This manual is for GNU tar (version
1.35, 22 August 2023), which creates and extracts files
from archives.
Copyright © 1992, 1994–1997, 1999–2001, 2003–2017, 2021–2023 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.”
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This manual is for GNU tar (version
1.35, 22 August 2023), which creates and extracts files
from archives.
Copyright © 1992, 1994–1997, 1999–2001, 2003–2017, 2021–2023 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.
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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 | ||
| 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 |
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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 (see section Tutorial Introduction to tar) 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; see section Tutorial Introduction to tar.)
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 (see section Date input formats) exists in its
entirety in other GNU manuals, and is mostly self-contained.
In addition, one section of this manual (see section Basic Tar Format) 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.)
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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.
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tar DoesThe 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
tape archiver. 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.
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!)
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.
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.
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tar Archives are NamedConventionally, 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.
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tar AuthorsGNU 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, François
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.
François 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 1.35 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 Performing Backups and Restoring Files.
In July, 2003 GNU tar was put on CVS at savannah.gnu.org
(see https://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.
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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.
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tarThis 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.
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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:
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’.
tar archives with tape drives.
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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’.
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tar Operations and Optionstar 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 (see section Old Option Style), and we discuss
the other two styles of writing options (See section Long Option Style, and
see section Short Option Style).
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 Advanced GNU tar Operations. 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.
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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 a new tar archive.
List the contents of an archive.
Extract one or more members from an archive.
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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 | ||
| The ‘--verbose’ Option | ||
| Getting Help: Using the ‘--help’ Option |
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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, see section Obtaining GNU tar default values). 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
Choosing and Naming Archive Files.
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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 ‘--verbose’ option also enables several warning messages, that tar does not issue otherwise, such as the warning about record size being used (see section The Blocking Factor of an Archive), selecting the decompress program and the like. If these are of no interest to you, you can suppress them using the ‘--warning’ option after ‘--verbose’, e.g.:
$ tar -c -v --warning=no-verbose -f afiles.tar apple angst aspic
See section verbose, for details.
The full output consists of six fields:
ls -l output (see Verbose listing in GNU core utilities).
Depending on the file type, the name can be followed by some additional information, described in the following table:
The file or archive member is a symbolic link and link-name is the name of file it links to.
The file or archive member is a hard link and link-name is the name of file it links to.
The archive member is an old GNU format long link. You will normally not encounter this.
The archive member is an old GNU format long name. You will normally not encounter this.
The archive member is a GNU volume header (see section Tape Files).
Encountered only at the beginning of a multi-volume archive (see section 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.
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
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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.
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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 | ||
| 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 |
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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 (see section Backup options, 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
How to Add Files to Existing Archives: ‘--append’ for information on how to do this.
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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 (see section Old Option Style), 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. See section Short Forms with ‘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.
See section Choosing Files and Names for tar, for the detailed discussion on these.) Now that they are
in the archive, they are called archive members, not files.
(see section members).
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. See section How to Add Files to Existing Archives: ‘--append’.
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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 three output
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.
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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 The Three Option 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.
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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; See section Absolute File Names. (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.)
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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 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
(See section Absolute File Names, 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 --list --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.
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 --list --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
See section Wildcards Patterns and Matching, for a detailed discussion of globbing patterns and related
tar command line options.
| Listing the Contents of a Stored Directory |
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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.
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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 | ||
| 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 |
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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
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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 (See section Commands That Will Fail, for more examples).
You can extract a file to standard output by combining the above options with the ‘--to-stdout’ (‘-O’) option (see section Writing to Standard Output).
If you give the ‘--verbose’ option, then ‘--extract’ will print the names of the archive members as it extracts them.
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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
see section Changing How tar Writes Files).
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.
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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’).
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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 (see section Modifying File and Member Names). For example,
$ tar --extract --file=music.tar --strip-components=1 folk
will extract the file ‘folk’ into the current working directory.
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(This message will disappear, once this node revised.)
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tarThis chapter is about how one invokes the GNU tar
command, from the command synopsis (see section General Synopsis of tar). There are
numerous options, and many styles for writing them. One mandatory
option specifies the operation tar should perform
(see section Operations), other options are meant to detail how
this operation should be performed (see section tar Options).
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 (see section The Three Option 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’ (see section GNU tar documentation), ‘--verbose’ (see section Checking tar progress)
and ‘--interactive’ options (see section Asking for Confirmation During Operations).
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tarThe 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). See section Absolute File Names, 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. See section Wildcards Patterns and Matching, 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:
‘Successful termination’.
‘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 (see section Comparing Archive Members with the File System). 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.
‘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 (see section Creating and Reading Compressed Archives) and the external compressor program
failed. Another example is rmt failure during backup to the
remote device (see section Remote Tape Server).
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tar OptionsGNU 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 The Three Most Frequently Used Operations and The Five Advanced tar 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 see section All tar 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.
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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(2). 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.
| 3.3.1 Long Option Style | ||
| 3.3.2 Short Option Style | ||
| 3.3.3 Old Option Style | ||
| 3.3.4 Mixing Option Styles |
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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’.
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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(3).
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.
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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
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All three styles may be intermixed in a single tar command,
so long as the rules for each style are fully
respected(4). 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.
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tar OptionsThe 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 | ||
3.4.2 tar Options | ||
| 3.4.3 Short Options Cross Reference | ||
| 3.4.4 Position-Sensitive Options |
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Appends files to the end of the archive. See section How to Add Files to Existing Archives: ‘--append’.
Same as ‘--concatenate’. See section Combining Archives with ‘--concatenate’.
Compares archive members with their counterparts in the file system, and reports differences in file size, mode, owner, modification date and contents. See section Comparing Archive Members with the File System.
Appends other tar archives to the end of the archive.
See section Combining Archives with ‘--concatenate’.
Creates a new tar archive. See section How to Create Archives.
Deletes members from the archive. Don’t try this on an archive on a tape! See section Removing Archive Members Using ‘--delete’.
Same as ‘--compare’. See section Comparing Archive Members with the File System.
Extracts members from the archive into the file system. See section How to Extract Members from an Archive.
Same as ‘--extract’. See section How to Extract Members from an Archive.
Lists the members in an archive. See section How to List Archives.
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. See section Updating an Archive.
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tar OptionsNormally 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. See section Absolute File Names.
Enable POSIX ACLs support. See section acls.
(See ‘--newer’, see section Operating Only on New Files)
A pattern must match an initial subsequence of the name’s components. See section Controlling Pattern-Matching.
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.
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’. See section Creating and Reading Compressed Archives.
Rather than deleting files from the file system, tar will
back them up using simple or numbered backups, depending upon
backup-type. See section Backup options.
With this option present, tar prints error messages for read errors
with the block number in the archive file. See block-number.
Sets the blocking factor tar uses to blocking x 512 bytes per
record. See section The Blocking Factor of an Archive.
This option tells tar to read or write archives through
bzip2. See section Creating and Reading Compressed Archives.
Check device numbers when creating a list of modified files for incremental archiving. This is the default. See device numbers, for a detailed description.
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
Checkpoints.
Instruct tar to execute an action upon hitting a
breakpoint. Here we give only a brief outline. See section Checkpoints,
for a complete description.
The action argument can be one of the following:
Produce an audible bell on the console.
Print a single dot on the standard listing stream.
Display a textual message on the standard error, with the status and number of the checkpoint. This is the default.
Display string on the standard error. Before output, the string is subject to meta-character expansion.
Execute the given command.
Wait for time seconds.
Output string on the current console (‘/dev/tty’).
Print statistics (see see totals).
Wait for signal signo.
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.
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 (5).
See section Hard Links.
tar will use the compress program when reading or
writing the archive. This allows you to directly act on archives
while saving space. See section Creating and Reading Compressed Archives.
(See ‘--mtime’.)
(See ‘--interactive’.) See section Asking for Confirmation During Operations.
Delay setting modification times and permissions of extracted directories until the end of extraction. See section Directory Modification Times and Permissions.
When reading or writing a file to be archived, tar accesses
the file that a symbolic link points to, rather than the symlink
itself. See section Symbolic Links.
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. See section Changing the Working Directory.
When performing operations, tar will skip files that match
pattern. See section Excluding Some Files.
Exclude backup and lock files. See section exclude-backups.
Similar to ‘--exclude’, except tar will use the list of
patterns in the file file. See section Excluding Some Files.
Exclude from dump any directory containing a valid cache directory tag file, but still dump the directory node and the tag file itself.
See section exclude-caches.
Exclude from dump any directory containing a valid cache directory tag file, but still dump the directory node itself.
See section Excluding Some Files.
Exclude from dump any directory containing a valid cache directory tag file. See section Excluding Some Files.
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. See section Excluding Some Files.
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.
See section Excluding Some Files.
Exclude from dump any directory containing file named file, but dump the directory node and file itself. See section exclude-tag.
Exclude from dump the contents of any directory containing file named file, but dump the directory node itself. See section exclude-tag-under.
Exclude from dump any directory containing file named file. See section exclude-tag-all.
Exclude from dump directories and files, that are internal for some widely used version control systems.
See exclude-vcs.
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. See exclude-vcs-ignores.
tar will use the file archive as the tar archive it
performs operations on, rather than tar’s compilation dependent
default. See section The ‘--file’ Option.
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. See section Reading Names from a File.
Forces tar to interpret the file name given to ‘--file’
as a local file, even if it looks like a remote tape drive name.
See local and remote archives.
Selects output archive format. Format may be one of the following:
Creates an archive that is compatible with Unix V7 tar.
Creates an archive that is compatible with GNU tar version
1.12 or earlier.
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.
Creates a POSIX.1-1988 compatible archive.
Creates a POSIX.1-2001 archive.
See section Controlling the Archive Format, for a detailed discussion of these formats.
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 (see section The ‘--verbose’ Option).
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. See section Overriding File Metadata.
Also see the ‘--group-map’ option and comments for the ‘--owner=user’ option.
Read owner group translation map from file. This option allows to translate only certain group names and/or UIDs. See section Overriding File Metadata, 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.
This option tells tar to read or write archives through
gzip, allowing tar to directly operate on several
kinds of compressed archives transparently. See section Creating and Reading Compressed Archives.
When creating an archive, dereference hard links and store the files they refer to, instead of creating usual hard link members.
See section Hard Links.
tar will print out a short message summarizing the operations and
options to tar and exit. See section GNU tar documentation.
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. See section Archiving Sparse Files.
Ignore case when matching member or file names with patterns. See section Controlling Pattern-Matching.
Ignore exit codes of subprocesses. See section Writing to an External Program.
Do not exit unsuccessfully merely because reading failed. See section Ignore Failed Read.
With this option, tar will ignore zeroed blocks in the
archive, which normally signals EOF. This option also suppresses
warnings about missing or incomplete zero blocks at the end of the
archive. See section Ignoring Blocks of Zeros.
Informs tar that it is working with an old
GNU-format incremental backup archive. It is intended
primarily for backwards compatibility only. See section Using tar to Perform Incremental Dumps,
for a detailed discussion of incremental archives.
Send verbose output to file instead of to standard output.
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. See info-script, for a detailed
discussion of this feature.
Specifies that tar should ask the user for confirmation before
performing potentially destructive options, such as overwriting files.
See section Asking for Confirmation During Operations.
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.
Do not replace existing files that are newer than their archive copies when extracting files from an archive.
Do not overwrite existing files when extracting files from an archive. Return error if such files exist. See also --skip-old-files.
See section Keep Old Files.
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. See section Tape Files.
Force incremental backup of level n. As of GNU tar version
1.35, the option ‘--level=0’ truncates the snapshot
file, thereby forcing the level 0 dump. Other values of n are
effectively ignored. See --level=0, for details and examples.
The use of this option is valid only in conjunction with the
‘--listed-incremental’ option. See section Using tar to Perform Incremental Dumps,
for a detailed description.
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. See section Using tar to Perform Incremental Dumps.
This option tells tar to read or write archives through
lzip. See section Creating and Reading Compressed Archives.
This option tells tar to read or write archives through
lzma. See section Creating and Reading Compressed Archives.
This option tells tar to read or write archives through
lzop. See section Creating and Reading Compressed Archives.
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. See section Overriding File Metadata.
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 (see section 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. See section Overriding File Metadata.
When --clamp-mtime is also specified, files with
modification times earlier than date will retain their actual
modification times, and date will be used only for files with
modification times later than date.
Informs tar that it should create or otherwise operate on a
multi-volume tar archive. See section Using Multiple Tapes.
(see ‘--info-script’)
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. See section Operating Only on New Files.
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). See section Operating Only on New Files.
Disable the POSIX ACLs support. See section acls.
An exclude pattern can match any subsequence of the name’s components. See section Controlling Pattern-Matching.
Disables automatic compressed format recognition based on the archive suffix. See --auto-compress. See section Creating and Reading Compressed Archives.
Do not check device numbers when creating a list of modified files for incremental archiving. See device numbers, for a detailed description.
Modification times and permissions of extracted directories are set when all files from this directory have been extracted. This is the default. See section Directory Modification Times and Permissions.
Use case-sensitive matching. See section Controlling Pattern-Matching.
Print warnings about subprocesses that terminated with a nonzero exit code. See section Writing to an External Program.
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.
Preserve metadata of existing directories when extracting files from an archive. See section Overwrite Old Files.
Remove characters listed in string from the list of quoted characters set by the previous ‘--quote-chars’ option (see section Quoting Member Names).
With this option, tar will not recurse into directories.
See section Descending into Directories.
When extracting an archive, do not attempt to preserve the owner
specified in the tar archive. This the default behavior
for ordinary users.
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.
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.
Disable SELinux context support. See section SELinux.
Treat all input file or member names literally, do not interpret escape sequences. See input name quoting.
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.
Do not use wildcards. See section Controlling Pattern-Matching.
Wildcards do not match ‘/’. See section Controlling Pattern-Matching.
Disable extended attributes support. See section xattrs.
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.
See section NUL-Terminated File Names.
See also verbatim-files-from.
This option will notify tar that it should use numeric user
and group IDs when creating a tar file, rather than names.
See section Handling File Attributes.
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.
See section Changes, for more information.
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.
Synonym for ‘--format=v7’.
Used when creating an archive. Prevents tar from recursing into
directories that are on different file systems from the current
directory.
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 See --auto-compress.
Overwrite existing files and directory metadata when extracting files from an archive. See section Overwrite Old Files.
Overwrite the metadata of existing directories when extracting files from an archive. See section Overwrite Old Files.
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.
See section Overriding File Metadata.
This option does not affect extraction from archives. See also ‘--owner-map’, below.
Read owner translation map from file. This option allows to translate only certain owner names or UIDs. See section Overriding File Metadata, 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.
This option enables creation of the archive in POSIX.1-2001
format (see section GNU tar and POSIX tar) and modifies the way tar handles the
extended header keywords. Keyword-list is a comma-separated
list of keyword options. See section Controlling Extended Header Keywords, for a detailed
discussion.
Synonym for ‘--format=v7’.
Same as ‘--format=posix’.
(See ‘--same-order’; see section Same Order.)
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. See section Setting Access Permissions.
Always quote characters from string, even if the selected quoting style would not quote them (see section Quoting Member Names).
Set quoting style to use when printing member and file names
(see section Quoting Member Names). Valid style values are:
literal, shell, shell-always, c,
escape, locale, and clocale. Default quoting
style is escape, unless overridden while configuring the
package.
Specifies that tar should reblock its input, for reading
from pipes on systems with buggy implementations. See section Options to Help Read Archives.
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. See Table 9.1,
for a list of valid suffixes. See section The Blocking Factor of an Archive, for a detailed
description of this option.
With this option, tar recurses into directories (default).
See section Descending into Directories.
Remove existing directory hierarchies before extracting directories of the same name from the archive. See section Recursive Unlink.
Directs tar to remove the source file from the file system after
appending it to an archive. See section Removing Files.
Disable use of some potentially harmful tar options.
Currently this option disables shell invocation from multi-volume menu
(see section Using Multiple Tapes).
Notifies tar that it should use cmd instead of
the default ‘/usr/libexec/rmt’ (see section Remote Tape Server).
Notifies tar that is should use cmd to communicate with remote
devices. See section Device Selection and Switching.
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. See section Same Order.
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. See section Handling File Attributes.
(See ‘--preserve-permissions’; see section Setting Access Permissions.)
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).
Enable the SELinux context support. See section selinux.
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. See section Obtaining GNU tar default values.
Instructs tar to mention the directories it is skipping when
operating on a tar archive. See show-omitted-dirs.
Displays the range of values allowed by this version of tar
for each field in the snapshot file, then exits successfully.
See section Format of the Incremental Snapshot Files.
Display file or member names after applying any transformations
(see section Modifying File and Member Names). 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. See listing member and file names.
Do not overwrite existing files when extracting files from an archive. See section 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 (see section Controlling Warning Messages).
Specify the directory sorting order when reading directories. Order may be one of the following:
No directory sorting is performed. This is the default.
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 more reproducible. See section Making tar Archives More Reproducible.
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.
Invokes a GNU extension when adding files to an archive that handles sparse files efficiently. See section Archiving Sparse Files.
Specifies the format version to use when archiving sparse files. Implies ‘--sparse’. See section Archiving Sparse Files. For the description of the supported sparse formats, See section Storing Sparse Files.
This option affects extraction only; tar will skip extracting
files in the archive until it finds one that matches name.
See section Coping with Scarce Resources.
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’.
See section Modifying File and Member Names.
Alters the suffix tar uses when backing up files from the default
‘~’. See section Backup options.
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. See Table 9.1, for a
list of allowed suffixes. See section Using Multiple Tapes, for a detailed
discussion of this option.
Reads the volume label. If an argument is specified, test whether it matches the volume label. See --test-label option.
During extraction tar will pipe extracted files to the
standard input of command. See section Writing to an External Program.
During extraction, tar will extract files to stdout rather
than to the file system. See section Writing to Standard Output.
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.
See totals.
Sets the data modification time of extracted files to the extraction time, rather than the data modification time stored in the archive. See section Setting Data Modification Times.
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, See section Modifying File and Member Names.
To see transformed member names in verbose listings, use ‘--show-transformed-names’ option (see show-transformed-names).
(See ‘--compress’, see section Creating and Reading Compressed Archives)
(See ‘--gzip’, see section Creating and Reading Compressed Archives)
Directs tar to remove the corresponding file from the file
system before extracting it from the archive. See section Unlink First.
Enable unquoting input file or member names (default). See input name quoting.
Instructs tar to access the archive through prog, which is
presumed to be a compression program of some sort. See section Creating and Reading Compressed Archives.
Display file modification dates in UTC. This option implies ‘--verbose’.
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.
See verbatim-files-from.
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.
See section Checking tar progress.
Verifies that the archive was correctly written when creating an archive. See section Verifying Data as It is Stored.
Print information about the program’s name, version, origin and legal
status, all on standard output, and then exit successfully.
See section GNU tar documentation.
Used in conjunction with ‘--multi-volume’. tar will
keep track of which volume of a multi-volume archive it is working in
file. See volno-file.
Enable or disable warning messages identified by keyword. The messages are suppressed if keyword is prefixed with ‘no-’. See section Controlling Warning Messages.
Use wildcards when matching member names with patterns. See section Controlling Pattern-Matching.
Wildcards match ‘/’. See section Controlling Pattern-Matching.
Enable extended attributes support. See section xattrs.
Specify exclude pattern for xattr keys. See section xattrs-exclude.
Specify include pattern for xattr keys. pattern is a globbing pattern, e.g. ‘--xattrs-include='user.*'’ to include only attributes from the user namespace. See section xattrs-include.
Use xz for compressing or decompressing the archives. See section Creating and Reading Compressed Archives.
Use zstd for compressing or decompressing the archives. See section Creating and Reading Compressed Archives.
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Here is an alphabetized list of all of the short option forms, matching them with the equivalent long option.
| Short Option | Reference |
|---|---|
| -A | --concatenate. |
| -B | --read-full-records. |
| -C | --directory. |
| -F | --info-script. |
| -G | --incremental. |
| -J | --xz. |
| -K | --starting-file. |
| -L | --tape-length. |
| -M | --multi-volume. |
| -N | --newer. |
| -O | --to-stdout. |
| -P | --absolute-names. |
| -R | --block-number. |
| -S | --sparse. |
| -T | --files-from. |
| -U | --unlink-first. |
| -V | --label. |
| -W | --verify. |
| -X | --exclude-from. |
| -Z | --compress. |
| -b | --blocking-factor. |
| -c | --create. |
| -d | --compare. |
| -f | --file. |
| -g | --listed-incremental. |
| -h | --dereference. |
| -i | --ignore-zeros. |
| -j | --bzip2. |
| -k | --keep-old-files. |
| -l | --check-links. |
| -m | --touch. |
| -o | When extracting, same as --no-same-owner. When creating,
– --old-archive.
The latter usage is deprecated. It is retained for compatibility with
the earlier versions of GNU |
| -p | --preserve-permissions. |
| -r | --append. |
| -s | --same-order. |
| -t | --list. |
| -u | --update. |
| -v | --verbose. |
| -w | --interactive. |
| -x | --extract. |
| -z | --gzip. |
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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 (see section Changing the Working 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 (see section Reading Names from a File). 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(6):
tar -cf a.tar --recursion /usr --no-recursion /var/*
During archive creation, GNU tar keeps track of positional options
used and arguments affected by them. If it finds out that any such
options are used in an obviously erroneous way, the fact is reported
and exit code is set to 2. E.g.:
$ tar -cf a.tar . --exclude '*.o' tar: The following options were used after any non-optional arguments in archive create or update mode. These options are positional and affect only arguments that follow them. Please, rearrange them properly. tar: --exclude '*.o' has no effect tar: Exiting with failure status due to previous errors
The following table summarizes all position-sensitive options.
See section Changing the Working Directory.
See section NUL-Terminated File Names.
See input name quoting.
See verbatim-files-from.
See section Descending into Directories.
See anchored patterns.
See section Controlling Pattern-Matching.
See section Controlling Pattern-Matching.
See section Excluding Some Files.
See section Excluding Some Files.
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tar documentationBeing 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) 1.35 Copyright (C) 2013-2020 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(7).
Another thing you might want to do is check the spelling or meaning
of some particular tar option, without resorting to this
manual, 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. See section 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
https://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.
Since 2014, GNU tar also has a man page.
It briefly explains all the options and operations.
This might be preferable when you don’t need any background.
But bear in mind that the authoritative source of
information about GNU tar is this Texinfo documentation.
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tar default valuesGNU 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 (see section Controlling the Archive Format), it uses standard
output as the archive, if no ‘--file’ option has been given
(see section The ‘--file’ Option), the default blocking factor is 20
(see section The Blocking Factor of an Archive). It also shows the default locations where
tar will look for rmt and rsh binaries.
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tar progressTypically, 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
(See 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)
Notice, that since tar operates on records, the number
of bytes reported can be rounded up to the nearest full record. This
can happen, in particular, when the last record in the archive is
partial. See section Blocking.
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:
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(8). 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 (see section 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). See section Backup options.
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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:
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.
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.
Print type of the checkpoint (‘write’ or ‘read’).
Print number of the checkpoint.
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.
Output current local time using fmt as format for strftime
(see strftime in strftime(3) man page). 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.
Pad output with spaces to the nth column. If the ‘{n}’ part is omitted, the current screen width is assumed.
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 (see 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 (see 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
The wait=signo action stops further execution until the
signal signo is delivered. Valid values for signo are:
SIGHUP, SIGQUIT, SIGINT, SIGUSR1 and
SIGUSR2. The ‘SIG’ prefix is optional. For example:
$ tar -c -f arc --checkpoint=1000 --checkpoint-action wait=USR1 .
In this example, GNU tar will stop archivation at each 1000th
checkpoint. wait until the ‘SIGUSR1’ signal is delivered,
and resume processing.
This action is used by the genfile utility to perform
modifications on the input files upon hitting certain checkpoints
(see section genfile).
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. See section Running External Commands, for more detail.
The command gets a copy of tar’s environment plus the
following variables:
TAR_VERSIONGNU tar version number.
TAR_ARCHIVEThe name of the archive tar is processing.
TAR_BLOCKING_FACTORCurrent blocking factor (see section Blocking).
TAR_CHECKPOINTNumber of the checkpoint.
TAR_SUBCOMMANDA short option describing the operation tar is executing.
See section The Five Advanced tar Operations, for a complete list of subcommand options.
TAR_FORMATFormat of the archive being processed. See section Controlling the Archive Format, 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.
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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:
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’ options accumulate.
By default, GNU tar enables all messages, except those that are
enabled in verbose mode (see section The ‘--verbose’ Option). See section Default Warning Settings, for details.
The subsections below discuss allowed values for keyword along with the warning messages they control.
3.9.1 Keywords controlling tar operation | Keywords applicable for tar --create.
| |
3.9.2 Keywords applicable for tar --create | ||
3.9.3 Keywords applicable for tar --extract | ||
| 3.9.4 Keywords controlling incremental extraction | ||
| 3.9.5 Warning Classes | Convenience keywords control multiple warnings. | |
| 3.9.6 Default Warning Settings | Default settings for warnings. |
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tar operationThese keywords control warnings that may appear in any GNU tar
operation mode:
‘file name read contains nul character’
‘%s: file name read contains nul character’
‘A lone zero block at %s’. Notice, that this warning is suppressed if ‘--ignore-zeros’ is in effect (see section Ignoring Blocks of Zeros).
‘Terminating zero blocks missing at %s’. This warning is suppressed if ‘--ignore-zeros’ is in effect (see section Ignoring Blocks of Zeros).
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tar --createThe following keywords control messages that can be issued while creating archives.
‘%s: contains a cache directory tag %s; %s’
‘%s: File shrank by %s bytes; padding with zeros’
‘%s: file is on a different filesystem; not dumped’
‘%s: Unknown file type; file ignored’
‘%s: socket ignored’
‘%s: door ignored’
‘%s: file is unchanged; not dumped’
‘%s: archive cannot contain itself; not dumped’
‘%s: File removed before we read it’
‘%s: file changed as we read it’
Suppresses warnings about read failures, which can occur if files or directories are unreadable, or if they change while being read. This keyword applies only if used together with the ‘--ignore-failed-read’ option. See section Ignore Failed Read.
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tar --extractThe following keywords control warnings that can be issued during archive extraction.
‘%s: skipping existing file’
‘%s: implausibly old time stamp %s’
‘%s: time stamp %s is %s s in the future’
‘Extracting contiguous files as regular files’
‘Attempting extraction of symbolic links as hard links’, warning message ‘Attempting extraction of symbolic links as hard links’
‘%s: Unknown file type '%c', extracted as normal file’
‘Current %s is newer or same age’
‘Ignoring unknown extended header keyword '%s'’
Controls verbose description of failures occurring when trying to run alternative decompressor programs (see 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 = %lu blocks’
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These keywords control warnings that may appear when extracting from incremental archives.
‘%s: Directory has been renamed from %s’
‘%s: Directory has been renamed’
‘%s: Directory is new’
‘%s: directory is on a different device: not purging’
‘Malformed dumpdir: 'X' never used’
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These convenience keywords define warning classes. When used, they affect several warnings at once.
Enable all warning messages.
Disable all warning messages.
A shorthand for all messages enabled when ‘--verbose’
(see section The ‘--verbose’ Option) is in effect. These are:
decompress-program, existing-file, new-directory,
record-size, rename-directory.
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GNU tar default settings correspond to:
--warning=all --warning=no-verbose --warning=no-missing-zero-blocks
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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 (see section Choosing Files and Names for tar), 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.
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Certain GNU tar operations imply running external commands that you
supply on the command line. One of such operations is checkpointing,
described above (see 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
(see 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.
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tar Operations4.1 Basic GNU tar Operations | ||
4.2 Advanced GNU tar Operations | ||
| 4.3 Options Used by ‘--create’ | ||
| 4.4 Options Used by ‘--extract’ | ||
| 4.5 Backup options | ||
| 4.6 Looking Ahead: The Rest of this Manual |
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tar OperationsThe 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.
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(9). The two most common errors are:
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 :-).
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
A socket is stored, within a GNU tar archive, as a pipe.
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.
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tar OperationsNow 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.
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tar OperationsIn 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 Preparing a Practice Directory 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:
Add new entries to an archive that already exists.
Add more recent copies of archive members to the end of an archive, if they exist.
Add one or more pre-existing archives to the end of another archive.
Delete items from an archive (does not work on tapes).
Compare archive members to their counterparts in the file system.
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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’, see section Updating an Archive.)
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(10). 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. See section —occurrence, 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 Removing Archive Members Using ‘--delete’ and Tapes and Other Archive Media, for more information.)
| 4.2.2.1 Appending Files to an Archive | ||
| 4.2.2.2 Multiple Members with the Same Name |
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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. See section Tapes and Other Archive 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
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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’; See section Updating an Archive, 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
See section Changing How tar Writes Files, for more information on ‘--extract’ and
see –occurrence, for a description of
‘--occurrence’ option.
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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. See section Multiple Members with the Same Name, for a detailed discussion.
| 4.2.3.1 How to Update an Archive Using ‘--update’ |
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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 that writing to the middle of a section of tape is a difficult
process. Tapes are not designed to go backward. See section Tapes and Other Archive 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 Performing Backups and Restoring Files.
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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(11).
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:
$ 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. See section Ignoring Blocks of Zeros, for further
information on dealing with archives improperly combined using the
cat shell utility.
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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. See section Tapes and Other Archive 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.
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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 Verifying Data as It is Stored.
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The previous chapter described the basics of how to use ‘--create’ (‘-c’) to create an archive from a set of files. See section How to Create Archives. This section described advanced options to be used with ‘--create’.
| 4.3.1 Overriding File Metadata | ||
| 4.3.2 Extended File Attributes | ||
| 4.3.3 Ignore Failed Read |
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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 Controlling Extended Header Keywords, for additional ways of controlling
metadata, stored in the archive.
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 (See File permissions in GNU core utilities. 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' .
When adding files to an archive, tar uses 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
(see section Date input formats) or a name of an existing file, starting
with ‘/’ or ‘.’. In the latter case, the modification time
of that file is used.
The following example sets the modification date to 00:00:00 UTC on January 1, 1970:
$ tar -c -f archive.tar --mtime='@0' .
When used with ‘--verbose’ (see section The ‘--verbose’ Option) GNU tar
converts the specified date back to a textual form and compares it
with the one given with ‘--mtime’.
If the two forms differ, tar prints both forms in a message,
to help the user check that the right date is being used.
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 sets the
modification date to date only on files whose actual modification
date is later than date. This makes it easier to build
reproducible archives given a common timestamp for generated files
while still retaining the original timestamps of untouched files.
See section Making tar Archives More Reproducible.
$ tar -c -f archive.tar --clamp-mtime --mtime="$SOURCE_EPOCH" .
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 .
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:
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.
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.
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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:
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.
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 ‘--xattrs’ is used, all names are stored in the archive (with ‘--create’), but only ‘user’ namespace is extracted (if using ‘--extract’). The reason for this behavior is that any other, system defined attributes don’t provide us sufficient compatibility promise. Storing all attributes is safe operation for the archiving purposes. Though extracting those (often security related) attributes on a different system than originally archived can lead to extraction failures, or even misinterpretations. This behavior can be controlled using the following options:
Specify exclude pattern for extended attributes.
Specify include pattern for extended attributes.
Here, the pattern is a globbing pattern. For example, the following command:
$ tar --xattrs --xattrs-exclude='user.*' -cf a.tar .
will include in the archive ‘a.tar’ all attributes, except those from the ‘user’ namespace.
Users shall check the attributes are binary compatible with the target system before any other namespace is extracted with an explicit ‘--xattrs-include’ option.
Any number of these options can be given, thereby creating lists of include and exclude patterns.
When both options are used, first ‘--xattrs-include’ 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-include’ 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:
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).
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 sign 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:
Enable the SELinux context support.
Disable SELinux context support.
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Do not exit with nonzero if there are mild problems while reading.
This option has effect only during creation. It instructs tar to treat as mild conditions any missing or unreadable files (directories), or files that change while reading. Such failures don’t affect the program exit code, and the corresponding diagnostic messages are marked as warnings, not errors. These warnings can be suppressed using the ‘--warning=failed-read’ option (see section Controlling Warning Messages).
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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
How to Extract Members from an Archive for more basic information about the
‘--extract’ operation.
| 4.4.1 Options to Help Read Archives | ||
4.4.2 Changing How tar Writes Files | ||
| 4.4.3 Coping with Scarce Resources |
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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.
See section 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. See section The Blocking Factor of an Archive.
| Reading Full Records | ||
| Ignoring Blocks of Zeros |
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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.
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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). This option also suppresses warnings
about missing or incomplete zero blocks at the end of the archive.
This can be turned on, if the need be, using the
‘--warning=alone-zero-block --warning=missing-zero-blocks’
options (see section Controlling Warning Messages).
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.
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’.
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tar Writes Files(This message will disappear, once this node revised.)
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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.
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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 the metadata of directories when extracting files from an archive, but remove other files before extracting.
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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.
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 (see exit status).
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 (see section Controlling Warning Messages).
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Do not replace existing files that are newer than their archive copies. This option is meaningless with ‘--list’ (‘-t’).
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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.
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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.
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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’).
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’).
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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.
Set modes of extracted archive members to those recorded in the archive, instead of current umask settings. Use in conjunction with ‘--extract’ (‘--get’, ‘-x’).
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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 (see section Using tar to Perform 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:
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.
Cancel the effect of the previous ‘--delay-directory-restore’.
Use this option if you have used ‘--delay-directory-restore’ in
TAR_OPTIONS variable (see TAR_OPTIONS) and wish to
temporarily disable it.
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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.
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.
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You can instruct tar to send the contents of each extracted
file to the standard input of an external program:
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 Running External Commands,
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_FILETYPEType 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_MODEFile mode, an octal number.
TAR_FILENAMEThe name of the file.
TAR_REALNAMEName of the file as stored in the archive.
TAR_UNAMEName of the file owner.
TAR_GNAMEName of the file owner group.
TAR_ATIMETime 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_MTIMETime of last modification.
TAR_CTIMETime of last status change.
TAR_SIZESize of the file.
TAR_UIDUID of the file owner.
TAR_GIDGID of the file owner.
Additionally, the following variables contain information about tar mode and the archive being processed:
TAR_VERSIONGNU tar version number.
TAR_ARCHIVEThe name of the archive tar is processing.
TAR_BLOCKING_FACTORCurrent blocking factor (see section Blocking).
TAR_VOLUMEOrdinal number of the volume tar is processing.
TAR_FORMATFormat of the archive being processed. See section Controlling the Archive Format, 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 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.
Cancel the effect of any previous ‘--ignore-command-error’
option. This option is useful if you have set
‘--ignore-command-error’ in TAR_OPTIONS
(see TAR_OPTIONS) and wish to temporarily cancel it.
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Remove files after adding them to the archive.
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(This message will disappear, once this node revised.)
| Starting File | ||
| Same Order |
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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
Asking for Confirmation During Operations, and Excluding Some Files.
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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.
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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.
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:
Always make numbered backups.
Make numbered backups of files that already have them, simple backups of the others.
Always make simple backups.
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.
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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.
See section Reading Names from a File.
There are various ways of causing tar to skip over some files,
and not archive them. See section Choosing Files and Names for tar.
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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.
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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’) (see section Using tar to Perform 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.
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tar to Perform Incremental DumpsIncremental 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:
Handle incremental backups with snapshot data in file.
To create an incremental backup, you would use ‘--listed-incremental’ together with ‘--create’ (see section How to Create Archives). 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:
Do not rely on device numbers when preparing a list of changed files for an incremental dump.
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
(see TAR_OPTIONS).
There is also another way to cope with changing device numbers. It is described in detail in 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 (see section Extracting Specific 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, see 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(12):
$ 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’ (see section How to List Archives), as usual. To obtain more information about the archive, use ‘--listed-incremental’ or ‘--incremental’ combined with two ‘--verbose’ options(13):
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). See section 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.
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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. See section Setting Parameters for Backups and Restoration, 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. See section Using tar to Perform Incremental Dumps, before
making such an attempt.
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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. See also
Bash Features in Bash Reference Manual.
The shell variables controlling behavior of backup and
restore are described in the following subsections.
| 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’ |
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The user name of the backup administrator. Backup scripts
sends a backup report to this address.
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 (see section Using the Backup Scripts).
The device tar writes the archive to. If TAPE_FILE
is a remote archive (see remote-dev), backup script will suppose
that your mt is able to access remote devices. If RSH
(see RSH) is set, ‘--rsh-command’ option will be added to
invocations of mt.
The blocking factor tar will use when writing the dump archive.
See section The Blocking Factor of an Archive.
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.
The name of the file that contains a list of file systems to backup or restore. By default it is ‘/etc/backup/dirs’.
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.
The name of the file that contains a list of individual files to backup or restore. By default it is ‘/etc/backup/files’.
Full file name of mt binary.
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.
Full file name of rsh binary on remote machines. This will
be passed via ‘--rsh-command’ option to the remote invocation
of GNU tar.
Name of temporary file to hold volume numbers. This needs to be accessible by all the machines which have file systems to be dumped.
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.
Time to sleep between dumps of any two successive file systems
This variable affects only backup.
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 change volume prompt.
Message to display on the terminal while waiting for dump time. Usually this will just be some literal text.
Full file name of the GNU tar executable. If this is not set, backup
scripts will search tar in the current shell path.
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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:
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
}
The name of rewind function. The default definition is as follows:
MT_REWIND=mt_rewind
mt_rewind() {
mt -f "$1" rewind
}
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
}
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
}
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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:
Its arguments are:
Current backup or restore level.
Name or IP address of the host machine being dumped or restored.
Full file name of the file system being dumped or restored.
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:
Dump begin function. It is executed before dumping the file system.
Executed after dumping the file system.
Executed before restoring the file system.
Executed after restoring the file system.
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The following is an example of ‘backup-specs’:
# site-specific parameters for file system backup. ADMINISTRATOR=friedman BACKUP_HOUR=1 TAPE_FILE=/dev/nrsmt0 # Usesshinstead of the less securershRSH=/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]"
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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)(14).
The ‘--time’ option determines when should the backup be run. Time may take three forms:
The dump must be run at hh hours mm minutes.
The dump must be run at hh hours.
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 (see section Using the Restore Script).
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. See section Format of the Incremental 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:
Do backup level level (default 0).
Force backup even if today’s log file already exists.
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.
Wait till time, then do backup.
Display short help message and exit.
Display information about the program’s name, version, origin and legal status, all on standard output, and then exit successfully.
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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’ (see section BACKUP_DIRS).
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:
Restore all file systems and files specified in ‘backup-specs’.
Start restoring from the given backup level, instead of the default 0.
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.
Display short help message and exit.
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. See section Tape Positions and Tape Marks, 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.
See section Using tar to Perform Incremental Dumps, for an explanation of how the script makes
that determination.
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tarCertain 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 | Choosing the Archive’s Name | |
| 6.2 Selecting Archive Members | ||
| 6.3 Reading Names from a File | ||
| 6.4 Excluding Some Files | ||
| 6.5 Wildcards Patterns and Matching | ||
| 6.6 Quoting Member Names | Ways of Quoting Special Characters in 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 |
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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.
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.
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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. See section The Five Advanced tar 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:
Enable unquoting input file or member names (default).
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.
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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.
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 400 blocks in length(15) 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; see section Creating and Reading Compressed Archives 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(16). Only a
subset of GNU tar options is allowed for use in file lists. For
a list of such options, Position-Sensitive Options.
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 (see section 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.
6.3.1 NUL-Terminated File Names |
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NUL-Terminated File NamesThe ‘--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’.
Only consider NUL-terminated file names, instead of files that
terminate in a newline.
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 800 blocks 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.
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To avoid operating on files whose names match a particular pattern, use the ‘--exclude’ or ‘--exclude-from’ options.
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.
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:
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.:
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.
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.
Contains shell globbing-patterns and regular expressions (if prefixed with ‘RE:’(17). Patterns affect the directory and all its subdirectories.
Any line beginning with a ‘#’ is a comment.
Contains POSIX regular expressions(18). 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.
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.
Same as ‘--exclude-ignore’, except that the patterns read affect both the directory where file resides and all its subdirectories.
Exclude files and directories used by following version control systems: ‘CVS’, ‘RCS’, ‘SCCS’, ‘SVN’, ‘Arch’, ‘Bazaar’, ‘Mercurial’, and ‘Darcs’.
As of version 1.35, the following files are excluded:
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:
Do not archive the contents of the directory, but archive the directory itself and the ‘CACHEDIR.TAG’ file.
Do not archive the contents of the directory, nor the ‘CACHEDIR.TAG’ file, archive only the directory itself.
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:
Do not dump the contents of the directory, but dump the directory itself and the file.
Do not dump the contents of the directory, nor the file, archive only the directory itself.
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
Problems with Using the exclude Options |
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exclude OptionsSome users find ‘exclude’ options confusing. Here are some common pitfalls:
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.
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
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.
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.
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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 |
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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 (19) 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:
Treat all member names as 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 (see section Absolute File Names), 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’.
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.
When ignoring case, upper-case patterns match lower-case names and vice versa. When not ignoring case (the default), matching is case-sensitive.
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 (see section Descending into Directories) 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’ |
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When displaying member names, tar takes care to avoid
ambiguities caused by certain characters. This is called name
quoting. The characters in question are:
| 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 |
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 and backslash.
GNU tar offers seven distinct quoting styles, which can be selected
using ‘--quoting-style’ option:
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:
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
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'
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'
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"
Control characters are printed using backslash notation, 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
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'
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:
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:
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 (see 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.
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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
Absolute File Names.
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 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:
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:
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 The ‘s’ Command in GNU sed.
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:
Apply the replacement to all matches to the regexp, not just the first.
Use case-insensitive matching.
regexp is an extended regular expression (see Extended regular expressions in GNU sed).
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:
Apply transformation to regular archive members.
Do not apply transformation to regular archive members.
Apply transformation to symbolic link targets.
Do not apply transformation to symbolic link targets.
Apply transformation to hard link targets.
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:
$ tar --transform='s,usr/,usr/local/,' -x -f arch.tar
$ tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar
$ tar --transform 's/.*/\L&/' -x -f arch.tar
$ tar --transform 's,^,/prefix/,' -x -f arch.tar
$ 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/,'
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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 want tar to make the date comparison based only 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.
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.
Act like ‘--after-date’, but look only 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 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’ checks only mtime and
disregards ctime. Neither option uses 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’
(see section The ‘--verbose’ Option) GNU tar converts the specified
date back to a textual form and compares that with the
one given with the option. If the two forms differ, tar
prints both forms in a message, to help the user check that the right
date is being used. 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. See section Using
tarto Perform Incremental Dumps, for proper way of creating incremental backups.
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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 (see find in GNU Find Manual)
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 Reading Names from a File, for more information on using find with
tar.
Prevents tar from recursively descending directories.
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 (see Type test in Finding Files), 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 --no-recursion -T -
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 (see section 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’.
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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.
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 | Changing Directory | |
| 6.10.2 Absolute File Names |
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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.
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’ (see section Reading Names from a File), 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.
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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:
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’(20).
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.
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
See section Integrity, for some of the security-related implications of using this option.
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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 | Common rules | |
| 7.2 Calendar date items | ‘14 Nov 2022’ | |
| 7.3 Time of day items | ‘9:02pm’ | |
| 7.4 Time zone items | ‘UTC’, ‘-0700’, ‘+0900’, … | |
| 7.5 Combined date and time of day items | ‘2022-11-14T21:02:42,000000-0500’ | |
| 7.6 Day of week items | ‘Monday’ and others | |
| 7.7 Relative items in date strings | ‘next tuesday, 2 years ago’ | |
| 7.8 Pure numbers in date strings | ‘20221114’, ‘2102’ | |
| 7.9 Seconds since the Epoch | ‘@1668477762’ | |
| 7.10 Specifying time zone rules | ‘TZ="America/New_York"’, ‘TZ="UTC0"’ | |
7.11 Authors of parse_datetime | Bellovin, Eggert, Salz, Berets, et al. |
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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:
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 Tue Nov 15 02:02:42 UTC 2022 $ TZ=UTC0 date +'%Y-%m-%d %H:%M:%SZ' 2022-11-15 02:02:42Z $ date --rfc-3339=ns # --rfc-3339 is a GNU extension. 2022-11-14 21:02:42.000000000-05:00 $ date --rfc-email # a GNU extension Mon, 14 Nov 2022 21:02:42 -0500 $ date +'%Y-%m-%d %H:%M:%S %z' # %z is a GNU extension. 2022-11-14 21:02:42 -0500 $ date +'@%s.%N' # %s and %N are GNU extensions. @1668477762.692722128
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 ‘2022-02-29’ 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.
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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:
2022-11-14 # ISO 8601.
22-11-14 # Assume 19xx for 69 through 99,
# 20xx for 00 through 68 (not recommended).
11/14/2022 # Common U.S. writing.
14 November 2022
14 Nov 2022 # Three-letter abbreviations always allowed.
November 14, 2022
14-nov-2022
14nov2022
The year can also be omitted. In this case, the last specified year is used, or the current year if none. For example:
11/14 nov 14
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
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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.
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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, and ‘A’ has different meaning as a military time zone than as an obsolete RFC 822 time zone. 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, timestamps are interpreted using the rules of the default time zone (see section Specifying time zone rules).
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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:
2022-09-24T20:02:00.052-05:00 2022-12-31T23:59:59,999999999+11:00 1970-01-01 00:00Z
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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.
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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, ‘2022-12-31 -1 month’ might evaluate to 2022-12-01, because 2022-11-31 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 Dec 2022 13:02:39 -0400 $ date --date='-1 month' +'Last month was %B?' Last month was December? $ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!' Last month was November!
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.
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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 (see section 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.
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If you precede a number with ‘@’, it represents an internal timestamp 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 timestamp.
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.
Most modern systems count seconds with 64-bit two’s-complement integers of seconds with nanosecond subcounts, which is a range that includes the known lifetime of the universe with nanosecond resolution. Some obsolescent 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. A few systems sport other time ranges.
On most hosts, these counts ignore the presence of leap seconds. For example, on most hosts ‘@1483228799’ represents 2016-12-31 23:59:59 UTC, ‘@1483228800’ represents 2017-01-01 00:00:00 UTC, and there is no way to represent the intervening leap second 2016-12-31 23:59:60 UTC.
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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, 2022?” 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" 2022-10-31 06:30' Mon Oct 31 01:30:00 EDT 2022
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 ‘2022-11-14
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 2022, 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.
A recent catalog of location names appears in the
TWiki Date and Time Gateway. 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 ‘Africa/Abidjan’ to ‘Pacific/Tongatapu’, 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. The previously-mentioned POSIX rule ‘UTC0’ says that the time zone abbreviation is ‘UTC’, the zone is zero hours away from Greenwich, and there is no daylight saving time. POSIX rules can also specify nonzero Greenwich offsets. For example, the following shell transcript answers the question “What time is it five and a half hours east of Greenwich when a clock seven hours west of Greenwich shows 9:50pm on July 12, 2022?”
$ TZ="<+0530>-5:30" date --date='TZ="<-07>+7" 2022-07-12 21:50' Wed Jul 13 10:20:00 +0530 2022
This example uses the somewhat-confusing POSIX convention for rules.
‘TZ="<-07>+7"’ says that the time zone abbreviation is ‘-07’
and the time zone is 7 hours west of Greenwich, and
‘TZ="<+0530>-5:30"’ says that the time zone abbreviation is ‘+0530’
and the time zone is 5 hours 30 minutes east of Greenwich.
(One should never use a setting like ‘TZ="UTC-5"’, since
this would incorrectly imply that local time is five hours east of
Greenwich and the time zone is called “UTC”.)
Although trickier POSIX TZ settings like
‘TZ="<-05>+5<-04>,M3.2.0/2,M11.1.0/2"’ can specify some daylight
saving regimes, location-based settings like
‘TZ="America/New_York"’ are typically simpler and more accurate
historically. See Specifying the Time Zone with TZ in The GNU C Library.
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parse_datetimeparse_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 François Pinard (pinard@iro.umontreal.ca) from the ‘parse_datetime.y’ source code, and then edited by K. Berry (kb@cs.umb.edu).
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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):
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.
Format used by GNU tar of versions prior to 1.12.
Archive format, compatible with the V7 implementation of tar. This format imposes a number of limitations. The most important of them are:
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 100
bytes long will not be able to use GNU tar 1.35 and
Automake prior to 1.9.
Archive format defined by POSIX.1-1988 and later. It stores symbolic ownership information. It is also able to store special files. However, it imposes several restrictions as well:
The format used by the late Jörg Schilling’s star
implementation. GNU tar is able to read ‘star’ archives but
currently does not produce them.
The format defined by POSIX.1-2001 and later. This is the
most flexible and feature-rich format. It does not impose arbitrary
restrictions on file sizes or file name lengths. This format is more
recent, so some tar implementations cannot handle it properly.
However, any tar implementation able to read ‘ustar’
archives should be able to read most ‘posix’ archives as well,
except that it will extract any additional information (such as long
file names) as extra plain text files.
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 | 8 GiB - 1 | 99 | n/a |
| ustar | 2097151 | 8 GiB - 1 | 255 | 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, a future version will
switch to ‘posix’.
| 8.1 Using Less Space through Compression | ||
| 8.2 Handling File Attributes | ||
8.3 Making tar Archives More Portable | ||
8.4 Making tar Archives More Reproducible | ||
8.5 Comparison of tar and cpio |
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| 8.1.1 Creating and Reading Compressed Archives | ||
| 8.1.2 Archiving Sparse Files |
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GNU tar is able to create and read compressed archives. It supports
a wide variety of compression programs, namely: gzip,
bzip2, lzip, lzma, lzop,
zstd, 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(21).
Creating a compressed archive is simple: you just specify a compression option along with the usual archive creation commands. Available compression options are summarized in the table below:
| Long | Short | Archive format |
|---|---|---|
| ‘--gzip’ | ‘-z’ | gzip |
| ‘--bzip2’ | ‘-j’ | bzip2 |
| ‘--xz’ | ‘-J’ | xz |
| ‘--lzip’ | lzip | |
| ‘--lzma’ | lzma | |
| ‘--lzop’ | lzop | |
| ‘--zstd’ | zstd | |
| ‘--compress’ | ‘-Z’ | compress |
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 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
(see 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 1.35 the following alternatives are
tried(22):
| 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:
Filter the archive through gzip.
Filter the archive through xz.
Filter the archive through bzip2.
Filter the archive through lzip.
Filter the archive through lzma.
Filter the archive through lzop.
Filter the archive through zstd.
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.
See section Using lbzip2 with GNU tar., 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:
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 |
| ‘.zst’ | zstd |
| ‘.tzst’ | zstd |
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 (see section Running External Commands).
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 (see gpg —- encryption and signing tool in GNU Privacy Guard Manual). 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 .
8.1.1.1 Using lbzip2 with GNU tar. |
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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.
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.
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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.
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). See section Using tar to Perform 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.
Select the format to store sparse files in. Valid version values are: ‘0.0’, ‘0.1’ and ‘1.0’. See section Storing Sparse Files, for a detailed description of each format.
Using ‘--sparse-format’ option implies ‘--sparse’.
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:
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.
When no ‘--hole-detection’ option is given, tar uses
the ‘seek’, if supported by the operating system.
Using ‘--hole-detection’ option implies ‘--sparse’.
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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.
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
(see section Using tar to Perform 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.
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’).
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.
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.
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.
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’).
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tar Archives More PortableCreating 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 | ||
| 8.3.2 Symbolic Links | ||
| 8.3.3 Hard Links | ||
| 8.3.4 Old V7 Archives | ||
| 8.3.5 Ustar Archive Format | Ustar Archives | |
8.3.6 GNU and old GNU tar format | GNU and old GNU format archives. | |
8.3.7 GNU tar and POSIX tar | POSIX archives | |
| 8.3.8 Checksumming Problems | ||
| 8.3.9 Large or Negative Values | Large files, negative time stamps, etc. | |
8.3.10 How to Extract GNU-Specific Data Using Other tar Implementations |
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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 on
case-insensitive file systems like FAT32,
you should not rely on case distinction for file names.
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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. See section Options Controlling the Overwriting of Existing Files.
The ‘--dereference’ option is unsafe if an untrusted user can
modify directories while tar is running. See section Security.
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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 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(23).
If you wish to avoid such problems at the cost of a bigger archive,
use the following option:
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
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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 tars 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.
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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 (see section ustar, 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’).
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tar formatGNU 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’.
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tar and POSIX tarStarting 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 |
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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=patternWhen 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.
If the pattern contains shell metacharacters like ‘*’, it should
be quoted to prevent the shell from expanding the pattern before
tar sees it.
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 (see section Wildcards Patterns and Matching). For example:
--pax-option 'delete=security.*'
would suppress security-related information.
exthdr.name=stringThis 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/%f
This default helps make the archive more reproducible.
See section Making tar Archives More Reproducible. POSIX recommends using
‘%d/PaxHeaders.%p/%f’ instead, which means the two archives
created with the same set of options and containing the same set
of files will be byte-to-byte different. This default will be used
if the environment variable POSIXLY_CORRECT is set.
exthdr.mtime=valueThis 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=stringThis 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.%n
If the environment variable POSIXLY_CORRECT is set, the
following value is used instead:
$TMPDIR/GlobalHead.%p.%n
In both cases, ‘$TMPDIR’ stands for the value of the TMPDIR
environment variable. If TMPDIR is not set, tar
uses ‘/tmp’.
globexthdr.mtime=valueThis 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=valueWhen 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:=valueWhen 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 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}'
As another example, the following option helps make the archive
more reproducible. See section Making tar Archives More Reproducible.
--pax-option delete=atime
If you extract files from such an archive and recreate the archive from them, you will also need to eliminate changes due to ctime:
--pax-option 'delete=atime,delete=ctime'
Normally tar saves an mtime value with subsecond resolution
in an extended header for any file with a timestamp that is not on a
one-second boundary. This is in addition to the traditional mtime
timestamp in the header block. Although you can suppress subsecond
timestamp resolution with ‘--pax-option delete=mtime’,
this hack will not work for timestamps before 1970 or after 2242-03-16
12:56:31 UTC.
If the environment variable POSIXLY_CORRECT is set, two POSIX
archives created using the same options on the same set of files might
not be byte-to-byte equivalent even with the above options. This is
because the POSIX default for extended header names includes
the tar process ID, which typically differs at each
run. To produce byte-to-byte equivalent archives in this case, either
unset POSIXLY_CORRECT, or use the following option, which can be
combined with the above options:
--pax-option exthdr.name=%d/PaxHeaders/%f
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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.
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(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 (see section Controlling the Archive Format) will
help you to do so.
In particular, when trying to archive files 8 GiB or larger, 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 exceptions are files 8 GiB or larger, or files dated before 1970-01-01 00:00:00 or after 2242-03-16 12:56:31 UTC
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tar ImplementationsIn 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 | Members Split Between Volumes | |
| 8.3.10.2 Extracting Sparse Members | Sparse Members |
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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 (see section Concatenate Volumes into a Single Archive).
This program is available from
GNU tar home page. 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/%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, then the member names will be:
var/longfile var/GNUFileParts/longfile.1 var/GNUFileParts/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/longfile.1 \ GNUFileParts/longfile.2 >> longfile $ rm -f GNUFileParts
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/longfile: Unknown file type 'x', extracted as normal file Unexpected EOF in archive $ tar xf vol-2.tar tmp/GlobalHead.1: Unknown file type 'g', extracted as normal file GNUFileParts/PaxHeaders/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.
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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.
Let’s begin with archive members in sparse format version 1.0(24), 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(25).
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:
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/sparsefile \ /home/gray/GNUSparseFile/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 Storing Sparse Files.) 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/name’.
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:
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’.
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.)
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.
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
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tar Archives More ReproducibleSometimes it is important for an archive to be reproducible, so that one can easily verify it to have been derived solely from its input. We call an archive reproducible, if an archive created from the same set of input files with the same command line options is byte-to-byte equivalent to the original one.
However, two archives created by GNU tar from two sets of input
files normally might differ even if the input files have the same
contents and GNU tar was invoked the same way on both sets of input.
This can happen if the inputs have different modification dates or
other metadata, or if the input directories’ entries are in different orders.
To avoid this problem when creating an archive, and thus make the
archive reproducible, you can run GNU tar in the C locale with
some or all of the following options:
Omit irrelevant information about directory entry order.
Avoid problems with large files or files with unusual timestamps. This also enables ‘--pax-option’ options mentioned below.
Omit the process ID of tar.
This option is needed only if POSIXLY_CORRECT is set in the environment.
Omit irrelevant information about file access or status change time.
Omit irrelevant information about file timestamps after ‘$SOURCE_EPOCH’, which should be a time no less than any timestamp of any source file.
Omit irrelevant information about user and group names.
Omit irrelevant information about file ownership and group.
Omit irrelevant information about file permissions.
When creating a reproducible archive from version-controlled source files,
it can be useful to set each file’s modification time
to be that of its last commit, so that the timestamps
are reproducible from the version-control repository.
If these timestamps are all on integer second boundaries, and if you use
‘--format=posix --pax-option='delete=atime,delete=ctime'
--clamp-mtime --mtime="$SOURCE_EPOCH"’
where $SOURCE_EPOCH is the the time of the most recent commit,
and if all non-source files have timestamps greater than $SOURCE_EPOCH,
then GNU tar should generate an archive in ustar format,
since no POSIX features will be needed and the archive will be in the
ustar subset of posix format.
Also, if compressing, use a reproducible compression format; e.g.,
with gzip you should use the ‘--no-name’ (‘-n’) option.
Here is an example set of shell commands to produce a reproducible
tarball with git and gzip, which you can tailor to
your project’s needs.
function get_commit_time() {
TZ=UTC0 git log -1 \
--format=tformat:%cd \
--date=format:%Y-%m-%dT%H:%M:%SZ \
"$@"
}
#
# Set each source file timestamp to that of its latest commit.
git ls-files | while read -r file; do
commit_time=$(get_commit_time "$file") &&
touch -md $commit_time "$file"
done
#
# Set timestamp of each directory under $FILES
# to the latest timestamp of any descendant.
find $FILES -depth -type d -exec sh -c \
'touch -r "$0/$(ls -At "$0" | head -n 1)" "$0"' \
{} ';'
#
# Create $ARCHIVE.tgz from $FILES, pretending that
# the modification time for each newer file
# is that of the most recent commit of any source file.
SOURCE_EPOCH=$(get_commit_time)
TARFLAGS="
--sort=name --format=posix
--pax-option=exthdr.name=%d/PaxHeaders/%f
--pax-option=delete=atime,delete=ctime
--clamp-mtime --mtime=$SOURCE_EPOCH
--numeric-owner --owner=0 --group=0
--mode=go+u,go-w
"
GZIPFLAGS="--no-name --best"
LC_ALL=C tar $TARFLAGS -cf - $FILES |
gzip $GZIPFLAGS > $ARCHIVE.tgz
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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);
cpios 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
cpios, 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.
tars 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; cpios
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
cpiowas made whentarwas 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
cpiowas made whentarwas 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 tarred as sparse files (which you can
easily test, because the resulting archive gets smaller, and
GNU cpio can no longer read it).
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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 | Device selection and switching | |
| 9.2 Remote Tape Server | ||
| 9.3 Some Common Problems and their Solutions | ||
| 9.4 Blocking | ||
| 9.5 Many Archives on One Tape | Many archives on one tape | |
| 9.6 Using Multiple Tapes | ||
| 9.7 Including a Label in the Archive | ||
| 9.8 Verifying Data as It is Stored | ||
| 9.9 Write Protection |
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(This message will disappear, once this node revised.)
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 (See section —rmt-command, for detailed description of this option. See section 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>’.
Archive file is local even if it contains a colon.
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.
Specify drive and density.
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.
See section Archives Longer than One Tape or Disk.
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.
Execute command at end of each tape. This implies ‘--multi-volume’ (‘-M’). See info-script, for a detailed description of this option.
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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.
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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
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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. TheJCL‘DD RECFORM=’ parameter specified this to the operating system.The Unix man page on
tarwas totally confused about this. When I wrotePD TAR, I used the historically correct terminology (tarwrites data records, which are grouped into blocks). It appears that the bogus terminology made it into POSIX (no surprise here), and now François 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.
See section Basic Tar Format.) 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(26). 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.
| 9.4.1 Format Variations | ||
| 9.4.2 The Blocking Factor of an Archive |
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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 (see section The Blocking Factor of an Archive), you must specify that
blocking-factor when operating on the archive. See section Controlling the Archive Format, for other
examples of format parameter considerations.
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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. See section How to Create Archives, 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’).
See section How to List Archives, for more information on the ‘--list’ (‘-t’)
operation. See section Options to Help Read Archives, for a more detailed explanation of that option.
Specifies the blocking factor of an archive. Can be used with any operation, but is usually not necessary with ‘--list’ (‘-t’).
Device blocking
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:
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.
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.
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.
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.
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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. See section Blocking.
| 9.5.1 Tape Positions and Tape Marks | ||
9.5.2 The mt Utility |
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(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 (see section The mt Utility). The restore script does
that automatically (see section Using the Restore Script).
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**----------------
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mt Utility(This message will disappear, once this node revised.)
See section The Blocking Factor of an Archive.
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:
Writes number tape marks at the current position on the tape.
Moves tape position forward number files.
Moves tape position back number files.
Rewinds the tape. (Ignores number.)
Rewinds the tape and takes the tape device off-line. (Ignores number.)
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.
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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
(see section Controlling the Archive Format): ‘GNU’ and ‘POSIX’.
| 9.6.1 Archives Longer than One Tape or Disk | ||
| 9.6.2 Tape Files | ||
| 9.6.3 Concatenate Volumes into a Single Archive | ||
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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 (see section How to Create Archives). 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.
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:
Set maximum length of a volume. The suf, if given, specifies units in which size is expressed, e.g. ‘2M’ mean 2 megabytes (see Table 9.1, 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(27):
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.
Request tar to exit immediately.
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(28).
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 Including a Label in the Archive, 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:
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.
See section Running External Commands, 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_VERSIONGNU tar version number.
TAR_ARCHIVEThe name of the archive tar is processing.
TAR_BLOCKING_FACTORCurrent blocking factor (see section Blocking).
TAR_VOLUMEOrdinal number of the volume tar is about to start.
TAR_SUBCOMMANDA short option describing the operation tar is executing.
See section The Five Advanced tar Operations, for a complete list of subcommand options.
TAR_FORMATFormat of the archive being processed. See section Controlling the Archive Format, for a complete list of archive format names.
TAR_FDFile 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. See section Extracting an Entire Archive, 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’ (see section Including a Label in the Archive) 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 Extracting Members Split Between Volumes.
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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’ (see section 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. See section Including a Label in the Archive.
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 The mt Utility.
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.
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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.
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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(29) in conjunction with the ‘--create’ operation to include a label entry in the archive as it is being created.
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. See section 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(30). 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. See section Excluding Some Files, for a precise description of how match
is attempted(31). 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:
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.
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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. See section Comparing Archive Members with the File System.
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. See section The Five Advanced tar Operations, for more
information on these operations.
Also, since tar normally strips leading ‘/’ from file
names (see section Absolute File Names), 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.
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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.
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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 | ||
| 10.2 Security |
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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 | ||
| 10.1.2 Data Corruption and Repair | ||
| 10.1.3 Race conditions |
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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.
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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.
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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.
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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 | ||
| 10.2.2 Integrity | ||
| 10.2.3 Dealing with Live Untrusted Data | ||
| 10.2.4 Security Rules of Thumb |
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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.
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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.
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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.
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This section briefly summarizes rules of thumb for avoiding security pitfalls.
$ 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.
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’.
tar.
tar to detect denial-of-service attacks.
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This appendix lists some important user-visible changes between
various versions of GNU tar. An up-to-date version of this document
is available at
the GNU tar documentation page.
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.
See section Wildcards Patterns and Matching, for the detailed discussion of the use of globbing
patterns by GNU tar.
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. See section v7, for the detailed discussion of this issue and its implications.
See Changing Automake’s Behavior in GNU Automake, 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’.
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.
These options are deprecated. Please use ‘--format=v7’ instead.
This option is deprecated. Please use ‘--format=posix’ instead.
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This appendix provides several recipes for performing common tasks
using GNU tar.
| B.1 Copying directory hierarchies | ||
| B.2 Restoring Intermediate Directories |
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This is a traditional way to copy a directory hierarchy preserving
the dates, modes, owners and link-structure of all the files therein.
It was used back when the cp command lacked the ‘-a’
option:
$ (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 same command 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=-
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A common concern is how to extract permissions and ownerships of intermediate directories when extracting only selected members from the archive. To illustrate this, consider the following archive:
# tar tvf A.tar drwxr-xr-x root/root 0 2017-11-16 14:39 foo/ dr-xr-x--- gray/user 0 2017-11-16 14:39 foo/bar/ -rw-r--r-- gray/user 10 2017-11-16 14:40 foo/bar/file
Suppose you extract only the file ‘foo/bar/file’, while being ‘root’:
# tar xvf A.tar foo/bar/file foo/bar/file
Now, let’s inspect the content of the created directories:
# find foo -ls 427257 0 drwxr-xr-x 3 root root 16 Nov 17 16:10 foo 427258 0 drwxr-xr-x 2 root root 17 Nov 17 16:10 foo/bar 427259 0 -rw-r--r-- 1 gray user 10 Nov 6 14:40 foo/bar/file
The requested file is restored, including its ownership and
permissions. The intermediate directories, however, are created with
the default permissions, current timestamp and owned by the current
user. This is because by the time tar has reached the requested file,
it had already skipped the entries for its parent directories, so it
has no iformation about their ownership and modes.
To restore meta information about the intermediate directories, you’ll need to specify them explicitly in the command line and use the ‘--no-recursive’ option (see section Descending into Directories) to avoid extracting their content.
To automate this process, Neal P. Murphy proposed the following shell script(32):
#! /bin/sh
(while read path
do
path=`dirname $path`
while [ -n "$path" -a "$path" != "." ]
do
echo $path
path=`dirname $path`
done
done < $2 | sort | uniq) |
tar -x --no-recursion -v -f $1 -T - -T $2
The script takes two arguments: the name of the archive file, and the name of the file list file.
To complete our example, the file list will contain single line:
foo/bar/file
Supposing its name is ‘file.list’ and the script is named ‘restore.sh’, you can invoke it as follows:
# sh restore.sh A.tar file.list
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Running tar --help displays the short tar option
summary (see section GNU tar documentation). 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:
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.
To assign true value to a variable, simply put this variable name. To
assign false value, prefix the variable name with ‘no-’. For
example:
# Assigntruevalue: dup-args # Assignfalsevalue: no-dup-args
Following variables are declared:
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.
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.
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
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
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
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.
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.
Indentation of wrapped usage lines. Affects ‘--usage’ output. Default is 12.
Right margin of the text output. Used for wrapping.
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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 (see section Format of the Incremental 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.)
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 (see 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.
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| Basic Tar Format | ||
| GNU Extensions to the Archive Format | ||
| Storing Sparse Files | ||
| Format of the Incremental Snapshot Files | ||
| Dumpdir |
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(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 Updating an Archive.
In addition to entries describing archive members, an archive may
contain entries which tar itself uses to store information.
See section Including a Label in the Archive, 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 does not treat missing end-of-file marker as an
error and silently ignores the fact. You can instruct it to issue
a warning, however, by using the ‘--warning=missing-zero-blocks’
option (see section missing-zero-blocks).
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' /* Jörg 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.
(In the extended GNU format, the numeric fields can take
other forms.)
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; for archive
members that are symbolic or hard links to another file, this field
is specified as zero.
The mtime field represents the data modification time of the file at
the time it was archived. It represents the integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.
The chksum field represents
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 filled with spaces (ASCII 32).
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_headers to describe a sparse file.
REGTYPEAREGTYPEThese 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.
LNKTYPEThis 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.
SYMTYPEThis represents a symbolic link to another file. The linked-to name
is specified in the linkname field with a trailing null.
CHRTYPEBLKTYPEThese 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.
DIRTYPEThis 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.
FIFOTYPEThis specifies a FIFO special file. Note that the archiving of a FIFO file archives the existence of this file and not its contents.
CONTTYPEThis 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 … ZThese 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.
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(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.
For fields containing numbers or timestamps that are out of range for the basic format, the GNU format uses a base-256 representation instead of an ASCII octal number. If the leading byte is 0xff (255), all the bytes of the field (including the leading byte) are concatenated in big-endian order, with the result being a negative number expressed in two’s complement form. If the leading byte is 0x80 (128), the non-leading bytes of the field are concatenated in big-endian order, with the result being a positive number expressed in binary form. Leading bytes other than 0xff, 0x80 and ASCII octal digits are reserved for future use, as are base-256 representations of values that would be in range for the basic format.
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.
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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
Archiving Sparse Files. 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 (see section GNU tar and POSIX tar). 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.
| E.0.1 Old GNU Format | ||
| E.0.2 PAX Format, Versions 0.0 and 0.1 | ||
| E.0.3 PAX Format, Version 1.0 |
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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.
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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.sizeReal size of the stored file;
GNU.sparse.numblocksNumber of blocks in the sparse map;
GNU.sparse.offsetOffset of the data block;
GNU.sparse.numbytesSize 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:
GNU.sparse.offset and
GNU.sparse.numbytes are conflicting with the POSIX specs.
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 tars will usually ignore the
unknown variables, which makes restoring the file more
difficult. See Extraction of sparse members in v.0.0 format, for the detailed description of how to
restore such members using non-GNU tars.
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.mapMap 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. See Extraction of sparse members in v.0.1 format, for the detailed description of how to
restore such members using non-GNU tars.
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 tars.
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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.majorMajor version
GNU.sparse.minorMinor 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 decimal numbers 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 tars and tars 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.
See section Extracting Sparse Members, for the detailed information on how to extract
sparse members without GNU tar.
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A snapshot file (or directory file) is created during
incremental backups (see section Using tar to Perform 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 1.35 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 1.35, 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.
[nfs]dev inode name
where:
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.)
Device number of the directory;
I-node number of the directory;
Name of the directory. Any special characters (white-space, backslashes, etc.) are quoted.
‘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’.
GNU tar-1.35-2
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; See section 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-snapshot-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.)
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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:
filename is contained in the archive.
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.
filename is a directory.
This code requests renaming of the filename to the name specified with the ‘T’ command, that immediately follows it.
Specify target file name for ‘R’ command (see below).
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:
R‘source’\0T‘dest’\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:
a b c
‘a’ became ‘b’ ‘b’ became ‘c’ ‘c’ became ‘a’
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 (see section Controlling the Archive Format):
In PAX archives, dumpdir is stored in the extended header of the
corresponding directory, in variable GNU.dumpdir.
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.
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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 four basic operation modes:
This is the default mode. In this mode, genfile
generates data files.
In this mode genfile displays status of specified files.
Set last access and modification times of files given in the command line.
In this mode genfile executes the given program with
‘--checkpoint’ option and executes a set of actions when
specified checkpoints are reached.
| F.1 Generate Mode | File Generation Mode. | |
| F.2 Status Mode | File Status Mode. | |
| F.3 Set File Time | Set File Time Mode. | |
| F.4 Exec Mode | Synchronous Execution Mode. |
| [ << ] | [ < ] | [ Up ] | [ > ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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:
The default pattern as described above.
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:
The number is expressed in kilobytes.
The number is expressed in megabytes.
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 (see section –pattern) 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.
| [ << ] | [ < ] | [ Up ] | [ > ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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:
The file name.
Device number in decimal.
Inode 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 ‘.’.
Number of hard links.
User ID of owner.
Group ID of owner.
File size in decimal.
The size in bytes of each file block.
Number of blocks allocated.
Time of last access.
Time of last modification
Time of last status change
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 *
By default, genfile follows symbolic links and returns
information about files pointed to by them. To get information about
the symlink files themselves, use the ‘--no-dereference’
(‘-h’) option.
| [ << ] | [ < ] | [ Up ] | [ > ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
This mode is requested by the ‘--set-time’ (‘-t’)
command line option. In this mode genfile operates
similarly to the touch command: for each file listed in the
command line, it sets its access and modification times to the current
timestamp or to the value given with the ‘--date’ option. The
‘--date’ option takes a date specification in
an almost arbitrary format as its argument (see section Date input formats), e.g.:
genfile --set-time --date='2 days ago' a b c
By default, genfile follows symbolic links and sets
times of the file they point to. This can be changed by supplying the
‘--no-dereference’ (‘-h’) option: if it is given,
genfile will change access and modification times of the
symbolic link itself. Notice, that not all operating systems allow
this.
| [ << ] | [ < ] | [ Up ] | [ > ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
This mode is designed for testing the behavior of paxutils
commands when some of the files change during archiving. It supposes
that the command being executed supports ‘--checkpoint’ and
‘--checkpoint-action’ options (see Checkpoints in GNU tar).
The ‘Exec Mode’ is enabled by ‘--run’ command line
option (or its alias ‘-r’). The non-optional arguments
supply the command line to be executed. Genfile modifies
this command line by inserting the following options between the
command name and first argument:
--checkpoint=n --checkpoint-action "echo=genfile checkpoint %u" --checkpoint-action "wait=SIGUSR1"
Here, n stands for the checkpoint granularity (for GNU
tar, it is the number of archive records read or written
between each pair of checkpoints). The default value is 1. This
value can be changed using the optional argument to the ‘--run’
option. For example, to run actions on each 10th checkpoint:
genfile --run=10 ...
If the command line contains options, it must be preceded by a
double-dash (‘--’), which will prevent these options from being
interpreted by genfile itself. For example:
genfile --run --checkpoint=2 --truncate foo -- tar -c -f a.tar .
Notice also, that when running tar, its command line may
not contain traditional options (cluster of letters without dash).
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
Truncate file to the size specified by previous ‘--length’ option (or 0, if it is not given).
Append data to file. The size of data and its pattern are given by previous ‘--length’ and ‘pattern’ options.
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 (see section Date input formats).
Modifies the action of the ‘--touch’ option. If both
options are given and file argument to the ‘--touch’
names a symbolic link, genfile will modify access and
modification times of the symbolic link file itself, instead the
file the symlink points to.
Execute given shell command.
Delete the named file or directory. If deleting the directory, it must be empty.
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.
In exec mode, genfile exits with the exit status of the
executed command.
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Version 1.3, 3 November 2008
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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
Short Options Cross Reference.
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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.
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.
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.
Before GNU tar version 1.11.6,
a bug prevented intermixing old style options with long options in
some cases.
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. See section Changes, for more information.
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/*
There are plans to merge the cpio and
tar packages into a single one which would be called
paxutils. So, who knows, one of these days
‘--version’ might output ‘tar (GNU paxutils) 3.2’.
This is actually a shortcut for ‘--checkpoint=n --checkpoint-action=dot’. See section dot.
This is well described in Unix-haters Handbook, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG Books, ISBN 1-56884-203-1.
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.
This can cause multiple members to have the same name. For information on how this affects reading the archive, see Multiple Members with the Same Name.
Notice, that since both archives were created without ‘-P’ option (see section Absolute File Names), these commands should be run from the root file system.
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.
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.
A file system block is usually 512 bytes, so this amounts to 200K. Use the ‘c’ suffix to specify size in bytes. Also, when using GNU find, you can specify other size units, such as ‘k’, ‘m’, etc. See Size in GNU Findutils, for details.
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.
According to the Bazaar docs,
globbing-patterns are Korn-shell style and regular expressions are
perl-style. As of GNU tar version 1.35, these are
treated as shell-style globs and POSIX extended regexps. This will be
fixed in future releases.
Support for perl-style regexps will appear in future releases.
Notice that earlier GNU tar versions used
globbing for inclusion members, which contradicted to UNIX98
specification and was not documented. See section Changes, for more
information on this and other changes.
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. See listing member and file names, for the information on how to handle this case.
It also had patent problems in the past.
To verbosely trace the decompressor selection, use the ‘--warning=decompress-program’ option (see section decompress-program).
There are plans to fix this in future releases.
See section PAX Format, Version 1.0.
Technically speaking, n is a
process ID of the tar process which created the
archive (see section Controlling Extended Header Keywords).
If this message is not needed, you can turn it off using the ‘--warning=no-record-size’ option.
If you run GNU tar under a different locale, the
translation to the locale’s language will be used.
See --restrict, for more information about this option.
Until version 1.10, that option was called ‘--volume’, but is not available under that name anymore.
Note that GNU tar versions up to 1.23 indicated
mismatch with an exit code 2 and printed a spurious diagnostics on
stderr.
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.
The original version of the script can be seen at http://lists.gnu.org/archive/html/bug-tar/2016-11/msg00024.html
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tar Operations and Optionstar
tartar Optionstar Options
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