattern are given by
previous '--length' and 'pattern' options.

'--touch FILE'
Update the access and modification times of FILE. These timestamps
are changed to the current time, unless '--date' option was given,
in which case they are changed to the specified time. Argument to
'--date' option is a date specification in an almost arbitrary
format (*note Date input formats::).

'--exec COMMAND'
Execute given shell command.

'--unlink FILE'
Unlink the FILE.

Option '--verbose' instructs 'genfile' to print on standard output
notifications about checkpoints being executed and to verbosely describe
exit status of the command.

While the command is being executed its standard output remains
connected to descriptor 1. All messages it prints to file descriptor 2,
except checkpoint notifications, are forwarded to standard error.

'Genfile' exits with the exit status of the executed command.

For compatibility with previous 'genfile' versions, the '--run'
option takes an optional argument. If used this way, its argument
supplies the command line to be executed. There should be no
non-optional arguments in the 'genfile' command line.

The actual command line is constructed by inserting the
'--checkpoint' option between the command name and its first argument
(if any). Due to this, the argument to '--run' may not use traditional
'tar' option syntax, i.e., the following is wrong:

# Wrong!
genfile --run='tar cf foo bar'

Use the following syntax instead:

genfile --run='tar -cf foo bar' ACTIONS...

The above command line is equivalent to

genfile ACTIONS... -- tar -cf foo bar

Notice, that the use of compatibility mode is deprecated.

Appendix F Free Software Needs Free Documentation
*************************************************

The biggest deficiency in the free software community today is not in
the software--it is the lack of good free documentation that we can
include with the free software. Many of our most important programs do
not come with free reference manuals and free introductory texts.
Documentation is an essential part of any software package; when an
important free software package does not come with a free manual and a
free tutorial, that is a major gap. We have many such gaps today.

Consider Perl, for instance. The tutorial manuals that people
normally use are non-free. How did this come about? Because the
authors of those manuals published them with restrictive terms--no
copying, no modification, source files not available--which exclude them
from the free software world.

That wasn't the first time this sort of thing happened, and it was
far from the last. Many times we have heard a GNU user eagerly describe
a manual that he is writing, his intended contribution to the community,
only to learn that he had ruined everything by signing a publication
contract to make it non-free.

Free documentation, like free software, is a matter of freedom, not
price. The problem with the non-free manual is not that publishers
charge a price for printed copies--that in itself is fine. (The Free
Software Foundation sells printed copies of manuals, too.) The problem
is the restrictions on the use of the manual. Free manuals are
available in source code form, and give you permission to copy and
modify. Non-free manuals do not allow this.

The criteria of freedom for a free manual are roughly the same as for
free software. Redistribution (including the normal kinds of commercial
redistribution) must be permitted, so that the manual can accompany
every copy of the program, both on-line and on paper.

Permission for modification of the technical content is crucial too.
When people modify the software, adding or changing features, if they
are conscientious they will change the manual too--so they can provide
accurate and clear documentation for the modified program. A manual
that leaves you no choice but to write a new manual to document a
changed version of the program is not really available to our community.

Some kinds of limits on the way modification is handled are
acceptable. For example, requirements to preserve the original author's
copyright notice, the distribution terms, or the list of authors, are
ok. It is also no problem to require modified versions to include
notice that they were modified. Even entire sections that may not be
deleted or changed are acceptable, as long as they deal with
nontechnical topics (like this one). These kinds of restrictions are
acceptable because they don't obstruct the community's normal use of the
manual.

However, it must be possible to modify all the _technical_ content of
the manual, and then distribute the result in all the usual media,
through all the usual channels. Otherwise, the restrictions obstruct
the use of the manual, it is not free, and we need another manual to
replace it.

Please spread the word about this issue. Our community continues to
lose manuals to proprietary publishing. If we spread the word that free
software needs free reference manuals and free tutorials, perhaps the
next person who wants to contribute by writing documentation will
realize, before it is too late, that only free manuals contribute to the
free software community.

If you are writing documentation, please insist on publishing it
under the GNU Free Documentation License or another free documentation
license. Remember that this decision requires your approval--you don't
have to let the publisher decide. Some commercial publishers will use a
free license if you insist, but they will not propose the option; it is
up to you to raise the issue and say firmly that this is what you want.
If the publisher you are dealing with refuses, please try other
publishers. If you're not sure whether a proposed license is free,
write to <licensing@gnu.org>.

You can encourage commercial publishers to sell more free, copylefted
manuals and tutorials by buying them, and particularly by buying copies
from the publishers that paid for their writing or for major
improvements. Meanwhile, try to avoid buying non-free documentation at
all. Check the distribution terms of a manual before you buy it, and
insist that whoever seeks your business must respect your freedom.
Check the history of the book, and try reward the publishers that have
paid or pay the authors to work on it.

The Free Software Foundation maintains a list of free documentation
published by other publishers, at
<http://www.fsf.org/doc/other-free-books.html>.

Appendix G GNU Free Documentation License
*****************************************

Version 1.3, 3 November 2008

Copyright (C) 2000-2002, 2007-2008, 2014, 2016 Free Software
Foundation, Inc.
<http://fsf.org/>

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

0. PREAMBLE

The purpose of this License is to make a manual, textbook, or other
functional and useful document "free" in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or
noncommercially. Secondarily, this License preserves for the
author and publisher a way to get credit for their work, while not
being considered responsible for modifications made by others.

This License is a kind of "copyleft", which means that derivative
works of the document must themselves be free in the same sense.
It complements the GNU General Public License, which is a copyleft
license designed for free software.

We have designed this License in order to use it for manuals for
free software, because free software needs free documentation: a
free program should come with manuals providing the same freedoms
that the software does. But this License is not limited to
software manuals; it can be used for any textual work, regardless
of subject matter or whether it is published as a printed book. We
recommend this License principally for works whose purpose is
instruction or reference.

1. APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium,
that contains a notice placed by the copyright holder saying it can
be distributed under the terms of this License. Such a notice
grants a world-wide, royalty-free license, unlimited in duration,
to use that work under the conditions stated herein. The
"Document", below, refers to any such manual or work. Any member
of the public is a licensee, and is addressed as "you". You accept
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The "Invariant Sections" are certain Secondary Sections whose
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listed, as Front-Cover Texts or Back-Cover Texts, in the notice
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plus such following pages as are needed to hold, legibly, the
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To "Preserve the Title" of such a section when you modify the
Document means that it remains a section "Entitled XYZ" according
to this definition.

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which states that this License applies to the Document. These
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this License, but only as regards disclaiming warranties: any other
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has no effect on the meaning of this License.

2. VERBATIM COPYING

You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License
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add no other conditions whatsoever to those of this License. You
may not use technical measures to obstruct or control the reading
or further copying of the copies you make or distribute. However,
you may accept compensation in exchange for copies. If you
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You may also lend copies, under the same conditions stated above,
and you may publicly display copies.

3. COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly
have printed covers) of the Document, numbering more than 100, and
the Document's license notice requires Cover Texts, you must
enclose the copies in covers that carry, clearly and legibly, all
these Cover Texts: Front-Cover Texts on the front cover, and
Back-Cover Texts on the back cover. Both covers must also clearly
and legibly identify you as the publisher of these copies. The
front cover must present the full title with all words of the title
equally prominent and visible. You may add other material on the
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long as they preserve the title of the Document and satisfy these
conditions, can be treated as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto
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If you publish or distribute Opaque copies of the Document
numbering more than 100, you must either include a machine-readable
Transparent copy along with each Opaque copy, or state in or with
each Opaque copy a computer-network location from which the general
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network protocols a complete Transparent copy of the Document, free
of added material. If you use the latter option, you must take
reasonably prudent steps, when you begin distribution of Opaque
copies in quantity, to ensure that this Transparent copy will
remain thus accessible at the stated location until at least one
year after the last time you distribute an Opaque copy (directly or
through your agents or retailers) of that edition to the public.

It is requested, but not required, that you contact the authors of
the Document well before redistributing any large number of copies,
to give them a chance to provide you with an updated version of the
Document.

4. MODIFICATIONS

You may copy and distribute a Modified Version of the Document
under the conditions of sections 2 and 3 above, provided that you
release the Modified Version under precisely this License, with the
Modified Version filling the role of the Document, thus licensing
distribution and modification of the Modified Version to whoever
possesses a copy of it. In addition, you must do these things in
the Modified Version:

A. Use in the Title Page (and on the covers, if any) a title
distinct from that of the Document, and from those of previous
versions (which should, if there were any, be listed in the
History section of the Document). You may use the same title
as a previous version if the original publisher of that
version gives permission.

B. List on the Title Page, as authors, one or more persons or
entities responsible for authorship of the modifications in
the Modified Version, together with at least five of the
principal authors of the Document (all of its principal
authors, if it has fewer than five), unless they release you
from this requirement.

C. State on the Title page the name of the publisher of the
Modified Version, as the publisher.

D. Preserve all the copyright notices of the Document.

E. Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

F. Include, immediately after the copyright notices, a license
notice giving the public permission to use the Modified
Version under the terms of this License, in the form shown in
the Addendum below.

G. Preserve in that license notice the full lists of Invariant
Sections and required Cover Texts given in the Document's
license notice.

H. Include an unaltered copy of this License.

I. Preserve the section Entitled "History", Preserve its Title,
and add to it an item stating at least the title, year, new
authors, and publisher of the Modified Version as given on the
Title Page. If there is no section Entitled "History" in the
Document, create one stating the title, year, authors, and
publisher of the Document as given on its Title Page, then add
an item describing the Modified Version as stated in the
previous sentence.

J. Preserve the network location, if any, given in the Document
for public access to a Transparent copy of the Document, and
likewise the network locations given in the Document for
previous versions it was based on. These may be placed in the
"History" section. You may omit a network location for a work
that was published at least four years before the Document
itself, or if the original publisher of the version it refers
to gives permission.

K. For any section Entitled "Acknowledgements" or "Dedications",
Preserve the Title of the section, and preserve in the section
all the substance and tone of each of the contributor
acknowledgements and/or dedications given therein.

L. Preserve all the Invariant Sections of the Document, unaltered
in their text and in their titles. Section numbers or the
equivalent are not considered part of the section titles.

M. Delete any section Entitled "Endorsements". Such a section
may not be included in the Modified Version.

N. Do not retitle any existing section to be Entitled
"Endorsements" or to conflict in title with any Invariant
Section.

O. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no
material copied from the Document, you may at your option designate
some or all of these sections as invariant. To do this, add their
titles to the list of Invariant Sections in the Modified Version's
license notice. These titles must be distinct from any other
section titles.

You may add a section Entitled "Endorsements", provided it contains
nothing but endorsements of your Modified Version by various
parties--for example, statements of peer review or that the text
has been approved by an organization as the authoritative
definition of a standard.

You may add a passage of up to five words as a Front-Cover Text,
and a passage of up to 25 words as a Back-Cover Text, to the end of
the list of Cover Texts in the Modified Version. Only one passage
of Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity. If the Document
already includes a cover text for the same cover, previously added
by you or by arrangement made by the same entity you are acting on
behalf of, you may not add another; but you may replace the old
one, on explicit permission from the previous publisher that added
the old one.

The author(s) and publisher(s) of the Document do not by this
License give permission to use their names for publicity for or to
assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

You may combine the Document with other documents released under
this License, under the terms defined in section 4 above for
modified versions, provided that you include in the combination all
of the Invariant Sections of all of the original documents,
unmodified, and list them all as Invariant Sections of your
combined work in its license notice, and that you preserve all
their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name
but different contents, make the title of each such section unique
by adding at the end of it, in parentheses, the name of the
original author or publisher of that section if known, or else a
unique number. Make the same adjustment to the section titles in
the list of Invariant Sections in the license notice of the
combined work.

In the combination, you must combine any sections Entitled
"History" in the various original documents, forming one section
Entitled "History"; likewise combine any sections Entitled
"Acknowledgements", and any sections Entitled "Dedications". You
must delete all sections Entitled "Endorsements."

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other
documents released under this License, and replace the individual
copies of this License in the various documents with a single copy
that is included in the collection, provided that you follow the
rules of this License for verbatim copying of each of the documents
in all other respects.

You may extract a single document from such a collection, and
distribute it individually under this License, provided you insert
a copy of this License into the extracted document, and follow this
License in all other respects regarding verbatim copying of that
document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other
separate and independent documents or works, in or on a volume of a
storage or distribution medium, is called an "aggregate" if the
copyright resulting from the compilation is not used to limit the
legal rights of the compilation's users beyond what the individual
works permit. When the Document is included in an aggregate, this
License does not apply to the other works in the aggregate which
are not themselves derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half
of the entire aggregate, the Document's Cover Texts may be placed
on covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic
form. Otherwise they must appear on printed covers that bracket
the whole aggregate.

8. TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section
4. Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also
include the original English version of this License and the
original versions of those notices and disclaimers. In case of a
disagreement between the translation and the original version of
this License or a notice or disclaimer, the original version will
prevail.

If a section in the Document is Entitled "Acknowledgements",
"Dedications", or "History", the requirement (section 4) to
Preserve its Title (section 1) will typically require changing the
actual title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense, or distribute it is void,
and will automatically terminate your rights under this License.

However, if you cease all violation of this License, then your
license from a particular copyright holder is reinstated (a)
provisionally, unless and until the copyright holder explicitly and
finally terminates your license, and (b) permanently, if the
copyright holder fails to notify you of the violation by some
reasonable means prior to 60 days after the cessation.

Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from
that copyright holder, and you cure the violation prior to 30 days
after your receipt of the notice.

Termination of your rights under this section does not terminate
the licenses of parties who have received copies or rights from you
under this License. If your rights have been terminated and not
permanently reinstated, receipt of a copy of some or all of the
same material does not give you any rights to use it.

10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of
the GNU Free Documentation License from time to time. Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns. See
<http://www.gnu.org/copyleft/>.

Each version of the License is given a distinguishing version
number. If the Document specifies that a particular numbered
version of this License "or any later version" applies to it, you
have the option of following the terms and conditions either of
that specified version or of any later version that has been
published (not as a draft) by the Free Software Foundation. If the
Document does not specify a version number of this License, you may
choose any version ever published (not as a draft) by the Free
Software Foundation. If the Document specifies that a proxy can
decide which future versions of this License can be used, that
proxy's public statement of acceptance of a version permanently
authorizes you to choose that version for the Document.

11. RELICENSING

"Massive Multiauthor Collaboration Site" (or "MMC Site") means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A
public wiki that anybody can edit is an example of such a server.
A "Massive Multiauthor Collaboration" (or "MMC") contained in the
site means any set of copyrightable works thus published on the MMC
site.

"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.

"Incorporate" means to publish or republish a Document, in whole or
in part, as part of another Document.

An MMC is "eligible for relicensing" if it is licensed under this
License, and if all works that were first published under this
License somewhere other than this MMC, and subsequently
incorporated in whole or in part into the MMC, (1) had no cover
texts or invariant sections, and (2) were thus incorporated prior
to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the
site under CC-BY-SA on the same site at any time before August 1,
2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

Copyright (C) YEAR YOUR NAME.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

with the Invariant Sections being LIST THEIR TITLES, with
the Front-Cover Texts being LIST, and with the Back-Cover Texts
being LIST.

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.

Appendix H Index of Command Line Options
****************************************

This appendix contains an index of all GNU 'tar' long command line
options. The options are listed without the preceding double-dash. For
a cross-reference of short command line options, see *note Short Option
Summary::.

* Menu:

* absolute-names: absolute. (line 7130)
* absolute-names, summary: Option Summary. (line 1619)
* acls, summary: Option Summary. (line 1627)
* add-file: files. (line 5950)
* after-date: after. (line 6903)
* after-date, summary: Option Summary. (line 1630)
* anchored: controlling pattern-matching.
(line 6406)
* anchored, summary: Option Summary. (line 1634)
* append: append. (line 3703)
* append <1>: appending files. (line 3762)
* append, summary: Operation Summary. (line 1555)
* atime-preserve: Attributes. (line 8083)
* atime-preserve, summary: Option Summary. (line 1638)
* auto-compress: gzip. (line 7883)
* auto-compress, summary: Option Summary. (line 1684)
* backup: backup. (line 4889)
* backup, summary: Option Summary. (line 1691)
* block-number: verbose. (line 3167)
* block-number, summary: Option Summary. (line 1697)
* blocking-factor: Blocking Factor. (line 9341)
* blocking-factor, summary: Option Summary. (line 1704)
* bzip2, summary: Option Summary. (line 1710)
* catenate: concatenate. (line 3909)
* catenate, summary: Operation Summary. (line 1560)
* check-device, described: Incremental Dumps. (line 5137)
* check-device, summary: Option Summary. (line 1716)
* check-links, described: hard links. (line 8275)
* check-links, summary: Option Summary. (line 1768)
* checkpoint: checkpoints. (line 3192)
* checkpoint, defined: checkpoints. (line 3199)
* checkpoint, summary: Option Summary. (line 1721)
* checkpoint-action: checkpoints. (line 3192)
* checkpoint-action, defined: checkpoints. (line 3208)
* checkpoint-action, summary: Option Summary. (line 1730)
* clamp-mtime, summary: Option Summary. (line 1785)
* compare: compare. (line 4030)
* compare, summary: Operation Summary. (line 1565)
* compress: gzip. (line 7846)
* compress, summary: Option Summary. (line 1777)
* concatenate: concatenate. (line 3909)
* concatenate, summary: Operation Summary. (line 1572)
* confirmation, summary: Option Summary. (line 1789)
* create, additional options: create options. (line 4062)
* create, complementary notes: Basic tar. (line 3576)
* create, introduced: Creating the archive.
(line 593)
* create, summary: Operation Summary. (line 1578)
* create, using with '--verbose': create verbose. (line 669)
* create, using with '--verify': verify. (line 10174)
* delay-directory-restore: Directory Modification Times and Permissions.
(line 4638)
* delay-directory-restore, summary: Option Summary. (line 1793)
* delete: delete. (line 3989)
* delete, summary: Operation Summary. (line 1583)
* delete, using before -append: append. (line 3744)
* dereference: dereference. (line 8228)
* dereference, summary: Option Summary. (line 1799)
* diff, summary: Operation Summary. (line 1588)
* directory: directory. (line 7055)
* directory, summary: Option Summary. (line 1806)
* exclude: exclude. (line 6025)
* exclude <1>: exclude. (line 6028)
* exclude, potential problems with: problems with exclude.
(line 6242)
* exclude, summary: Option Summary. (line 1814)
* exclude-backups: exclude. (line 6133)
* exclude-backups, summary: Option Summary. (line 1819)
* exclude-caches: exclude. (line 6153)
* exclude-caches, summary: Option Summary. (line 1828)
* exclude-caches-all: exclude. (line 6161)
* exclude-caches-all, summary: Option Summary. (line 1843)
* exclude-caches-under: exclude. (line 6157)
* exclude-caches-under, summary: Option Summary. (line 1836)
* exclude-from: exclude. (line 6025)
* exclude-from <1>: exclude. (line 6039)
* exclude-from, summary: Option Summary. (line 1822)
* exclude-ignore: exclude. (line 6095)
* exclude-ignore, summary: Option Summary. (line 1848)
* exclude-ignore-recursive: exclude. (line 6100)
* exclude-ignore-recursive, summary: Option Summary. (line 1853)
* exclude-tag: exclude. (line 6170)
* exclude-tag, summary: Option Summary. (line 1858)
* exclude-tag-all: exclude. (line 6178)
* exclude-tag-all, summary: Option Summary. (line 1870)
* exclude-tag-under: exclude. (line 6174)
* exclude-tag-under, summary: Option Summary. (line 1864)
* exclude-vcs: exclude. (line 6104)
* exclude-vcs, summary: Option Summary. (line 1875)
* exclude-vcs-ignores: exclude. (line 6061)
* exclude-vcs-ignores, summary: Option Summary. (line 1882)
* extract: extract. (line 941)
* extract, additional options: extract options. (line 4346)
* extract, complementary notes: Basic tar. (line 3614)
* extract, summary: Operation Summary. (line 1593)
* extract, using with '--listed-incremental': Incremental Dumps.
(line 5150)
* file: file. (line 5724)
* file, short description: file. (line 5733)
* file, summary: Option Summary. (line 1890)
* file, tutorial: file tutorial. (line 375)
* files-from: files. (line 5885)
* files-from, summary: Option Summary. (line 1897)
* force-local, short description: Device. (line 9063)
* force-local, summary: Option Summary. (line 1904)
* format, summary: Option Summary. (line 1910)
* full-time, summary: Option Summary. (line 1935)
* get, summary: Operation Summary. (line 1599)
* group: override. (line 4162)
* group, summary: Option Summary. (line 1953)
* group-map, summary: Option Summary. (line 1963)
* gunzip, summary: Option Summary. (line 1973)
* gzip: gzip. (line 7824)
* gzip, summary: Option Summary. (line 1973)
* hard-dereference, described: hard links. (line 8303)
* hard-dereference, summary: Option Summary. (line 1982)
* help: help tutorial. (line 528)
* help, introduction: help. (line 2978)
* help, summary: Option Summary. (line 1988)
* hole-detection: sparse. (line 8049)
* hole-detection, summary: Option Summary. (line 1994)
* ignore-case: controlling pattern-matching.
(line 6413)
* ignore-case, summary: Option Summary. (line 1999)
* ignore-command-error: Writing to an External Program.
(line 4792)
* ignore-command-error, summary: Option Summary. (line 2003)
* ignore-failed-read: Ignore Failed Read.
(line 4341)
* ignore-failed-read, summary: Option Summary. (line 2007)
* ignore-zeros: Ignore Zeros. (line 4392)
* ignore-zeros, short description: Blocking Factor. (line 9485)
* ignore-zeros, summary: Option Summary. (line 2012)
* incremental, summary: Option Summary. (line 2018)
* incremental, using with '--list': Incremental Dumps. (line 5215)
* index-file, summary: Option Summary. (line 2026)
* info-script: Multi-Volume Archives.
(line 9840)
* info-script, short description: Device. (line 9114)
* info-script, summary: Option Summary. (line 2030)
* interactive: interactive. (line 3508)
* interactive, summary: Option Summary. (line 2039)
* keep-directory-symlink, summary: Option Summary. (line 2047)
* keep-newer-files: Keep Newer Files. (line 4520)
* keep-newer-files, summary: Option Summary. (line 2061)
* keep-old-files: Keep Old Files. (line 4502)
* keep-old-files, introduced: Dealing with Old Files.
(line 4428)
* keep-old-files, summary: Option Summary. (line 2066)
* label: Tape Files. (line 9984)
* label <1>: label. (line 10035)
* label, summary: Option Summary. (line 2075)
* level, described: Incremental Dumps. (line 5105)
* level, summary: Option Summary. (line 2083)
* list: list. (line 807)
* list, summary: Operation Summary. (line 1604)
* list, using with '--incremental': Incremental Dumps. (line 5215)
* list, using with '--listed-incremental': Incremental Dumps.
(line 5215)
* list, using with '--verbose': list. (line 835)
* list, using with file name arguments: list. (line 826)
* listed-incremental, described: Incremental Dumps. (line 5044)
* listed-incremental, summary: Option Summary. (line 2093)
* listed-incremental, using with '--extract': Incremental Dumps.
(line 5150)
* listed-incremental, using with '--list': Incremental Dumps.
(line 5215)
* lzip: gzip. (line 7837)
* lzip, summary: Option Summary. (line 2102)
* lzma: gzip. (line 7840)
* lzma, summary: Option Summary. (line 2107)
* lzop: gzip. (line 7843)
* mode: override. (line 4077)
* mode, summary: Option Summary. (line 2117)
* mtime: override. (line 4093)
* mtime, summary: Option Summary. (line 2124)
* multi-volume: Multi-Volume Archives.
(line 9763)
* multi-volume, short description: Device. (line 9081)
* multi-volume, summary: Option Summary. (line 2139)
* new-volume-script: Multi-Volume Archives.
(line 9840)
* new-volume-script, short description: Device. (line 9114)
* new-volume-script, summary: Option Summary. (line 2030)
* new-volume-script, summary <1>: Option Summary. (line 2145)
* newer: after. (line 6903)
* newer, summary: Option Summary. (line 2149)
* newer-mtime: after. (line 6914)
* newer-mtime, summary: Option Summary. (line 2158)
* no-acls, summary: Option Summary. (line 2164)
* no-anchored: controlling pattern-matching.
(line 6406)
* no-anchored, summary: Option Summary. (line 2168)
* no-auto-compress, summary: Option Summary. (line 2172)
* no-check-device, described: Incremental Dumps. (line 5133)
* no-check-device, summary: Option Summary. (line 2177)
* no-delay-directory-restore: Directory Modification Times and Permissions.
(line 4644)
* no-delay-directory-restore, summary: Option Summary. (line 2182)
* no-ignore-case: controlling pattern-matching.
(line 6413)
* no-ignore-case, summary: Option Summary. (line 2188)
* no-ignore-command-error: Writing to an External Program.
(line 4797)
* no-ignore-command-error, summary: Option Summary. (line 2191)
* no-null, described: nul. (line 5978)
* no-null, summary: Option Summary. (line 2195)
* no-overwrite-dir, summary: Option Summary. (line 2201)
* no-quote-chars, summary: Option Summary. (line 2206)
* no-recursion: recurse. (line 6967)
* no-recursion, summary: Option Summary. (line 2211)
* no-same-owner: Attributes. (line 8136)
* no-same-owner, summary: Option Summary. (line 2216)
* no-same-permissions, summary: Option Summary. (line 2223)
* no-seek, summary: Option Summary. (line 2229)
* no-selinux, summary: Option Summary. (line 2235)
* no-unquote: Selecting Archive Members.
(line 5844)
* no-unquote, summary: Option Summary. (line 2239)
* no-verbatim-files-from: files. (line 5946)
* no-verbatim-files-from, summary: Option Summary. (line 2243)
* no-wildcards: controlling pattern-matching.
(line 6369)
* no-wildcards, summary: Option Summary. (line 2257)
* no-wildcards-match-slash: controlling pattern-matching.
(line 6419)
* no-wildcards-match-slash, summary: Option Summary. (line 2260)
* no-xattrs, summary: Option Summary. (line 2263)
* null, described: nul. (line 5974)
* null, summary: Option Summary. (line 2267)
* numeric-owner: Attributes. (line 8142)
* numeric-owner, summary: Option Summary. (line 2280)
* occurrence, described: append. (line 3731)
* occurrence, summary: Option Summary. (line 2298)
* old-archive, summary: Option Summary. (line 2313)
* one-file-system: one. (line 7031)
* one-file-system, summary: Option Summary. (line 2316)
* one-top-level, summary: Option Summary. (line 2321)
* overwrite: Overwrite Old Files.
(line 4470)
* overwrite, introduced: Dealing with Old Files.
(line 4444)
* overwrite, summary: Option Summary. (line 2332)
* overwrite-dir: Overwrite Old Files.
(line 4492)
* overwrite-dir, introduced: Dealing with Old Files.
(line 4418)
* overwrite-dir, summary: Option Summary. (line 2337)
* owner: override. (line 4130)
* owner, summary: Option Summary. (line 2342)
* owner-map, summary: Option Summary. (line 2352)
* pax-option: PAX keywords. (line 8383)
* pax-option, summary: Option Summary. (line 2362)
* portability, summary: Option Summary. (line 2368)
* posix, summary: Option Summary. (line 2372)
* preserve-order: Same Order. (line 4834)
* preserve-order, summary: Option Summary. (line 2375)
* preserve-permissions: Setting Access Permissions.
(line 4572)
* preserve-permissions, short description: Attributes. (line 8182)
* preserve-permissions, summary: Option Summary. (line 2379)
* quote-chars, summary: Option Summary. (line 2390)
* quoting-style: quoting styles. (line 6481)
* quoting-style, summary: Option Summary. (line 2394)
* read-full-records: Reading. (line 4357)
* read-full-records <1>: read full records. (line 4383)
* read-full-records, short description: Blocking Factor. (line 9501)
* read-full-records, summary: Option Summary. (line 2401)
* record-size, summary: Option Summary. (line 2407)
* recursion: recurse. (line 6978)
* recursion, summary: Option Summary. (line 2415)
* recursive-unlink: Recursive Unlink. (line 4537)
* recursive-unlink, summary: Option Summary. (line 2420)
* remove-files: remove files. (line 4806)
* remove-files, summary: Option Summary. (line 2425)
* restrict, summary: Option Summary. (line 2430)
* rmt-command, summary: Option Summary. (line 2436)
* rsh-command: Device. (line 9066)
* rsh-command, summary: Option Summary. (line 2441)
* same-order: Same Order. (line 4834)
* same-order, summary: Option Summary. (line 2446)
* same-owner: Attributes. (line 8117)
* same-owner, summary: Option Summary. (line 2455)
* same-permissions: Setting Access Permissions.
(line 4572)
* same-permissions, short description: Attributes. (line 8182)
* same-permissions, summary: Option Summary. (line 2379)
* same-permissions, summary <1>: Option Summary. (line 2462)
* seek, summary: Option Summary. (line 2466)
* selinux, summary: Option Summary. (line 2476)
* show-defaults: defaults. (line 3039)
* show-defaults, summary: Option Summary. (line 2480)
* show-omitted-dirs: verbose. (line 3160)
* show-omitted-dirs, summary: Option Summary. (line 2493)
* show-snapshot-field-ranges: Snapshot Files. (line 11596)
* show-snapshot-field-ranges, summary: Option Summary. (line 2498)
* show-stored-names: list. (line 869)
* show-stored-names, summary: Option Summary. (line 2504)
* show-transformed-names: transform. (line 6698)
* show-transformed-names, summary: Option Summary. (line 2504)
* skip-old-files, introduced: Dealing with Old Files.
(line 4440)
* skip-old-files, summary: Option Summary. (line 2513)
* sort, summary: Option Summary. (line 2526)
* sparse: sparse. (line 8007)
* sparse, summary: Option Summary. (line 2543)
* sparse-version: sparse. (line 8042)
* sparse-version, summary: Option Summary. (line 2549)
* starting-file: Starting File. (line 4817)
* starting-file, summary: Option Summary. (line 2555)
* strip-components: transform. (line 6678)
* strip-components, summary: Option Summary. (line 2562)
* suffix: backup. (line 4915)
* suffix, summary: Option Summary. (line 2573)
* tape-length: Multi-Volume Archives.
(line 9790)
* tape-length, short description: Device. (line 9089)
* tape-length, summary: Option Summary. (line 2578)
* test-label: label. (line 10064)
* test-label, summary: Option Summary. (line 2588)
* to-command: Writing to an External Program.
(line 4690)
* to-command, summary: Option Summary. (line 2593)
* to-stdout: Writing to Standard Output.
(line 4661)
* to-stdout, summary: Option Summary. (line 2598)
* totals: verbose. (line 3100)
* totals, summary: Option Summary. (line 2604)
* touch: Data Modification Times.
(line 4559)
* touch <1>: Attributes. (line 8106)
* touch, summary: Option Summary. (line 2610)
* transform: transform. (line 6727)
* transform, summary: Option Summary. (line 2617)
* uncompress: gzip. (line 7846)
* uncompress, summary: Option Summary. (line 1777)
* uncompress, summary <1>: Option Summary. (line 2631)
* ungzip: gzip. (line 7824)
* ungzip, summary: Option Summary. (line 1973)
* ungzip, summary <1>: Option Summary. (line 2635)
* unlink-first: Unlink First. (line 4527)
* unlink-first, introduced: Dealing with Old Files.
(line 4463)
* unlink-first, summary: Option Summary. (line 2639)
* unquote: Selecting Archive Members.
(line 5841)
* unquote, summary: Option Summary. (line 2645)
* update: update. (line 3854)
* update <1>: how to update. (line 3873)
* update, summary: Operation Summary. (line 1609)
* usage: help. (line 3005)
* use-compress-program: gzip. (line 7905)
* use-compress-program, summary: Option Summary. (line 2649)
* utc, summary: Option Summary. (line 2655)
* verbatim-files-from: files. (line 5941)
* verbatim-files-from, summary: Option Summary. (line 2660)
* verbose: verbose. (line 3073)
* verbose, introduced: verbose tutorial. (line 404)
* verbose, summary: Option Summary. (line 2682)
* verbose, using with '--create': create verbose. (line 669)
* verbose, using with '--list': list. (line 835)
* verify, short description: verify. (line 10158)
* verify, summary: Option Summary. (line 2690)
* verify, using with '--create': verify. (line 10174)
* version: help. (line 2958)
* version, summary: Option Summary. (line 2696)
* volno-file: Multi-Volume Archives.
(line 9831)
* volno-file, summary: Option Summary. (line 2702)
* warning, explained: warnings. (line 3403)
* warning, summary: Option Summary. (line 2708)
* wildcards: controlling pattern-matching.
(line 6366)
* wildcards, summary: Option Summary. (line 2714)
* wildcards-match-slash: controlling pattern-matching.
(line 6419)
* wildcards-match-slash, summary: Option Summary. (line 2718)
* xattrs, summary: Option Summary. (line 2721)
* xattrs-exclude, summary: Option Summary. (line 2725)
* xattrs-include, summary: Option Summary. (line 2729)
* xform: transform. (line 6727)
* xform, summary: Option Summary. (line 2617)
* xz: gzip. (line 7829)
* xz, summary: Option Summary. (line 2735)

Appendix I Index
****************

* Menu:

* '%s: Directory has been renamed from %s', warning message: warnings.
(line 3487)
* '%s: Directory has been renamed', warning message: warnings.
(line 3487)
* '%s: Directory is new', warning message: warnings. (line 3489)
* '%s: directory is on a different device: not purging', warning message: warnings.
(line 3491)
* '%s: skipping existing file', warning message: warnings. (line 3453)
* -after-date and -update compared: after. (line 6898)
* -newer-mtime and -update compared: after. (line 6898)
* -quite, option: Generate Mode. (line 11860)
* .bzrignore: exclude. (line 6082)
* .cvsignore: exclude. (line 6069)
* .gitignore: exclude. (line 6074)
* .hgignore: exclude. (line 6089)
* 'A lone zero block at', warning message: warnings. (line 3424)
* abbreviations for months: Calendar date items.
(line 7342)
* absolute file names: absolute. (line 7126)
* absolute file names <1>: Remote Tape Server.
(line 9135)
* Adding archives to an archive: concatenate. (line 3909)
* Adding files to an Archive: appending files. (line 3762)
* ADMINISTRATOR: General-Purpose Variables.
(line 5292)
* Age, excluding files by: after. (line 6885)
* ago in date strings: Relative items in date strings.
(line 7483)
* all: warnings. (line 3419)
* alone-zero-block: warnings. (line 3424)
* alternative decompression programs: gzip. (line 7787)
* am in date strings: Time of day items. (line 7378)
* Appending files to an Archive: appending files. (line 3762)
* appending files to existing archive: append. (line 3703)
* Arch, excluding files: exclude. (line 6104)
* archive: Definitions. (line 76)
* Archive creation: file. (line 5752)
* archive member: Definitions. (line 85)
* Archive Name: file. (line 5724)
* Archive, creation of: create. (line 537)
* Archives, Appending files to: appending files. (line 3762)
* archives, binary equivalent: PAX keywords. (line 8513)
* Archiving Directories: create dir. (line 747)
* archiving files: Top. (line 20)
* ARGP_HELP_FMT, environment variable: Configuring Help Summary.
(line 10565)
* arguments to long options: Long Options. (line 1360)
* arguments to old options: Old Options. (line 1440)
* arguments to short options: Short Options. (line 1383)
* 'Attempting extraction of symbolic links as hard links', warning message: warnings.
(line 3459)
* attributes, files: Attributes. (line 8079)
* authors of 'parse_datetime': Authors of parse_datetime.
(line 7619)
* Avoiding recursion in directories: recurse. (line 6962)
* backup options: backup. (line 4854)
* backup suffix: backup. (line 4915)
* backups: backup. (line 4889)
* backups <1>: Backups. (line 4977)
* BACKUP_DIRS: General-Purpose Variables.
(line 5316)
* BACKUP_FILES: General-Purpose Variables.
(line 5344)
* BACKUP_HOUR: General-Purpose Variables.
(line 5296)
* bad-dumpdir: warnings. (line 3493)
* basic operations: Operations. (line 3656)
* Bazaar, excluding files: exclude. (line 6104)
* Bazaar, ignore files: exclude. (line 6056)
* beginning of time, for POSIX: Seconds since the Epoch.
(line 7554)
* 'bell', checkpoint action: checkpoints. (line 3292)
* Bellovin, Steven M.: Authors of parse_datetime.
(line 7619)
* Berets, Jim: Authors of parse_datetime.
(line 7619)
* Berry, K.: Authors of parse_datetime.
(line 7632)
* binary equivalent archives, creating: PAX keywords. (line 8513)
* block: Blocking. (line 9208)
* Block number where error occurred: verbose. (line 3167)
* BLOCKING: General-Purpose Variables.
(line 5311)
* Blocking Factor: Blocking Factor. (line 9339)
* blocking factor: Blocking Factor. (line 9522)
* Blocks per record: Blocking Factor. (line 9339)
* bug reports: Reports. (line 209)
* Bytes per record: Blocking Factor. (line 9339)
* bzip2: gzip. (line 7739)
* cachedir: warnings. (line 3431)
* calendar date item: Calendar date items.
(line 7310)
* case, ignored in dates: General date syntax.
(line 7297)
* 'cat' vs 'concatenate': concatenate. (line 3966)
* Changing directory mid-stream: directory. (line 7050)
* Character class, excluding characters from: wildcards. (line 6317)
* checkpoints, defined: checkpoints. (line 3192)
* Choosing an archive file: file. (line 5724)
* combined date and time of day item: Combined date and time of day items.
(line 7428)
* comments, in dates: General date syntax.
(line 7297)
* compress: gzip. (line 7739)
* Compressed archives: gzip. (line 7739)
* 'concatenate' vs 'cat': concatenate. (line 3966)
* Concatenating Archives: concatenate. (line 3909)
* 'contains a cache directory tag', warning message: warnings.
(line 3431)
* contiguous-cast: warnings. (line 3457)
* corrupted archives: Full Dumps. (line 5002)
* corrupted archives <1>: gzip. (line 7873)
* Creation of the archive: create. (line 537)
* 'Current %s is newer or same age', warning message: warnings.
(line 3463)
* CVS, excluding files: exclude. (line 6104)
* CVS, ignore files: exclude. (line 6056)
* Darcs, excluding files: exclude. (line 6104)
* DAT blocking: Blocking Factor. (line 9532)
* Data Modification time, excluding files by: after. (line 6885)
* Data modification times of extracted files: Data Modification Times.
(line 4550)
* date and time of day format, ISO 8601: Combined date and time of day items.
(line 7428)
* date format, ISO 8601: Calendar date items.
(line 7334)
* date input formats: Date input formats.
(line 7209)
* day in date strings: Relative items in date strings.
(line 7475)
* day in date strings <1>: Relative items in date strings.
(line 7489)
* day of week item: Day of week items. (line 7447)
* decompress-program: warnings. (line 3467)
* Deleting files from an archive: delete. (line 3989)
* Deleting from tape archives: delete. (line 4000)
* dereferencing hard links: hard links. (line 8250)
* Descending directories, avoiding: recurse. (line 6962)
* Device numbers, changing: Fixing Snapshot Files.
(line 10709)
* Device numbers, using in incremental backups: Incremental Dumps.
(line 5119)
* Directories, Archiving: create dir. (line 747)
* Directories, avoiding recursion: recurse. (line 6962)
* Directory, changing mid-stream: directory. (line 7050)
* DIRLIST: General-Purpose Variables.
(line 5339)
* displacement of dates: Relative items in date strings.
(line 7466)
* doc-opt-col: Configuring Help Summary.
(line 10639)
* 'door ignored', warning message: warnings. (line 3436)
* 'dot', checkpoint action: checkpoints. (line 3316)
* Double-checking a write operation: verify. (line 10156)
* dumps, full: Full Dumps. (line 5002)
* DUMP_BEGIN: User Hooks. (line 5492)
* DUMP_END: User Hooks. (line 5496)
* DUMP_REMIND_SCRIPT: General-Purpose Variables.
(line 5398)
* dup-args: Configuring Help Summary.
(line 10596)
* dup-args-note: Configuring Help Summary.
(line 10613)
* 'echo', checkpoint action: checkpoints. (line 3211)
* Eggert, Paul: Authors of parse_datetime.
(line 7619)
* End-of-archive blocks, ignoring: Ignore Zeros. (line 4392)
* End-of-archive info script: Multi-Volume Archives.
(line 9840)
* entry: Naming tar Archives.
(line 165)
* epoch, for POSIX: Seconds since the Epoch.
(line 7554)
* Error message, block number of: verbose. (line 3177)
* Exabyte blocking: Blocking Factor. (line 9532)
* exclude: exclude. (line 6031)
* exclude-caches: exclude. (line 6141)
* exclude-from: exclude. (line 6044)
* exclude-tag: exclude. (line 6164)
* Excluding characters from a character class: wildcards. (line 6317)
* Excluding file by age: after. (line 6885)
* Excluding files by file system: exclude. (line 6025)
* Excluding files by name and pattern: exclude. (line 6025)
* Exec Mode, 'genfile': Exec Mode. (line 11950)
* 'exec', checkpoint action: checkpoints. (line 3337)
* existing backup method: backup. (line 4907)
* existing-file: warnings. (line 3453)
* exit status: Synopsis. (line 1223)
* 'Extracting contiguous files as regular files', warning message: warnings.
(line 3457)
* extracting Nth copy of the file: append. (line 3731)
* extraction: Definitions. (line 92)
* Extraction: extract. (line 941)
* file archival: Top. (line 20)
* file attributes: Attributes. (line 8079)
* 'file changed as we read it', warning message: warnings. (line 3446)
* 'file is on a different filesystem', warning message: warnings.
(line 3434)
* 'file is the archive; not dumped', warning message: warnings.
(line 3442)
* 'file is the archive; not dumped', warning message <1>: warnings.
(line 3442)
* 'file is unchanged; not dumped', warning message: warnings.
(line 3440)
* File lists separated by NUL characters: Generate Mode. (line 11773)
* file name: Definitions. (line 85)
* File Name arguments, alternatives: files. (line 5877)
* File name arguments, using '--list' with: list. (line 826)
* 'file name read contains nul character', warning message: warnings.
(line 3422)
* file names, absolute: absolute. (line 7126)
* File names, excluding files by: exclude. (line 6025)
* File names, terminated by 'NUL': nul. (line 5969)
* File names, using hard links: hard links. (line 8250)
* File names, using symbolic links: dereference. (line 8228)
* 'File removed before we read it', warning message: warnings.
(line 3444)
* 'File shrank by %s bytes', warning message: warnings. (line 3432)
* File system boundaries, not crossing: one. (line 7023)
* file-changed: warnings. (line 3446)
* file-ignored: warnings. (line 3436)
* file-removed: warnings. (line 3444)
* file-shrank: warnings. (line 3432)
* file-unchanged: warnings. (line 3440)
* FILELIST: General-Purpose Variables.
(line 5355)
* filename-with-nuls: warnings. (line 3422)
* 'find', using with 'tar': files. (line 5877)
* 'find', using with 'tar' <1>: recurse. (line 6967)
* first in date strings: General date syntax.
(line 7259)
* format 0, snapshot file: Snapshot Files. (line 11509)
* format 1, snapshot file: Snapshot Files. (line 11536)
* format 2, snapshot file: Snapshot Files. (line 11558)
* Format Options: Format Variations. (line 9321)
* Format Parameters: Format Variations. (line 9321)
* Format, old style: old. (line 8322)
* fortnight in date strings: Relative items in date strings.
(line 7475)
* free documentation: Free Software Needs Free Documentation.
(line 12025)
* full dumps: Full Dumps. (line 5002)
* future time stamps: Large or Negative Values.
(line 8564)
* general date syntax: General date syntax.
(line 7243)
* Generate Mode, 'genfile': Generate Mode. (line 11746)
* genfile: Genfile. (line 11720)
* 'genfile', create file: Generate Mode. (line 11746)
* 'genfile', creating sparse files: Generate Mode. (line 11795)
* 'genfile', generate mode: Generate Mode. (line 11746)
* 'genfile', reading a list of file names: Generate Mode. (line 11762)
* 'genfile', seeking to a given offset: Generate Mode. (line 11758)
* Getting program version number: help. (line 2958)
* git, excluding files: exclude. (line 6104)
* Git, ignore files: exclude. (line 6056)
* GNU archive format: gnu. (line 8356)
* GNU.sparse.major, extended header variable: PAX 1. (line 11457)
* GNU.sparse.map, extended header variable: PAX 0. (line 11426)
* GNU.sparse.minor, extended header variable: PAX 1. (line 11460)
* GNU.sparse.name, extended header variable: PAX 0. (line 11434)
* GNU.sparse.name, extended header variable, in v.1.0: PAX 1.
(line 11467)
* GNU.sparse.numblocks, extended header variable: PAX 0. (line 11381)
* GNU.sparse.numbytes, extended header variable: PAX 0. (line 11387)
* GNU.sparse.offset, extended header variable: PAX 0. (line 11384)
* GNU.sparse.realsize, extended header variable: PAX 1. (line 11467)
* GNU.sparse.size, extended header variable: PAX 0. (line 11377)
* gnupg, using with tar: gzip. (line 7929)
* gpg, using with tar: gzip. (line 7929)
* gzip: gzip. (line 7739)
* hard links, dereferencing: hard links. (line 8250)
* header-col: Configuring Help Summary.
(line 10685)
* hole detection: sparse. (line 8049)
* hook: User Hooks. (line 5473)
* hour in date strings: Relative items in date strings.
(line 7475)
* ignore-archive: warnings. (line 3442)
* ignore-archive <1>: warnings. (line 3442)
* ignore-newer: warnings. (line 3463)
* Ignoring end-of-archive blocks: Ignore Zeros. (line 4392)
* 'Ignoring unknown extended header keyword '%s'', warning message: warnings.
(line 3465)
* 'implausibly old time stamp %s', warning message: warnings.
(line 3454)
* Info script: Multi-Volume Archives.
(line 9840)
* Interactive operation: interactive. (line 3500)
* ISO 8601 date and time of day format: Combined date and time of day items.
(line 7428)
* ISO 8601 date format: Calendar date items.
(line 7334)
* items in date strings: General date syntax.
(line 7243)
* Labeling an archive: label. (line 10035)
* labeling archives: Tape Files. (line 9984)
* Labeling multi-volume archives: label. (line 10035)
* Labels on the archive media: label. (line 10035)
* language, in dates: General date syntax.
(line 7273)
* language, in dates <1>: General date syntax.
(line 7277)
* Large lists of file names on small machines: Same Order. (line 4834)
* large values: Large or Negative Values.
(line 8564)
* last DAY: Day of week items. (line 7456)
* last in date strings: General date syntax.
(line 7259)
* Laszlo Ersek: lbzip2. (line 7963)
* lbzip2: lbzip2. (line 7963)
* leap seconds: General date syntax.
(line 7302)
* leap seconds <1>: Time of day items. (line 7371)
* leap seconds <2>: Seconds since the Epoch.
(line 7567)
* Listing all 'tar' options: help. (line 2978)
* listing member and file names: list. (line 846)
* Listing volume label: label. (line 10056)
* Lists of file names: files. (line 5877)
* Local and remote archives: file. (line 5788)
* long options: Long Options. (line 1335)
* long options with mandatory arguments: Long Options. (line 1360)
* long options with optional arguments: Long Options. (line 1368)
* long-opt-col: Configuring Help Summary.
(line 10631)
* lzip: gzip. (line 7739)
* lzma: gzip. (line 7739)
* lzop: gzip. (line 7739)
* MacKenzie, David: Authors of parse_datetime.
(line 7619)
* 'Malformed dumpdir: 'X' never used', warning message: warnings.
(line 3493)
* member: Definitions. (line 85)
* member name: Definitions. (line 85)
* members, multiple: multiple. (line 3798)
* Members, replacing with other members: append. (line 3744)
* Mercurial, excluding files: exclude. (line 6104)
* Mercurial, ignore files: exclude. (line 6056)
* Meyering, Jim: Authors of parse_datetime.
(line 7619)
* Middle of the archive, starting in the: Starting File. (line 4822)
* midnight in date strings: Time of day items. (line 7378)
* minute in date strings: Relative items in date strings.
(line 7475)
* minutes, time zone correction by: Time of day items. (line 7386)
* Modes of extracted files: Setting Access Permissions.
(line 4568)
* Modification time, excluding files by: after. (line 6885)
* Modification times of extracted files: Data Modification Times.
(line 4550)
* month in date strings: Relative items in date strings.
(line 7475)
* month names in date strings: Calendar date items.
(line 7342)
* months, written-out: General date syntax.
(line 7269)
* MT: General-Purpose Variables.
(line 5360)
* MT_BEGIN: Magnetic Tape Control.
(line 5424)
* MT_OFFLINE: Magnetic Tape Control.
(line 5444)
* MT_REWIND: Magnetic Tape Control.
(line 5434)
* MT_STATUS: Magnetic Tape Control.
(line 5454)
* Multi-volume archives: Multi-Volume Archives.
(line 9763)
* Multi-volume archives in PAX format, extracting using non-GNU tars: Split Recovery.
(line 8620)
* Multi-volume archives, extracting using non-GNU tars: Split Recovery.
(line 8609)
* multiple members: multiple. (line 3798)
* Naming an archive: file. (line 5724)
* negative time stamps: Large or Negative Values.
(line 8564)
* new-directory: warnings. (line 3489)
* next DAY: Day of week items. (line 7456)
* next in date strings: General date syntax.
(line 7259)
* none: warnings. (line 3420)
* noon in date strings: Time of day items. (line 7378)
* now in date strings: Relative items in date strings.
(line 7493)
* ntape device: Many. (line 9565)
* 'NUL'-terminated file names: nul. (line 5969)
* Number of blocks per record: Blocking Factor. (line 9339)
* Number of bytes per record: Blocking Factor. (line 9339)
* numbered backup method: backup. (line 4903)
* numbers, written-out: General date syntax.
(line 7259)
* Obtaining help: help. (line 2978)
* Obtaining total status information: verbose. (line 3100)
* Old GNU archive format: gnu. (line 8356)
* Old GNU sparse format: Old GNU Format. (line 11325)
* old option style: Old Options. (line 1420)
* old options with mandatory arguments: Old Options. (line 1440)
* Old style archives: old. (line 8322)
* Old style format: old. (line 8322)
* opt-doc-col: Configuring Help Summary.
(line 10671)
* option syntax, traditional: Old Options. (line 1420)
* optional arguments to long options: Long Options. (line 1368)
* optional arguments to short options: Short Options. (line 1391)
* options for use with '--extract': extract options. (line 4346)
* Options when reading archives: Reading. (line 4357)
* Options, archive format specifying: Format Variations. (line 9321)
* Options, format specifying: Format Variations. (line 9321)
* options, GNU style: Long Options. (line 1335)
* options, long style: Long Options. (line 1335)
* options, mixing different styles: Mixing. (line 1482)
* options, mnemonic names: Long Options. (line 1335)
* options, old style: Old Options. (line 1420)
* options, short style: Short Options. (line 1376)
* options, traditional: Short Options. (line 1376)
* ordinal numbers: General date syntax.
(line 7259)
* Overwriting old files, prevention: Dealing with Old Files.
(line 4428)
* parse_datetime: Date input formats.
(line 7209)
* pattern, 'genfile': Generate Mode. (line 11779)
* PAX archive format: posix. (line 8373)
* Permissions of extracted files: Setting Access Permissions.
(line 4568)
* Pinard, F.: Authors of parse_datetime.
(line 7632)
* pm in date strings: Time of day items. (line 7378)
* POSIX archive format: posix. (line 8373)
* Progress information: verbose. (line 3137)
* Protecting old files: Dealing with Old Files.
(line 4448)
* pure numbers in date strings: Pure numbers in date strings.
(line 7527)
* RCS, excluding files: exclude. (line 6104)
* Reading file names from a file: files. (line 5877)
* Reading incomplete records: Reading. (line 4357)
* record: Blocking. (line 9208)
* Record Size: Blocking Factor. (line 9339)
* 'Record size = %lu blocks', warning message: warnings. (line 3480)
* record-size: warnings. (line 3480)
* Records, incomplete: Reading. (line 4357)
* Recursion in directories, avoiding: recurse. (line 6962)
* relative items in date strings: Relative items in date strings.
(line 7466)
* Remote devices: file. (line 5778)
* remote tape drive: Remote Tape Server.
(line 9124)
* Removing files from an archive: delete. (line 3989)
* rename-directory: warnings. (line 3487)
* Replacing members with other members: append. (line 3744)
* reporting bugs: Reports. (line 209)
* RESTORE_BEGIN: User Hooks. (line 5499)
* RESTORE_END: User Hooks. (line 5502)
* Resurrecting files from an archive: extract. (line 941)
* Retrieving files from an archive: extract. (line 941)
* return status: Synopsis. (line 1223)
* rmargin: Configuring Help Summary.
(line 10703)
* rmt: Remote Tape Server.
(line 9124)
* RSH: General-Purpose Variables.
(line 5364)
* RSH_COMMAND: General-Purpose Variables.
(line 5369)
* Running out of space: Scarce. (line 4814)
* Salz, Rich: Authors of parse_datetime.
(line 7619)
* SCCS, excluding files: exclude. (line 6104)
* short options: Short Options. (line 1376)
* short options with mandatory arguments: Short Options. (line 1383)
* short options with optional arguments: Short Options. (line 1391)
* short-opt-col: Configuring Help Summary.
(line 10623)
* simple backup method: backup. (line 4912)
* SIMPLE_BACKUP_SUFFIX: backup. (line 4915)
* 'sleep', checkpoint action: checkpoints. (line 3331)
* SLEEP_MESSAGE: General-Purpose Variables.
(line 5407)
* SLEEP_TIME: General-Purpose Variables.
(line 5392)
* Small memory: Scarce. (line 4814)
* snapshot file field ranges: Snapshot Files. (line 11596)
* snapshot file, format 0: Snapshot Files. (line 11509)
* snapshot file, format 1: Snapshot Files. (line 11536)
* snapshot file, format 2: Snapshot Files. (line 11558)
* snapshot files, editing: Fixing Snapshot Files.
(line 10709)
* snapshot files, fixing device numbers: Fixing Snapshot Files.
(line 10709)
* 'socket ignored', warning message: warnings. (line 3436)
* Sparse Files: sparse. (line 7989)
* sparse files v.0.0, extracting with non-GNU tars: Sparse Recovery.
(line 8781)
* sparse files v.0.1, extracting with non-GNU tars: Sparse Recovery.
(line 8781)
* sparse files v.1.0, extracting with non-GNU tars: Sparse Recovery.
(line 8706)
* Sparse files, creating using 'genfile': Generate Mode. (line 11795)
* sparse files, extracting with non-GNU tars: Sparse Recovery.
(line 8695)
* sparse formats: Sparse Formats. (line 11302)
* sparse formats, defined: sparse. (line 8035)
* sparse formats, Old GNU: Old GNU Format. (line 11325)
* sparse formats, v.0.0: PAX 0. (line 11373)
* sparse formats, v.0.1: PAX 0. (line 11418)
* sparse formats, v.1.0: PAX 1. (line 11449)
* sparse versions: Sparse Formats. (line 11302)
* Specifying archive members: Selecting Archive Members.
(line 5808)
* Specifying files to act on: Selecting Archive Members.
(line 5808)
* Standard input and output: file. (line 5757)
* Standard output, writing extracted files to: Writing to Standard Output.
(line 4653)
* Storing archives in compressed format: gzip. (line 7739)
* SVN, excluding files: exclude. (line 6104)
* Symbolic link as file name: dereference. (line 8228)
* symlink-cast: warnings. (line 3459)
* TAPE: file tutorial. (line 383)
* tape blocking: Blocking Factor. (line 9522)
* tape marks: Many. (line 9602)
* tape positioning: Many. (line 9585)
* Tapes, using '--delete' and: delete. (line 4000)
* TAPE_FILE: General-Purpose Variables.
(line 5304)
* tar: What tar Does. (line 107)
* TAR: General-Purpose Variables.
(line 5412)
* tar archive: Definitions. (line 76)
* Tar archive formats: Formats. (line 7639)
* tar entry: Naming tar Archives.
(line 165)
* tar file: Naming tar Archives.
(line 165)
* tar to a remote device: file. (line 5778)
* tar to standard input and output: file. (line 5757)
* tar-snapshot-edit: Fixing Snapshot Files.
(line 10720)
* tarcat: Tarcat. (line 10014)
* TAR_ARCHIVE, checkpoint script environment: checkpoints. (line 3353)
* TAR_ARCHIVE, info script environment variable: Multi-Volume Archives.
(line 9862)
* TAR_ARCHIVE, to-command environment: Writing to an External Program.
(line 4760)
* TAR_ATIME, to-command environment: Writing to an External Program.
(line 4733)
* TAR_BLOCKING_FACTOR, checkpoint script environment: checkpoints.
(line 3356)
* TAR_BLOCKING_FACTOR, info script environment variable: Multi-Volume Archives.
(line 9865)
* TAR_BLOCKING_FACTOR, to-command environment: Writing to an External Program.
(line 4763)
* TAR_CHECKPOINT, checkpoint script environment: checkpoints.
(line 3359)
* TAR_CTIME, to-command environment: Writing to an External Program.
(line 4742)
* TAR_FD, info script environment variable: Multi-Volume Archives.
(line 9879)
* TAR_FILENAME, to-command environment: Writing to an External Program.
(line 4721)
* TAR_FILETYPE, to-command environment: Writing to an External Program.
(line 4705)
* TAR_FORMAT, checkpoint script environment: checkpoints. (line 3366)
* TAR_FORMAT, info script environment variable: Multi-Volume Archives.
(line 9875)
* TAR_FORMAT, to-command environment: Writing to an External Program.
(line 4769)
* TAR_GID, to-command environment: Writing to an External Program.
(line 4751)
* TAR_GNAME, to-command environment: Writing to an External Program.
(line 4730)
* TAR_MODE, to-command environment: Writing to an External Program.
(line 4718)
* TAR_MTIME, to-command environment: Writing to an External Program.
(line 4739)
* TAR_OPTIONS, environment variable: using tar options. (line 1285)
* TAR_REALNAME, to-command environment: Writing to an External Program.
(line 4724)
* TAR_SIZE, to-command environment: Writing to an External Program.
(line 4745)
* TAR_SUBCOMMAND, checkpoint script environment: checkpoints.
(line 3362)
* TAR_SUBCOMMAND, info script environment variable: Multi-Volume Archives.
(line 9871)
* TAR_UID, to-command environment: Writing to an External Program.
(line 4748)
* TAR_UNAME, to-command environment: Writing to an External Program.
(line 4727)
* TAR_VERSION, checkpoint script environment: checkpoints. (line 3350)
* TAR_VERSION, info script environment variable: Multi-Volume Archives.
(line 9859)
* TAR_VERSION, to-command environment: Writing to an External Program.
(line 4757)
* TAR_VOLUME, info script environment variable: Multi-Volume Archives.
(line 9868)
* TAR_VOLUME, to-command environment: Writing to an External Program.
(line 4766)
* this in date strings: Relative items in date strings.
(line 7493)
* time of day item: Time of day items. (line 7363)
* 'time stamp %s is %s s in the future', warning message: warnings.
(line 3454)
* time zone correction: Time of day items. (line 7386)
* time zone item: General date syntax.
(line 7277)
* time zone item <1>: Time zone items. (line 7405)
* timestamp: warnings. (line 3454)
* today in date strings: Relative items in date strings.
(line 7493)
* tomorrow in date strings: Relative items in date strings.
(line 7489)
* 'totals', checkpoint action: checkpoints. (line 3326)
* 'ttyout', checkpoint action: checkpoints. (line 3297)
* TZ: Specifying time zone rules.
(line 7575)
* Ultrix 3.1 and write failure: Remote Tape Server.
(line 9158)
* 'Unknown file type '%c', extracted as normal file', warning message: warnings.
(line 3461)
* 'Unknown file type; file ignored', warning message: warnings.
(line 3436)
* unknown-cast: warnings. (line 3461)
* unknown-keyword: warnings. (line 3465)
* unpacking: Definitions. (line 92)
* Updating an archive: update. (line 3854)
* usage-indent: Configuring Help Summary.
(line 10699)
* Using encrypted archives: gzip. (line 7929)
* ustar archive format: ustar. (line 8344)
* uuencode: Applications. (line 4925)
* v7 archive format: old. (line 8322)
* VCS, excluding files: exclude. (line 6104)
* VCS, excluding patterns from ignore files: exclude. (line 6056)
* VCS, ignore files: exclude. (line 6056)
* Verbose operation: verbose. (line 3073)
* Verifying a write operation: verify. (line 10156)
* Verifying the currency of an archive: compare. (line 4030)
* version control system, excluding files: exclude. (line 6104)
* Version of the 'tar' program: help. (line 2958)
* version-control Emacs variable: backup. (line 4897)
* VERSION_CONTROL: backup. (line 4889)
* volno file: Multi-Volume Archives.
(line 9831)
* VOLNO_FILE: General-Purpose Variables.
(line 5375)
* Volume label, listing: label. (line 10056)
* Volume number file: Multi-Volume Archives.
(line 9831)
* week in date strings: Relative items in date strings.
(line 7475)
* Where is the archive?: file. (line 5724)
* Working directory, specifying: directory. (line 7050)
* Writing extracted files to standard output: Writing to Standard Output.
(line 4653)
* Writing new archives: file. (line 5752)
* xdev: warnings. (line 3434)
* xdev <1>: warnings. (line 3491)
* XLIST: General-Purpose Variables.
(line 5381)
* xsparse: Sparse Recovery. (line 8702)
* year in date strings: Relative items in date strings.
(line 7475)
* yesterday in date strings: Relative items in date strings.
(line 7489)

GNU tar: an archiver tool
1 Introduction
2 Tutorial Introduction to 'tar'
3 Invoking GNU 'tar'
4 GNU 'tar' Operations
5 Performing Backups and Restoring Files
6 Choosing Files and Names for 'tar'
7 Date input formats
8 Controlling the Archive Format
9 Tapes and Other Archive Media
10 Reliability and Security
Appendix A Changes
Appendix B Configuring Help Summary
Appendix C Fixing Snapshot Files
Appendix D Tar Internals
Appendix E Genfile
Appendix F Free Software Needs Free Documentation
Appendix G GNU Free Documentation License
Appendix H Index of Command Line Options
Appendix I Index
GNU tar: an archiver tool
1 Introduction
1.1 What this Book Contains
1.2 Some Definitions
1.3 What 'tar' Does
1.4 How 'tar' Archives are Named
1.5 GNU 'tar' Authors
1.6 Reporting bugs or suggestions
2 Tutorial Introduction to 'tar'
2.1 Assumptions this Tutorial Makes
2.2 Stylistic Conventions
2.3 Basic 'tar' Operations and Options
2.4 The Three Most Frequently Used Operations
2.5 Two Frequently Used Options
The '--file' Option
The '--verbose' Option
Getting Help: Using the '--help' Option
2.6 How to Create Archives
2.6.1 Preparing a Practice Directory for Examples
2.6.2 Creating the Archive
2.6.3 Running '--create' with '--verbose'
2.6.4 Short Forms with 'create'
2.6.5 Archiving Directories
2.7 How to List Archives
Listing the Contents of a Stored Directory
2.8 How to Extract Members from an Archive
2.8.1 Extracting an Entire Archive
2.8.2 Extracting Specific Files
2.8.3 Extracting Files that are Directories
2.8.4 Extracting Archives from Untrusted Sources
2.8.5 Commands That Will Fail
2.9 Going Further Ahead in this Manual
3 Invoking GNU 'tar'
3.1 General Synopsis of 'tar'
3.2 Using 'tar' Options
3.3 The Three Option Styles
3.3.1 Long Option Style
3.3.2 Short Option Style
3.3.3 Old Option Style
3.3.4 Mixing Option Styles
3.4 All 'tar' Options
3.4.1 Operations
3.4.2 'tar' Options
3.4.3 Short Options Cross Reference
3.4.4 Position-Sensitive Options
3.5 GNU 'tar' documentation
3.6 Obtaining GNU 'tar' default values
3.7 Checking 'tar' progress
3.8 Checkpoints
3.9 Controlling Warning Messages
3.10 Asking for Confirmation During Operations
3.11 Running External Commands
4 GNU 'tar' Operations
4.1 Basic GNU 'tar' Operations
4.2 Advanced GNU 'tar' Operations
4.2.1 The Five Advanced 'tar' Operations
4.2.2 How to Add Files to Existing Archives: '--append'
4.2.2.1 Appending Files to an Archive
4.2.2.2 Multiple Members with the Same Name
4.2.3 Updating an Archive
4.2.3.1 How to Update an Archive Using '--update'
4.2.4 Combining Archives with '--concatenate'
4.2.5 Removing Archive Members Using '--delete'
4.2.6 Comparing Archive Members with the File System
4.3 Options Used by '--create'
4.3.1 Overriding File Metadata
4.3.2 Extended File Attributes
4.3.3 Ignore Fail Read
4.4 Options Used by '--extract'
4.4.1 Options to Help Read Archives
Reading Full Records
Ignoring Blocks of Zeros
4.4.2 Changing How 'tar' Writes Files
Options Controlling the Overwriting of Existing Files
Overwrite Old Files
Keep Old Files
Keep Newer Files
Unlink First
Recursive Unlink
Setting Data Modification Times
Setting Access Permissions
Directory Modification Times and Permissions
Writing to Standard Output
Writing to an External Program
Removing Files
4.4.3 Coping with Scarce Resources
Starting File
Same Order
4.5 Backup options
4.6 Notable 'tar' Usages
4.7 Looking Ahead: The Rest of this Manual
5 Performing Backups and Restoring Files
5.1 Using 'tar' to Perform Full Dumps
5.2 Using 'tar' to Perform Incremental Dumps
5.3 Levels of Backups
5.4 Setting Parameters for Backups and Restoration
5.4.1 General-Purpose Variables
5.4.2 Magnetic Tape Control
5.4.3 User Hooks
5.4.4 An Example Text of 'Backup-specs'
5.5 Using the Backup Scripts
5.6 Using the Restore Script
6 Choosing Files and Names for 'tar'
6.1 Choosing and Naming Archive Files
6.2 Selecting Archive Members
6.3 Reading Names from a File
6.3.1 'NUL'-Terminated File Names
6.4 Excluding Some Files
Problems with Using the 'exclude' Options
6.5 Wildcards Patterns and Matching
Controlling Pattern-Matching
6.6 Quoting Member Names
6.7 Modifying File and Member Names
6.8 Operating Only on New Files
6.9 Descending into Directories
6.10 Crossing File System Boundaries
6.10.1 Changing the Working Directory
6.10.2 Absolute File Names
7 Date input formats
7.1 General date syntax
7.2 Calendar date items
7.3 Time of day items
7.4 Time zone items
7.5 Combined date and time of day items
7.6 Day of week items
7.7 Relative items in date strings
7.8 Pure numbers in date strings
7.9 Seconds since the Epoch
7.10 Specifying time zone rules
7.11 Authors of 'parse_datetime'
8 Controlling the Archive Format
8.1 Using Less Space through Compression
8.1.1 Creating and Reading Compressed Archives
8.1.1.1 Using lbzip2 with GNU 'tar'.
8.1.2 Archiving Sparse Files
8.2 Handling File Attributes
8.3 Making 'tar' Archives More Portable
8.3.1 Portable Names
8.3.2 Symbolic Links
8.3.3 Hard Links
8.3.4 Old V7 Archives
8.3.5 Ustar Archive Format
8.3.6 GNU and old GNU 'tar' format
8.3.7 GNU 'tar' and POSIX 'tar'
8.3.7.1 Controlling Extended Header Keywords
8.3.8 Checksumming Problems
8.3.9 Large or Negative Values
8.3.10 How to Extract GNU-Specific Data Using Other 'tar' Implementations
8.3.10.1 Extracting Members Split Between Volumes
8.3.10.2 Extracting Sparse Members
8.4 Comparison of 'tar' and 'cpio'
9 Tapes and Other Archive Media
9.1 Device Selection and Switching
9.2 Remote Tape Server
9.3 Some Common Problems and their Solutions
9.4 Blocking
9.4.1 Format Variations
9.4.2 The Blocking Factor of an Archive
9.5 Many Archives on One Tape
9.5.1 Tape Positions and Tape Marks
9.5.2 The 'mt' Utility
9.6 Using Multiple Tapes
9.6.1 Archives Longer than One Tape or Disk
9.6.2 Tape Files
9.6.3 Concatenate Volumes into a Single Archive
9.7 Including a Label in the Archive
9.8 Verifying Data as It is Stored
9.9 Write Protection
10 Reliability and Security
10.1 Reliability
10.1.1 Permissions Problems
10.1.2 Data Corruption and Repair
10.1.3 Race conditions
10.2 Security
10.2.1 Privacy
10.2.2 Integrity
10.2.3 Dealing with Live Untrusted Data
10.2.4 Security Rules of Thumb
Appendix A Changes
Appendix B Configuring Help Summary
Appendix C Fixing Snapshot Files
Appendix D Tar Internals
Basic Tar Format
GNU Extensions to the Archive Format
Storing Sparse Files
Old GNU Format
PAX Format, Versions 0.0 and 0.1
PAX Format, Version 1.0
Format of the Incremental Snapshot Files
Dumpdir
Appendix E Genfile
E.1 Generate Mode
E.2 Status Mode
E.3 Exec Mode
Appendix F Free Software Needs Free Documentation
Appendix G GNU Free Documentation License
Appendix H Index of Command Line Options
Appendix I Index
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