This manual is for GNU tar (version 1.15.92, 26 June 2006), which creates and extracts files from archives.
Copyright © 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001, 2003, 2004, 2005, 2006 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 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, 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 are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom.”
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.
Appendices
--- The Detailed Node Listing ---
Introduction
Tutorial Introduction to tar
Two Frequently Used Options
How to Create Archives
How to List Archives
How to Extract Members from an Archive
Invoking GNU tar
The Three Option Styles
All tar Options
GNU tar Operations
Advanced GNU tar Operations
How to Add Files to Existing Archives: --append
Updating an Archive
Options Used by --create
Options Used by --extract
Options to Help Read Archives
Changing How tar Writes Files
Coping with Scarce Resources
Performing Backups and Restoring Files
Setting Parameters for Backups and Restoration
Choosing Files and Names for tar
Reading Names from a File
Excluding Some Files
Wildcards Patterns and Matching
Crossing File System Boundaries
Date input formats
Controlling the Archive Format
Making tar Archives More Portable
GNU tar and POSIX tar
How to Extract GNU-Specific Data Using Other tar Implementations
Using Less Space through Compression
Tapes and Other Archive Media
Blocking
Many Archives on One Tape
Using Multiple Tapes
Tar Internals
Storing Sparse Files
Genfile
Copying This Manual
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.
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 Tutorial) which provides a gentle introduction for people who are new to using tar. It is meant to be self contained, not requiring any reading from subsequent chapters to make sense. It moves from topic to topic in a logical, progressive order, building on information already explained.
Although the tutorial is paced and structured to allow beginners to learn how to use tar, it is not intended solely for beginners. The tutorial explains how to use the three most frequently used operations (‘create’, ‘list’, and ‘extract’) as well as two frequently used options (‘file’ and ‘verbose’). The other chapters do not refer to the tutorial frequently; however, if a section discusses something which is a complex variant of a basic concept, there may be a cross reference to that basic concept. (The entire book, including the tutorial, assumes that the reader understands some basic concepts of using a Unix-type operating system; see Tutorial.)
The third chapter presents the remaining five operations, and information about using tar options and option syntax.
The other chapters are meant to be used as a reference. Each chapter presents everything that needs to be said about a specific topic.
One of the chapters (see Date input formats) exists in its entirety in other GNU manuals, and is mostly self-contained. In addition, one section of this manual (see Standard) contains a big quote which is taken directly from tar sources.
In general, we give both long and short (abbreviated) option names at least once in each section where the relevant option is covered, so that novice readers will become familiar with both styles. (A few options have no short versions, and the relevant sections will indicate this.)
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.
The tar program provides the ability to create tar archives, as well as various other kinds of manipulation. For example, you can use tar on previously created archives to extract files, to store additional files, or to update or list files which were already stored.
Initially, tar archives were used to store files conveniently on
magnetic tape. The name tar comes from this use; it stands for
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.
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!)
Conventionally, tar archives are given names ending with ‘.tar’. This is not necessary for tar to operate properly, but this manual follows that convention in order to accustom readers to it and to make examples more clear.
Often, people refer to tar archives as “tar files,” and archive members as “files” or “entries”. For people familiar with the operation of tar, this causes no difficulty. However, in this manual, we consistently refer to “archives” and “archive members” to make learning to use tar easier for novice users.
GNU tar was originally written by John Gilmore, and modified by many people. The GNU enhancements were written by Jay Fenlason, then Joy Kendall, and the whole package has been further maintained by Thomas Bushnell, n/BSG, 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 1.15.92 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 Backups.
In July, 2003 GNU tar was put on CVS at savannah.gnu.org (see http://savannah.gnu.org/projects/tar), and active development and maintenance work has started again. Currently GNU tar is being maintained by Paul Eggert, Sergey Poznyakoff and Jeff Bailey.
Support for POSIX archives was added by Sergey Poznyakoff.
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. .
This chapter guides you through some basic examples of three tar operations: --create, --list, and --extract. If you already know how to use some other version of tar, then you may not need to read this chapter. This chapter omits most complicated details about how tar works.
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:
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’.
tar can take a wide variety of arguments which specify and define the actions it will have on the particular set of files or the archive. The main types of arguments to tar fall into one of two classes: operations, and options.
Some arguments fall into a class called operations; exactly one of these is both allowed and required for any instance of using tar; you may not specify more than one. People sometimes speak of operating modes. You are in a particular operating mode when you have specified the operation which specifies it; there are eight operations in total, and thus there are eight operating modes.
The other arguments fall into the class known as options. You are not required to specify any options, and you are allowed to specify more than one at a time (depending on the way you are using tar at that time). Some options are used so frequently, and are so useful for helping you type commands more carefully that they are effectively “required”. We will discuss them in this chapter.
You can write most of the tar operations and options in any of three forms: long (mnemonic) form, short form, and old style. Some of the operations and options have no short or “old” forms; however, the operations and options which we will cover in this tutorial have corresponding abbreviations. We will indicate those abbreviations appropriately to get you used to seeing them. (Note that the “old style” option forms exist in GNU tar for compatibility with Unix tar. In this book we present a full discussion of this way of writing options and operations (see Old Options), and we discuss the other two styles of writing options (See Long Options, and see Short Options).
In the examples and in the text of this tutorial, we usually use the long forms of operations and options; but the “short” forms produce the same result and can make typing long tar commands easier. For example, instead of typing
tar --create --verbose --file=afiles.tar apple angst aspic
you can type
tar -c -v -f afiles.tar apple angst aspic
or even
tar -cvf afiles.tar apple angst aspic
For more information on option syntax, see Advanced tar. In discussions in the text, when we name an option by its long form, we also give the corresponding short option in parentheses.
The term, “option”, can be confusing at times, since “operations” are often lumped in with the actual, optional “options” in certain general class statements. For example, we just talked about “short and long forms of options and operations”. However, experienced tar users often refer to these by shorthand terms such as, “short and long options”. This term assumes that the “operations” are included, also. Context will help you determine which definition of “options” to use.
Similarly, the term “command” can be confusing, as it is often used in two different ways. People sometimes refer to tar “commands”. A tar command is the entire command line of user input which tells tar what to do — including the operation, options, and any arguments (file names, pipes, other commands, etc). However, you will also sometimes hear the term “the tar command”. When the word “command” is used specifically like this, a person is usually referring to the tar operation, not the whole line. Again, use context to figure out which of the meanings the speaker intends.
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.
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.)
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 the 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 defaults). If there is no tape drive attached, or the default is not meaningful, then tar will print an error message. The error message might look roughly like one of the following:
tar: can't open /dev/rmt8 : No such device or address
tar: can't open /dev/rsmt0 : I/O error
To avoid confusion, we recommend that you always specify an archive file name by using --file=archive-name (-f archive-name) when writing your tar commands. For more information on using the --file=archive-name (-f archive-name) option, see file.
--verbose (-v) shows details about the results of running tar. This can be especially useful when the results might not be obvious. For example, if you want to see the progress of tar as it writes files into the archive, you can use the --verbose option. In the beginning, you may find it useful to use --verbose at all times; when you are more accustomed to tar, you will likely want to use it at certain times but not at others. We will use --verbose at times to help make something clear, and we will give many examples both using and not using --verbose to show the differences.
Each instance of --verbose on the command line increases the verbosity level by one, so if you need more details on the output, specify it twice.
When reading archives (--list, --extract, --diff), tar by default prints only the names of the members being extracted. Using --verbose will show a full, ls style member listing.
In contrast, when writing archives (--create, --append, --update), tar does not print file names by default. So, a single --verbose option shows the file names being added to the archive, while two --verbose options enable the full listing.
For example, to create an archive in verbose mode:
$ tar -cvf afiles.tar apple angst aspic
apple
angst
aspic
Creating the same archive with the verbosity level 2 could give:
$ tar -cvvf afiles.tar apple angst aspic
-rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
-rw-r--r-- gray/staff 11481 2006-06-09 12:06 angst
-rw-r--r-- gray/staff 23152 2006-06-09 12:06 aspic
This works equally well using short or long forms of options. Using long forms, you would simply write out the mnemonic form of the option twice, like this:
$ tar --create --verbose --verbose ...
Note that you must double the hyphens properly each time.
Later in the tutorial, we will give examples using --verbose --verbose.
The full output consists of six fields:
Depending on the file type, the name can be followed by some additional information, described in the following table:
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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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.
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 path 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 backup, for the information on how to do so). To add files to an existing archive, you need to use a different option, such as --append (-r); see append for information on how to do this.
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. You could also say:
$ tar blues --create folk --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 short create, for more information on this.
In this example, you type the command as shown above: --create is the operation which creates the new archive (collection.tar), and --file is the option which lets you give it the name you chose. The files, blues, folk, and jazz, are now members of the archive, collection.tar (they are file name arguments to the --create operation. See Choosing, for the detailed discussion on these.) Now that they are in the archive, they are called archive members, not files. (see 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 append.
If you include the --verbose (-v) option on the command line,
tar will list the files it is acting on as it is working. In
verbose mode, the create example above would appear as:
$ tar --create --verbose --file=collection.tar blues folk jazz
blues
folk
jazz
This example is just like the example we showed which did not use --verbose, except that tar generated the remaining lines
In the rest of the examples in this chapter, we will frequently use
verbose mode so we can show actions or tar responses that
you would otherwise not see, and which are important for you to
understand.
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 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 blues --create folk --file=collection.tar jazz
is confusing as it is. When shown using short forms, however, it becomes much more so:
$ tar blues -c folk -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.
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 absolute. (Note also that collection.tar, the original archive file, has itself been archived. tar will accept any file as a file to be archived, regardless of its content. When music.tar is extracted, the archive file collection.tar will be re-written into the file system).
If you give tar a command such as
$ tar --create --file=foo.tar .
tar will report ‘tar: ./foo.tar is the archive; not dumped’. This happens because tar creates the archive foo.tar in the current directory before putting any files into it. Then, when tar attempts to add all the files in the directory . to the archive, it notices that the file ./foo.tar is the same as the archive foo.tar, and skips it. (It makes no sense to put an archive into itself.) GNU tar will continue in this case, and create the archive normally, except for the exclusion of that one file. (Please note: Other implementations of tar may not be so clever; they will enter an infinite loop when this happens, so you should not depend on this behavior unless you are certain you are running GNU tar. In general, it is wise to always place the archive outside of the directory being dumped.
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, 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
The archive bfiles.tar would list as follows:
./birds
baboon
./box
Be sure to use a --file=archive-name (-f archive-name) option just as with --create (-c) to specify the name of the archive.
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 absolute, for more information). In other words, in verbose mode GNU tar shows file names when creating an archive and member names when listing it. Consider this example:
$ tar cfv archive /etc/mail
tar: Removing leading `/' from member names
/etc/mail/
/etc/mail/sendmail.cf
/etc/mail/aliases
$ tar tf archive
etc/mail/
etc/mail/sendmail.cf
etc/mail/aliases
This default behavior can sometimes be inconvenient. You can force GNU tar show member names when creating archive by supplying --show-stored-names option.
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=afiles.tar apple would only print apple.
Because tar preserves paths, file names must be specified as they appear in the archive (i.e., relative to the directory from which the archive was created). Therefore, it is essential when specifying member names to tar that you give the exact member names. For example, tar --list --file=bfiles.tar birds would produce an error message something like ‘tar: birds: Not found in archive’, because there is no member named birds, only one named ./birds. While the names birds and ./birds name the same file, member names by default are compared verbatim.
However, tar --list --file=bfiles.tar baboon would respond with baboon, because this exact member name is in the archive file bfiles.tar. If you are not sure of the exact file name, use globbing patterns, for example:
$ tar --list --file=bfiles.tar --wildcards '*b*'
will list all members whose name contains ‘b’. See wildcards, for a detailed discussion of globbing patterns and related tar command line options.
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.
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-- 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
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. tar --extract --file=bfiles.tar birds will fail, because there is no member named birds. To extract the member named ./birds, you must specify tar --extract --file=bfiles.tar ./birds. If you don't remember the exact member names, use --list (-t) option (see list). You can also extract those members that match a specific globbing pattern. For example, to extract from bfiles.tar all files that begin with ‘b’, no matter their directory prefix, you could type:
$ tar -x -f bfiles.tar --wildcards --no-anchored 'b*'
Here, --wildcards instructs tar to treat command line arguments as globbing patterns and --no-anchored informs it that the patterns apply to member names after any ‘/’ delimiter. The use of globbing patterns is discussed in detail in See wildcards.
You can extract a file to standard output by combining the above options with the --to-stdout (-O) option (see Writing to Standard Output).
If you give the --verbose option, then --extract will print the names of the archive members as it extracts them.
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 Writing).
However, if a file was stored with a directory name as part of its file name, and that directory does not exist under the working directory when the file is extracted, tar will create the directory.
We can demonstrate how to use --extract to extract a directory file with an example. Change to the practice directory if you weren't there, and remove the files folk and jazz. Then, go back to the parent directory and extract the archive music.tar. You may either extract the entire archive, or you may extract only the files you just deleted. To extract the entire archive, don't give any file names as arguments after the archive name music.tar. To extract only the files you deleted, use the following command:
$ tar -xvf music.tar practice/folk practice/jazz
practice/folk
practice/jazz
If you were to specify two --verbose (-v) options, tar would have displayed more detail about the extracted files, as shown in the example below:
$ tar -xvvf music.tar practice/folk practice/jazz
-rw-r--r-- me user 28 1996-10-18 16:31 practice/jazz
-rw-r--r-- me user 20 1996-09-23 16:44 practice/folk
Because you created the directory with practice as part of the file names of each of the files by archiving the practice directory as practice, you must give practice as part of the file names when you extract those files from the archive.
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).
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/folk
practice/jazz
practice/rock
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.
(This message will disappear, once this node revised.)
This chapter is about how one invokes the GNU tar command, from the command synopsis (see Synopsis). There are numerous options, and many styles for writing them. One mandatory option specifies the operation tar should perform (see Operation Summary), other options are meant to detail how this operation should be performed (see Option Summary). Non-option arguments are not always interpreted the same way, depending on what the operation is.
You will find in this chapter everything about option styles and rules for writing them (see 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 help), --verbose (see verbose) and --interactive options (see interactive).
The GNU tar program is invoked as either one of:
tar option... [name]...
tar letter... [argument]... [option]... [name]...
The second form is for when old options are being used.
You can use tar to store files in an archive, to extract them from an archive, and to do other types of archive manipulation. The primary argument to tar, which is called the operation, specifies which action to take. The other arguments to tar are either options, which change the way tar performs an operation, or file names or archive members, which specify the files or members tar is to act on.
You can actually type in arguments in any order, even if in this manual the options always precede the other arguments, to make examples easier to understand. Further, the option stating the main operation mode (the tar main command) is usually given first.
Each name in the synopsis above is interpreted as an archive member name when the main command is one of --compare (--diff, -d), --delete, --extract (--get, -x), --list (-t) or --update (-u). When naming archive members, you must give the exact name of the member in the archive, as it is printed by --list. For --append (-r) and --create (-c), these name arguments specify the names of either files or directory hierarchies to place in the archive. These files or hierarchies should already exist in the file system, prior to the execution of the tar command.
tar interprets relative file names as being relative to the working directory. tar will make all file names relative (by removing leading slashes when archiving or restoring files), unless you specify otherwise (using the --absolute-names option). See absolute, for more information about --absolute-names.
If you give the name of a directory as either a file name or a member name, then tar acts recursively on all the files and directories beneath that directory. For example, the name / identifies all the files in the file system to tar.
The distinction between file names and archive member names is especially important when shell globbing is used, and sometimes a source of confusion for newcomers. See wildcards, for more information about globbing. The problem is that shells may only glob using existing files in the file system. Only tar itself may glob on archive members, so when needed, you must ensure that wildcard characters reach tar without being interpreted by the shell first. Using a backslash before ‘*’ or ‘?’, or putting the whole argument between quotes, is usually sufficient for this.
Even if names are often specified on the command line, they can also be read from a text file in the file system, using the --files-from=file-of-names (-T file-of-names) option.
If you don't use any file name arguments, --append (-r), --delete and --concatenate (--catenate, -A) will do nothing, while --create (-c) will usually yield a diagnostic and inhibit tar execution. The other operations of tar (--list, --extract, --compare, and --update) will act on the entire contents of the archive.
Besides successful exits, GNU tar may fail for
many reasons. Some reasons correspond to bad usage, that is, when the
tar command is improperly written. Errors may be
encountered later, while encountering an error 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 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.
GNU tar returns only a few exit statuses. I'm really aiming simplicity in that area, for now. If you are not using the --compare --diff, -d) option, zero means that everything went well, besides maybe innocuous warnings. Nonzero means that something went wrong. Right now, as of today, “nonzero” is almost always 2, except for remote operations, where it may be 128.
GNU tar has a total of eight operating modes which allow you to perform a variety of tasks. You are required to choose one operating mode each time you employ the tar program by specifying one, and only one operation as an argument to the tar command (two lists of four operations each may be found at frequent operations and 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 All Options.)
The TAR_OPTIONS environment variable specifies default options to
be placed in front of any explicit options. For example, if
TAR_OPTIONS is ‘-v --unlink-first’, tar behaves as
if the two options -v and --unlink-first had been
specified before any explicit options. Option specifications are
separated by whitespace. A backslash escapes the next character, so it
can be used to specify an option containing whitespace or a backslash.
Note that tar options are case sensitive. For example, the options -T and -t are different; the first requires an argument for stating the name of a file providing a list of names, while the second does not require an argument and is another way to write --list (-t).
In addition to the eight operations, there are many options to tar, and three different styles for writing both: long (mnemonic) form, short form, and old style. These styles are discussed below. Both the options and the operations can be written in any of these three styles.
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. (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.) 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.
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 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.
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 argument2.
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.
(This message will disappear, once this node revised.)
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 them3. 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.
On the other hand, this old style syntax makes 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.
Old options are kept for compatibility with old versions of tar.
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
As far as we know, all tar programs, GNU and non-GNU, support old options. GNU tar supports them not only for historical reasons, but also because many people are used to them. For compatibility with Unix tar, the first argument is always treated as containing command and option letters even if it doesn't start with ‘-’. Thus, ‘tar c’ is equivalent to ‘tar -c’: both of them specify the --create (-c) command to create an archive.
All three styles may be intermixed in a single tar command, so long as the rules for each style are fully respected4. 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.
The coming manual sections contain an alphabetical listing of all tar operations and options, with brief descriptions and cross references to more in-depth explanations in the body of the manual. They also contain an alphabetically arranged table of the short option forms with their corresponding long option. You can use this table as a reference for deciphering tar commands in scripts.
--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 system does not support --atime-preserve=system, you might be able to preserve access times reliably by 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.
bzip2. See gzip.
See Formats, for a detailed discussion of these formats.
Also see the comments for the --owner=user option.
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 the future releases.
See Changes, for more information.
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.
This option does not affect extraction from archives.
$ 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 transform.
To see transformed member names in verbose listings, use --show-transformed-names option (see show-transformed-names).
literal, shell, shell-always, c,
escape, locale, and clocale. Default quoting
style is escape, unless overridden while configuring the
package.
$ tar --show-defaults
--format=gnu -f- -b20 --quoting-style=escape \
--rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
tar --extract --file archive.tar --strip-components=2
would extract this file to file name.
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.
|
| -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 creating, –no-same-owner, when extracting —
–portability.
The later usage is deprecated. It is retained for compatibility with the earlier versions of GNU tar. In the future releases -o will be equivalent to --no-same-owner only. |
| -p | –preserve-permissions.
|
| -r | –append.
|
| -s | –same-order.
|
| -t | –list.
|
| -u | –update.
|
| -v | –verbose.
|
| -w | –interactive.
|
| -x | –extract.
|
| -z | –gzip.
|
Being careful, the first thing is really checking that you are using GNU tar, indeed. The --version option causes tar to print information about its name, version, origin and legal status, all on standard output, and then exit successfully. For example, ‘tar --version’ might print:
tar (GNU tar) 1.15.92
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software. You may redistribute copies of it under the terms
of the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
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 contains8.
Another thing you might want to do is checking the spelling or meaning of some particular tar option, without resorting to this manual, for once you have carefully read it. GNU tar has a short help feature, triggerable through the --help option. By using this option, tar will print a usage message listing all available options on standard output, then exit successfully, without doing anything else and ignoring all other options. Even if this is only a brief summary, it may be several screens long. So, if you are not using some kind of scrollable window, you might prefer to use something like:
$ tar --help | less
presuming, here, that you like using less for a pager. Other popular pagers are more and pg. If you know about some keyword which interests you and do not want to read all the --help output, another common idiom is doing:
tar --help | grep keyword
for getting only the pertinent lines. Notice, however, that some tar options have long description lines and the above command will list only the first of them.
The exact look of the option summary displayed by tar --help is configurable. See 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 option without accompanying explanations.
The short help output is quite succinct, and you might have to get back to the full documentation for precise points. If you are reading this paragraph, you already have the tar manual in some form. This manual is available in a variety of forms from http://www.gnu.org/software/tar/manual. It may be printed out of the GNU tar distribution, provided you have TeX already installed somewhere, and a laser printer around. Just configure the distribution, execute the command ‘make dvi’, then print doc/tar.dvi the usual way (contact your local guru to know how). If GNU tar has been conveniently installed at your place, this manual is also available in interactive, hypertextual form as an Info file. Just call ‘info tar’ or, if you do not have the info program handy, use the Info reader provided within GNU Emacs, calling ‘tar’ from the main Info menu.
There is currently no man page for GNU tar.
If you observe such a man page on the system you are running,
either it does not belong to GNU tar, or it has not
been produced by GNU. Some package maintainers convert
tar --help output to a man page, using help2man. In
any case, please bear in mind that the authoritative source of
information about GNU tar is this Texinfo documentation.
GNU tar has some predefined defaults that are used when you do not explicitely 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 Formats), it uses standard output as the archive, if no --file option has been given (see file tutorial), the default blocking factor is 20 (see Blocking Factor). It also shows the default locations where tar will look for rmt and rsh binaries.
Typically, tar performs most operations without reporting any information to the user except error messages. When using tar with many options, particularly ones with complicated or difficult-to-predict behavior, it is possible to make serious mistakes. tar provides several options that make observing tar easier. These options cause tar to print information as it progresses in its job, and you might want to use them just for being more careful about what is going on, or merely for entertaining yourself. If you have encountered a problem when operating on an archive, however, you may need more information than just an error message in order to solve the problem. The following options can be helpful diagnostic tools.
Normally, the --list (-t) command to list an archive prints just the file names (one per line) and the other commands are silent. When used with most operations, the --verbose (-v) option causes tar to print the name of each file or archive member as it is processed. This and the other options which make tar print status information can be useful in monitoring tar.
With --create or --extract, --verbose used once just prints the names of the files or members as they are processed. Using it twice causes tar to print a longer listing (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 cfv -’, or even ‘tar cv’—if the installer let standard output be the default archive). In that case tar writes verbose output to the standard error stream.
If --index-file=file is specified, tar sends verbose output to file rather than to standard output or standard error.
The --totals option causes tar to print on the standard error the total amount of bytes transferred when processing an archive. When creating or appending to an archive, this option prints the number of bytes written to the archive and the average speed at which they have been written, e.g.:
$ tar -c -f archive.tar --totals /home
Total bytes written: 7924664320 (7.4GiB, 85MiB/s)
When reading an archive, this option displays the number of bytes read:
$ tar -x -f archive.tar --totals
Total bytes read: 7924664320 (7.4GiB, 95MiB/s)
Finally, when deleting from an archive, the --totals option displays both numbers plus number of bytes removed from the archive:
$ tar --delete -f foo.tar --totals --wildcards '*~'
Total bytes read: 9543680 (9.2MiB, 201MiB/s)
Total bytes written: 3829760 (3.7MiB, 81MiB/s)
Total bytes deleted: 1474048
You can also obtain this information on request. When --totals is used with an argument, this argument is interpreted as a symbolic name of a signal, upon delivery of which the statistics is to be printed:
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. For example:
$ tar -c --checkpoint=.1000 /var
...
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 if 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 backup.
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 Choosing), or by performing an operation interactively, using the --interactive (-w) option. tar also accepts --confirmation for this option.
When the --interactive (-w) option is specified, before reading, writing, or deleting files, tar first prints a message for each such file, telling what operation it intends to take, then asks for confirmation on the terminal. The actions which require confirmation include adding a file to the archive, extracting a file from the archive, deleting a file from the archive, and deleting a file from disk. To confirm the action, you must type a line of input beginning with ‘y’. If your input line begins with anything other than ‘y’, tar skips that file.
If tar is reading the archive from the standard input, tar opens the file /dev/tty to support the interactive communications.
Verbose output is normally sent to standard output, separate from
other error messages. However, if the archive is produced directly
on standard output, then verbose output is mixed with errors on
stderr. Producing the archive on standard output may be used
as a way to avoid using disk space, when the archive is soon to be
consumed by another process reading it, say. Some people felt the need
of producing an archive on stdout, still willing to segregate between
verbose output and error output. A possible approach would be using a
named pipe to receive the archive, and having the consumer process to
read from that named pipe. This has the advantage of letting standard
output free to receive verbose output, all separate from errors.
The basic tar operations, --create (-c), --list (-t) and --extract (--get, -x), are currently presented and described in the tutorial chapter of this manual. This section provides some complementary notes for these operations.
create 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 catastrophical 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 cfT empty-archive.tar /dev/null
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.
Now that you have learned the basics of using GNU tar, you may want to learn about further ways in which tar can help you.
This chapter presents five, more advanced operations which you probably won't use on a daily basis, but which serve more specialized functions. We also explain the different styles of options and why you might want to use one or another, or a combination of them in your tar commands. Additionally, this chapter includes options which allow you to define the output from tar more carefully, and provide help and error correction in special circumstances.
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In the last chapter, you learned about the first three operations to tar. This chapter presents the remaining five operations to tar: --append, --update, --concatenate, --delete, and --compare.
You are not likely to use these operations as frequently as those covered in the last chapter; however, since they perform specialized functions, they are quite useful when you do need to use them. We will give examples using the same directory and files that you created in the last chapter. As you may recall, the directory is called practice, the files are ‘jazz’, ‘blues’, ‘folk’, ‘rock’, 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 prepare for examples. (Below in this section, we will remind you of the state of the examples where the last chapter left them.)
The five operations that we will cover in this chapter are:
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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 update.)
If you use --append to add a file that has the same name as an archive member to an archive containing that archive member, then the old member is not deleted. What does happen, however, is somewhat complex. tar allows you to have infinite number of files with the same name. Some operations treat these same-named members no differently than any other set of archive members: for example, if you view an archive with --list (-t), you will see all of those members listed, with their data modification times, owners, etc.
Other operations don't deal with these members as perfectly as you might prefer; if you were to use --extract to extract the archive, only the most recently added copy of a member with the same name as four 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 this10. 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 —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 delete and Media, for more information.)
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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 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
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 update, for more information. We describe this use of --append here for the sake of completeness.) When you extract the archive, the older version will be effectively lost. This works because files are extracted from an archive in the order in which they were archived. Thus, when the archive is extracted, a file archived later in time will replace a file of the same name which was archived earlier, even though the older version of the file will remain in the archive unless you delete all versions of the file.
Supposing you change the file blues and then append the changed version to collection.tar. As you saw above, the original blues is in the archive collection.tar. If you change the file and append the new version of the file to the archive, there will be two copies in the archive. When you extract the archive, the older version of the file will be extracted first, and then replaced by the newer version when it is extracted.
You can append the new, changed copy of the file blues to the archive in this way:
$ tar --append --verbose --file=collection.tar blues
blues
Because you specified the --verbose option, tar has printed the name of the file being appended as it was acted on. Now list the contents of the archive:
$ tar --list --verbose --file=collection.tar
-rw-r--r-- me user 28 1996-10-18 16:31 jazz
-rw-r--r-- me user 21 1996-09-23 16:44 blues
-rw-r--r-- me user 20 1996-09-23 16:44 folk
-rw-r--r-- me user 20 1996-09-23 16:44 rock
-rw-r--r-- me user 58 1996-10-24 18:30 blues
The newest version of blues is now at the end of the archive (note the different creation dates and file sizes). If you extract the archive, the older version of the file blues will be replaced by the newer version. You can confirm this by extracting the archive and running ‘ls’ on the directory.
If you wish to extract the first occurrence of the file blues from the archive, use --occurrence option, as shown in the following example:
$ tar --extract -vv --occurrence --file=collection.tar blues
-rw-r--r-- me user 21 1996-09-23 16:44 blues
See Writing, for more information on --extract and See –occurrence, for the 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 multiple, for a detailed discussion.
You must use file name arguments with the --update (-u) operation. If you don't specify any files, tar won't act on any files and won't tell you that it didn't do anything (which may end up confusing you).
To see the --update option at work, create a new file, classical, in your practice directory, and some extra text to the file blues, using any text editor. Then invoke tar with the ‘update’ operation and the --verbose (-v) option specified, using the names of all the files in the practice directory as file name arguments:
$ tar --update -v -f collection.tar blues folk rock classical
blues
classical
$
Because we have specified verbose mode, tar prints out the names of the files it is working on, which in this case are the names of the files that needed to be updated. If you run ‘tar --list’ and look at the archive, you will see blues and classical at its end. There will be a total of two versions of the member ‘blues’; the one at the end will be newer and larger, since you added text before updating it.
(The reason tar does not overwrite the older file when updating it is because writing to the middle of a section of tape is a difficult process. Tapes are not designed to go backward. See 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 Backups.
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:
$ cd ..
$ tar --concatenate --file=bluesrock.tar jazzfolk.tar
If you now list the contents of the bluesrock.tar, you will see that now it also contains the archive members of jazzfolk.tar:
$ tar --list --file=bluesrock.tar
blues
rock
folk
jazz
When you use --concatenate, the source and target archives must already exist and must have been created using compatible format parameters. Notice, that tar does not check whether the archives it concatenates have compatible formats, it does not even check if the files are really tar archives.
Like --append (-r), this operation cannot be performed on some tape drives, due to deficiencies in the formats those tape drives use.
It may seem more intuitive to you to want or try to use cat to concatenate two archives instead of using the --concatenate operation; after all, cat is the utility for combining files.
However, tar archives incorporate an end-of-file marker which must be removed if the concatenated archives are to be read properly as one archive. --concatenate removes the end-of-archive marker from the target archive before each new archive is appended. If you use cat to combine the archives, the result will not be a valid tar format archive. If you need to retrieve files from an archive that was added to using the cat utility, use the --ignore-zeros (-i) option. See Ignore 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 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 later goal, See verify.
The previous chapter described the basics of how to use --create (-c) to create an archive from a set of files. See create. This section described advanced options to be used with --create.
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 PAX keywords, for additional ways of controlling metadata, stored in the archive.
$ tar -c -f archive.tar --mode='a+rw' .
The following example will set the modification date to 00:00:00 UTC, January 1, 1970:
$ tar -c -f archive.tar --mtime='1970-01-01' .
When used with --verbose (see verbose tutorial) GNU tar will try to convert the specified date back to its textual representation and compare it with the one given with --mtime options. If the two dates differ, tar will print a warning saying what date it will use. This is to help user ensure he is using the right date.
For example:
$ tar -c -f archive.tar -v --mtime=yesterday .
tar: Option --mtime: Treating date `yesterday' as 2006-06-20
13:06:29.152478
...
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 .
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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 extract for more basic information about the --extract operation.
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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 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 Blocking Factor.
Normally, tar stops reading when it encounters a block of zeros between file entries (which usually indicates the end of the archive). --ignore-zeros (-i) allows tar to completely read an archive which contains a block of zeros before the end (i.e., a damaged archive, or one that was created by concatenating several archives together).
The --ignore-zeros (-i) option is turned off by default because many versions of tar write garbage after the end-of-archive entry, since that part of the media is never supposed to be read. GNU tar does not write after the end of an archive, but seeks to maintain compatiblity among archiving utilities.
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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.
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.
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.
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.
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).
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.
After sucessfully 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 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 remebers all restored directories, and restores their meta-data only after the entire archive has been processed. Notice, that you do not need to specity 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 delay-directory-restore command line option:
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.
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
Hovewer, --to-command may be more convenient for use with multiple files. See the next section.
You can instruct tar to send the contents of each extracted file to the standard input of an external program:
sh -c. 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:
| f | Regular file
|
| d | Directory
|
| l | Symbolic link
|
| h | Hard link
|
| b | Block device
|
| c | Character device
|
Currently only regular files are supported.
In addition to these variables, TAR_VERSION contains the GNU tar version number.
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:
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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 restart the same tar operation. In this case, --starting-file is not necessary. See Incremental Dumps, See interactive, and exclude.)
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.
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 has having similar names through the side-effect of name truncation. (This is true only if we have a good scheme for truncated backup names, which I'm not sure at all: I suspect work is needed in this area.) 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.
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:
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You can easily use archive files to transport a group of files from one system to another: put all relevant files into an archive on one computer system, transfer the archive to another system, and extract the contents there. The basic transfer medium might be magnetic tape, Internet FTP, or even electronic mail (though you must encode the archive with uuencode in order to transport it properly by mail). Both machines do not have to use the same operating system, as long as they both support the tar program.
For example, here is how you might copy a directory's contents from one disk to another, while preserving the dates, modes, owners and link-structure of all the files therein. In this case, the transfer medium is a pipe, which is one a Unix redirection mechanism:
$ (cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)
You can avoid subshells by using -C option:
$ tar -C sourcedir -cf - . | tar -C targetdir -xf -
The command also works using short option forms:
$ (cd sourcedir; tar --create --file=- . ) \
| (cd targetdir; tar --extract --file=-)
# Or:
$ tar --directory sourcedir --create --file=- . ) \
| tar --directory targetdir --extract --file=-
This is one of the easiest methods to transfer a tar archive.
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 files.
There are various ways of causing tar to skip over some files, and not archive them. See Choosing.
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GNU tar is distributed along with the scripts which the Free Software Foundation uses for performing backups. There is no corresponding scripts available yet for doing restoration of files. 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 at these places:
http://www.cs.umd.edu/projects/amanda/amanda.html
ftp://ftp.cs.umd.edu/pub/amanda
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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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 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.
Incremental backup is a special form of GNU tar archive that stores additional metadata so that exact state of the file system can be restored when extracting the archive.
GNU tar currently offers two options for handling incremental backups: --listed-incremental=snapshot-file (-g snapshot-file) and --incremental (-G).
The option --listed-incremental instructs tar to operate on an incremental archive with additional metadata stored in a standalone file, called a snapshot file. The purpose of this file is to help determine which files have been changed, added or deleted since the last backup, so that the next incremental backup will contain only modified files. The name of the snapshot file is given as an argument to the option:
To create an incremental backup, you would use --listed-incremental together with --create (see create). For example:
$ tar --create \
--file=archive.1.tar \
--listed-incremental=/var/log/usr.snar \
/usr
This will create in archive.1.tar an incremental backup of the /usr file system, storing additional metadata in the file /var/log/usr.snar. If this file does not exist, it will be created. The created archive will then be a level 0 backup; please see the next section for more on backup levels.
Otherwise, if the file /var/log/usr.snar exists, it determines which files are modified. In this case only these files will be stored in the archive. Suppose, for example, that after running the above command, you delete file /usr/doc/old and create directory /usr/local/db with the following contents:
$ ls /usr/local/db
/usr/local/db/data
/usr/local/db/index
Some time later you create another incremental backup. You will then see:
$ tar --create \
--file=archive.2.tar \
--listed-incremental=/var/log/usr.snar \
/usr
tar: usr/local/db: Directory is new
usr/local/db/
usr/local/db/data
usr/local/db/index
The created archive archive.2.tar will contain only these three members. This archive is called a level 1 backup. Notice that /var/log/usr.snar will be updated with the new data, so if you plan to create more ‘level 1’ backups, it is necessary to create a working copy of the snapshot file before running tar. The above example will then be modified as follows:
$ cp /var/log/usr.snar /var/log/usr.snar-1
$ tar --create \
--file=archive.2.tar \
--listed-incremental=/var/log/usr.snar-1 \
/usr
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 is supposed to be a non-volatile value. 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 considers 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.
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 extracting files). In this case, tar does not need to access snapshot file, since all the data necessary for extraction are stored in the archive itself. So, when extracting, you can give whatever argument to --listed-incremental, the usual practice is to use --listed-incremental=/dev/null. Alternatively, you can use --incremental, which needs no arguments. In general, --incremental (-G) can be used as a shortcut for --listed-incremental when listing or extracting incremental backups (for more information, regarding this option, 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 do12:
$ 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 list), as usual. To obtain more information about the archive, use --listed-incremental or --incremental combined with two --verbose options13:
tar --list --incremental --verbose --verbose 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 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.
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 Backup Parameters, for its detailed description. Once the backup parameters are set, you can perform backups or restoration by running the appropriate script.
The name of the backup script is backup. The name of the
restore script is restore. The following sections describe
their use in detail.
Please Note: The backup and restoration scripts are designed to be used together. While it is possible to restore files by hand from an archive which was created using a backup script, and to create an archive by hand which could then be extracted using the restore script, it is easier to use the scripts. See Incremental Dumps, before making such an attempt.
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.
The shell variables controlling behavior of backup and
restore are described in the following subsections.
The user name of the backup administrator.
Backupscripts 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 Scripted Backups).
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 Blocking Factor.
A list of file systems to be dumped (for
backup), or restored (forrestore). 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
DIRLISTvariable.
A path to the file containing the list of the 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 (forrestore). These should be accessible from the machine on which the backup script is run.If the list of file systems 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
FILELISTvariable.
A path to the file containing the list of the individual files to backup or restore. By default it is /etc/backup/files.
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 mashines. 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.
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 }
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:
- level
- Current backup or restore level.
- host
- Name or IP address of the host machine being dumped or restored.
- fs
- Full path name to the file system being dumped or restored.
- fsname
- File system name with directory separators replaced with colons. This is useful, e.g., for creating unique files.
Following variables keep the names of user hook functions
The following is an example of backup-specs:
# site-specific parameters for file system backup.
ADMINISTRATOR=friedman
BACKUP_HOUR=1
TAPE_FILE=/dev/nrsmt0
# Use ssh instead of the less secure rsh
RSH=/usr/bin/ssh
RSH_COMMAND=/usr/bin/ssh
# Override MT_STATUS function:
my_status() {
mts -t $TAPE_FILE
}
MT_STATUS=my_status
# Disable MT_OFFLINE function
MT_OFFLINE=:
BLOCKING=124
BACKUP_DIRS="
albert:/fs/fsf
apple-gunkies:/gd
albert:/fs/gd2
albert:/fs/gp
geech:/usr/jla
churchy:/usr/roland
albert:/
albert:/usr
apple-gunkies:/
apple-gunkies:/usr
gnu:/hack
gnu:/u
apple-gunkies:/com/mailer/gnu
apple-gunkies:/com/archive/gnu"
BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
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:
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 Scripted Restoration).
The backup scripts write two files on the file system. The first is a record file in /etc/tar-backup/, which is used by the scripts to store and retrieve information about which files were dumped. This file is not meant to be read by humans, and should not be deleted by them. See 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:
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 BACKUP_DIRS).
You may select the file systems (and/or files) to restore by
giving restore 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:
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 Tape Positioning, for a discussion of tape positioning.
Warning: The script will delete files from the active file system if they were not in the file system when the archive was made.
See Incremental Dumps, for an explanation of how the script makes that determination.
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Certain options to tar enable you to specify a name for your archive. Other options let you decide which files to include or exclude from the archive, based on when or whether files were modified, whether the file names do or don't match specified patterns, or whether files are in specified directories.
This chapter discusses these options in detail.
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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.
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 complete the remote connection, if possible, and prompt you for a username and password. If you use --file=@hostname:/dev/file-name, tar will complete the remote connection, if possible, 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, were 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.
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 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:
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.
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.
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 can not use
both --files-from=- and --file=- (-f -) in the same
command.
Any number of -T options can be given in the command line.
The following example shows how to use find to generate a list of files smaller than 400K in length and put that list into a file called small-files. You can then use the -T option to tar to specify the files from that file, small-files, to create the archive little.tgz. (The -z option to tar compresses the archive with gzip; see gzip for more information.)
$ find . -size -400 -print > small-files
$ tar -c -v -z -T small-files -f little.tgz
In the file list given by -T option, any file name beginning with ‘-’ character is considered a tar option and is processed accordingly.15 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
Notice that the option parsing algorithm used with -T is stricter than the one used by shell. Namely, when specifying option arguments, you should observe the following rules:
-Cdir.
--directory=dir.
--directory
dir
and
-C
dir
If you happen to have a file whose name starts with ‘-’,
precede it with --add-file option to prevent it from
being recognized as an option. For example: --add-file=--my-file.
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.
NUL terminated file names, instead of files that
terminate in a newline.
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.
This example shows how to use find to generate a list of files
larger than 800K in length and put that list into a file called
long-files. The -print0 option to find is just
like -print, except that it separates files with a NUL
rather than with a newline. You can then run tar with both the
--null and -T options to specify that tar get 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
(This message will disappear, once this node revised.)
To avoid operating on files whose names match a particular pattern, use the --exclude or --exclude-from options.
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.
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.
When creating an archive, the --exclude-caches option 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.
exclude OptionsSome users find ‘exclude’ options confusing. Here are some common pitfalls:
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.
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.
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 16 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:
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 cancelled 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 absolute), patterns and names are used as-is. For example, trailing ‘/’ is not trimmed from a user-specified name before deciding whether to exclude it.
However, this matching procedure can be altered by the options listed below. These options accumulate. For example:
--ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
ignores case when excluding ‘makefile’, but not when excluding ‘readme’.
The --recursion and --no-recursion options (see recurse) also affect how member patterns are interpreted. If recursion is in effect, a pattern matches a name if it matches any of the name's parent directories.
The following table summarizes pattern-matching default values:
| Members | Default settings
|
|---|---|
| Inclusion | --no-wildcards --anchored --no-wildcards-match-slash
|
| Exclusion | --wildcards --no-anchored --wildcards-match-slash
|
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, space and backslash. Using this quoting style, control characters are represented as listed in column ‘Character’ in the above table, a space is printed as ‘\ ’ and a backslash as ‘\\’.
GNU tar offers seven distinct quoting styles, which can be selected using --quoting-style option:
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:
$ tar tf arch.tar --quoting-style=literal
./
./a space
./a'single'quote
./a"double"quote
./a\backslash
./a tab
./a
newline
$ tar tf arch.tar --quoting-style=shell
./
'./a space'
'./a'\''single'\''quote'
'./a"double"quote'
'./a\backslash'
'./a tab'
'./a
newline'
$ tar tf arch.tar --quoting-style=shell-always
'./'
'./a space'
'./a'\''single'\''quote'
'./a"double"quote'
'./a\backslash'
'./a tab'
'./a
newline'
$ tar tf arch.tar --quoting-style=c
"./"
"./a space"
"./a'single'quote"
"./a\"double\"quote"
"./a\\backslash"
"./a\ttab"
"./a\nnewline"
$ tar tf arch.tar --quoting-style=escape
./
./a space
./a'single'quote
./a"double"quote
./a\\backslash
./a\ttab
./a\nnewline
For example:
$ tar tf arch.tar --quoting-style=locale
`./'
`./a space'
`./a\'single\'quote'
`./a"double"quote'
`./a\\backslash'
`./a\ttab'
`./a\nnewline'
$ 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:
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:
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.
Tar archives contain detailed information about files stored in them and full file names are part of that information. When storing file to an archive, its file name is recorded in the archive 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.
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 two options for these needs.
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 to the above invocation --verbose (-v) option, 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:
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 is stdlib.h 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:
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.
Supported flags are:
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 the interaction is defined to be: ignore matches before the numberth, and then match and replace all matches from the numberth on.
Any delimiter can be used in lieue 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/\//-/.
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,^,/prefix/,' -x -f arch.tar
$ tar --transform 's/.*/\L&/' -x -f arch.tar
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.
(This message will disappear, once this node revised.)
The --after-date=date (--newer=date, -N date) option causes tar to only work on files whose data modification or status change times are newer than the date given. If date starts with ‘/’ or ‘.’, it is taken to be a file name; the data modification time of that file is used as the date. If you use this option when creating or appending to an archive, the archive will only include new files. If you use --after-date when extracting an archive, tar will only extract files newer than the date you specify.
If you only want tar to make the date comparison based on modification of the file's data (rather than status changes), then use the --newer-mtime=date option.
You may use these options with any operation. Note that these options differ from the --update (-u) operation in that they allow you to specify a particular date against which tar can compare when deciding whether or not to archive the files.
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.
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 only checks mtime and
disregards ctime. Neither does it use atime (the last time the
contents of the file were looked at).
Date specifiers can have embedded spaces. Because of this, you may need to quote date arguments to keep the shell from parsing them as separate arguments. For example, the following command will add to the archive all the files modified less than two days ago:
$ tar -cf foo.tar --newer-mtime '2 days ago'
When any of these options is used with the option --verbose (see verbose tutorial) GNU tar will try to convert the specified date back to its textual representation and compare that with the one given with the option. If the two dates differ, tar will print a warning saying what date it will use. This is to help user ensure he is using the right date. For example:
$ tar -c -f archive.tar --after-date='10 days ago' .
tar: Option --after-date: Treating date `10 days ago' as 2006-06-11
13:19:37.232434
Please Note: --after-date and --newer-mtime should not be used for incremental backups. See Incremental Dumps, for proper way of creating incremental backups.
(This message will disappear, once this node revised.)
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 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 files, for more information on using find with tar, or look.
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), as they usually do not want all the files in a directory. They then use the --files-from option to archive the files located via find.
The problem when restoring files archived in this manner is that the directories themselves are not in the archive; so the --same-permissions (--preserve-permissions, -p) option does not affect them—while users might really like it to. Specifying --no-recursion is a way to tell tar to grab only the directory entries given to it, adding no new files on its own. To summarize, if you use find to create a list of files to be stored in an archive, use it as follows:
$ find dir tests | \
tar -cf archive -T - --no-recursion
The --no-recursion option also applies when extracting: it causes tar to extract only the matched directory entries, not the files under those directories.
The --no-recursion option also affects how globbing patterns are interpreted (see 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.
(This message will disappear, once this node revised.)
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.
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.
(This message will disappear, once this node revised.)
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.
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 orange-colored.
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 files), you can put various tar options (including -C) in the file list. Notice, however, that in this case the option and its argument may not be separated by whitespace. If you use short option, its argument must either follow the option letter immediately, without any intervening whitespace, or occupy the next line. Otherwise, if you use long option, separate its argument by an equal sign.
For instance, the file list for the above example will be:
-C
/etc
passwd
hosts
-C
/lib
libc.a
To use it, you would invoke tar as follows:
$ tar -c -f foo.tar --files-from list
Notice also that you can only use the short option variant in the file list, i.e., always use -C, not --directory.
The interpretation of --directory is disabled by --null option.
(This message will disappear, once this node revised.)
By default, GNU tar drops a leading ‘/’ on input or output, and complains about file names containing a .. component. This option turns off this behavior.
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.17
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.
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:
$ (cd / && tar -c -f archive.tar home)
# or:
$ tar -c -f archive.tar -C / home
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
get_date function) is not described here.
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
Mon Mar 1 00:21:42 UTC 2004
$ TZ=UTC0 date +'%Y-%m-%d %H:%M:%SZ'
2004-03-01 00:21:42Z
$ date --iso-8601=ns | tr T ' ' # --iso-8601 is a GNU extension.
2004-02-29 16:21:42,692722128-0800
$ date --rfc-2822 # a GNU extension
Sun, 29 Feb 2004 16:21:42 -0800
$ date +'%Y-%m-%d %H:%M:%S %z' # %z is a GNU extension.
2004-02-29 16:21:42 -0800
$ date +'@%s.%N' # %s and %N are GNU extensions.
@1078100502.692722128
Alphabetic case is completely ignored in dates. Comments may be introduced between round parentheses, as long as included parentheses are properly nested. Hyphens not followed by a digit are currently ignored. Leading zeros on numbers are ignored.
Invalid dates like ‘2005-02-29’ or times like ‘24:00’ are rejected. In the typical case of a host that does not support leap seconds, a time like ‘23:59:60’ is rejected even if it corresponds to a valid leap second.
A calendar date item specifies a day of the year. It is specified differently, depending on whether the month is specified numerically or literally. All these strings specify the same calendar date:
1972-09-24 # iso 8601.
72-9-24 # Assume 19xx for 69 through 99,
# 20xx for 00 through 68.
72-09-24 # Leading zeros are ignored.
9/24/72 # Common U.S. writing.
24 September 1972
24 Sept 72 # September has a special abbreviation.
24 Sep 72 # Three-letter abbreviations always allowed.
Sep 24, 1972
24-sep-72
24sep72
The year can also be omitted. In this case, the last specified year is used, or the current year if none. For example:
9/24
sep 24
Here are the rules.
For numeric months, the iso 8601 format ‘year-month-day’ is allowed, where year is any positive number, month is a number between 01 and 12, and day is a number between 01 and 31. A leading zero must be present if a number is less than ten. If year is 68 or smaller, then 2000 is added to it; otherwise, if year is less than 100, then 1900 is added to it. The construct ‘month/day/year’, popular in the United States, is accepted. Also ‘month/day’, omitting the year.
Literal months may be spelled out in full: ‘January’, ‘February’, ‘March’, ‘April’, ‘May’, ‘June’, ‘July’, ‘August’, ‘September’, ‘October’, ‘November’ or ‘December’. Literal months may be abbreviated to their first three letters, possibly followed by an abbreviating dot. It is also permitted to write ‘Sept’ instead of ‘September’.
When months are written literally, the calendar date may be given as any of the following:
day month year
day month
month day year
day-month-year
Or, omitting the year:
month day
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. 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). The minute part of the time of day may not be elided when a time zone correction is used. This is the best way to specify a time zone correction by fractional parts of an hour.
Either ‘am’/‘pm’ or a time zone correction may be specified, but not both.
A time zone item specifies an international time zone, indicated by a small set of letters, e.g., ‘UTC’ or ‘Z’ for Coordinated Universal Time. Any included periods are ignored. By following a non-daylight-saving time zone by the string ‘DST’ in a separate word (that is, separated by some white space), the corresponding daylight saving time zone may be specified. Alternatively, a non-daylight-saving time zone can be followed by a time zone correction, to add the two values. This is normally done only for ‘UTC’; for example, ‘UTC+05:30’ is equivalent to ‘+05:30’.
Time zone items other than ‘UTC’ and ‘Z’ are obsolescent and are not recommended, because they are ambiguous; for example, ‘EST’ has a different meaning in Australia than in the United States. Instead, it's better to use unambiguous numeric time zone corrections like ‘-0500’, as described in the previous section.
If neither a time zone item nor a time zone correction is supplied, time stamps are interpreted using the rules of the default time zone (see Specifying time zone rules).
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.
Relative items adjust a date (or the current date if none) forward or backward. The effects of relative items accumulate. Here are some examples:
1 year
1 year ago
3 years
2 days
The unit of time displacement may be selected by the string ‘year’ or ‘month’ for moving by whole years or months. These are fuzzy units, as years and months are not all of equal duration. More precise units are ‘fortnight’ which is worth 14 days, ‘week’ worth 7 days, ‘day’ worth 24 hours, ‘hour’ worth 60 minutes, ‘minute’ or ‘min’ worth 60 seconds, and ‘second’ or ‘sec’ worth one second. An ‘s’ suffix on these units is accepted and ignored.
The unit of time may be preceded by a multiplier, given as an optionally signed number. Unsigned numbers are taken as positively signed. No number at all implies 1 for a multiplier. Following a relative item by the string ‘ago’ is equivalent to preceding the unit by a multiplier with value -1.
The string ‘tomorrow’ is worth one day in the future (equivalent to ‘day’), the string ‘yesterday’ is worth one day in the past (equivalent to ‘day ago’).
The strings ‘now’ or ‘today’ are relative items corresponding to zero-valued time displacement, these strings come from the fact a zero-valued time displacement represents the current time when not otherwise changed by previous items. They may be used to stress other items, like in ‘12:00 today’. The string ‘this’ also has the meaning of a zero-valued time displacement, but is preferred in date strings like ‘this thursday’.
When a relative item causes the resulting date to cross a boundary where the clocks were adjusted, typically for daylight saving time, the resulting date and time are adjusted accordingly.
The fuzz in units can cause problems with relative items. For example, ‘2003-07-31 -1 month’ might evaluate to 2003-07-01, because 2003-06-31 is an invalid date. To determine the previous month more reliably, you can ask for the month before the 15th of the current month. For example:
$ date -R
Thu, 31 Jul 2003 13:02:39 -0700
$ date --date='-1 month' +'Last month was %B?'
Last month was July?
$ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!'
Last month was June!
Also, take care when manipulating dates around clock changes such as daylight saving leaps. In a few cases these have added or subtracted as much as 24 hours from the clock, so it is often wise to adopt universal time by setting the TZ environment variable to ‘UTC0’ before embarking on calendrical calculations.
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 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.
If you precede a number with ‘@’, it represents an internal time stamp as a count of seconds. The number can contain an internal decimal point (either ‘.’ or ‘,’); any excess precision not supported by the internal representation is truncated toward minus infinity. Such a number cannot be combined with any other date item, as it specifies a complete time stamp.
Internally, computer times are represented as a count of seconds since an epoch—a well-defined point of time. On GNU and POSIX systems, the epoch is 1970-01-01 00:00:00 utc, so ‘@0’ represents this time, ‘@1’ represents 1970-01-01 00:00:01 utc, and so forth. GNU and most other POSIX-compliant systems support such times as an extension to POSIX, using negative counts, so that ‘@-1’ represents 1969-12-31 23:59:59 utc.
Traditional Unix systems count seconds with 32-bit two's-complement integers and can represent times from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 utc. More modern systems use 64-bit counts of seconds with nanosecond subcounts, and can represent all the times in the known lifetime of the universe to a resolution of 1 nanosecond.
On most hosts, these counts ignore the presence of leap seconds. For example, on most hosts ‘@915148799’ represents 1998-12-31 23:59:59 utc, ‘@915148800’ represents 1999-01-01 00:00:00 utc, and there is no way to represent the intervening leap second 1998-12-31 23:59:60 utc.
Normally, dates are interpreted using the rules of the current time zone, which in turn are specified by the TZ environment variable, or by a system default if TZ is not set. To specify a different set of default time zone rules that apply just to one date, start the date with a string of the form ‘TZ="rule"’. The two quote characters (‘"’) must be present in the date, and any quotes or backslashes within rule must be escaped by a backslash.
For example, with the GNU date command you can answer the question “What time is it in New York when a Paris clock shows 6:30am on October 31, 2004?” by using a date beginning with ‘TZ="Europe/Paris"’ as shown in the following shell transcript:
$ export TZ="America/New_York"
$ date --date='TZ="Europe/Paris" 2004-10-31 06:30'
Sun Oct 31 01:30:00 EDT 2004
In this example, the --date operand begins with its own TZ setting, so the rest of that operand is processed according to ‘Europe/Paris’ rules, treating the string ‘2004-10-31 06:30’ as if it were in Paris. However, since the output of the date command is processed according to the overall time zone rules, it uses New York time. (Paris was normally six hours ahead of New York in 2004, but this example refers to a brief Halloween period when the gap was five hours.)
A TZ value is a rule that typically names a location in the ‘tz’ database. 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 ‘Arctic/Longyearbyen’ to ‘Antarctica/South_Pole’, but
if you are at sea and have your own private time zone, or if you are
using a non-GNU host that does not support the ‘tz’
database, you may need to use a POSIX rule instead. Simple
POSIX rules like ‘UTC0’ specify a time zone without
daylight saving time; other rules can specify simple daylight saving
regimes. See Specifying the Time Zone with TZ.
get_date
get_date was 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.
This chapter was originally produced by François Pinard (pinard@iro.umontreal.ca) from the getdate.y source code, and then edited by K. Berry (kb@cs.umb.edu).
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):
Archives in ‘gnu’ format are able to hold pathnames of unlimited
length.
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 filenames more than 99
characters long will not be able to use GNU tar 1.15.92 and
Automake prior to 1.9.
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 | Path Name | Devn
|
|---|---|---|---|---|
| gnu | 1.8e19 | Unlimited | Unlimited | 63
|
| oldgnu | 1.8e19 | Unlimited | Unlimited | 63
|
| v7 | 2097151 | 8GB | 99 | n/a
|
| ustar | 2097151 | 8GB | 256 | 21
|
| posix | Unlimited | Unlimited | Unlimited | Unlimited
|
The default format for GNU tar is defined at compilation time. You may check it by running tar --help, and examining the last lines of its output. Usually, GNU tar is configured to create archives in ‘gnu’ format, however, future version will switch to ‘posix’.
GNU tar is able to create and read compressed archives. It supports gzip and bzip2 compression programs. For backward compatibilty, it also supports compress command, although we strongly recommend against using it, since there is a patent covering the algorithm it uses and you could be sued for patent infringement merely by running compress! Besides, it is less effective than gzip and bzip2.
Creating a compressed archive is simple: you just specify a compression option along with the usual archive creation commands. The compression option is -z (--gzip) to create a gzip compressed archive, -j (--bzip2) to create a bzip2 compressed archive, and -Z (--compress) to use compress program. For example:
$ tar cfz archive.tar.gz .
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 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 tfz -
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 (-u)) them or delete (--delete) members from them. Likewise, you cannot append another tar archive to a compressed archive using --append (-r)). Secondly, multi-volume archives cannot be compressed.
The following table summarizes compression options used by GNU tar.
You can use --gzip and --gunzip 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; if you need to override them, set GZIP environment variable, e.g.:
$ GZIP=--best tar cfz archive.tar.gz subdir
Another way would be to avoid the --gzip (--gunzip, --ungzip, -z) option and run gzip explicitly:
$ tar cf - subdir | gzip --best -c - > archive.tar.gz
About corrupted compressed archives: gzip'ed files have no redundancy, for maximum compression. 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.
There are pending suggestions for having a per-volume or per-file compression in GNU tar. This would allow for viewing the contents without decompression, and for resynchronizing decompression at every volume or file, in case of corrupted archives. Doing so, we might lose some compressibility. But this would have make recovering easier. So, there are pros and cons. We'll see!
bzip2. Otherwise like --gzip.
The GNU Project recommends you not use compress, because there is a patent covering the algorithm it uses. You could be sued for patent infringement merely by running compress.
First, when called without options, it should read data from standard input, compress it and output it on standard output.
Secondly, if called with -d argument, 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 --use-compress-program option, in particular, lets you implement your own filters, not necessarily dealing with compression/decomression. For example, suppose you wish to implement PGP encryption on top of compression, using gpg (see gpg). 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 commpressed archive signed with your private key:
$ tar -cf foo.tar.gpgz --use-compress=gpgz .
Likewise, the following command will list its contents:
$ tar -tf foo.tar.gpgz --use-compress=gpgz .
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 consecutive stretches of zeros. It then records in the archive for the file where the 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 holes created wherever the continuous stretches of zeros were found. Thus, if you use --sparse, tar archives won't take more space than the original.
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 Incremental Dumps.
However, be aware that --sparse option presents a serious drawback. Namely, in order to determine if the file is sparse tar has to read it before trying to archive it, so in total the file is read twice. So, always bear in mind that the time needed to process all files with this option is roughly twice the time needed to archive them without it.
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.
Using --sparse-format option implies --sparse.
(This message will disappear, once this node revised.)
When tar reads files, it updates their access times. To avoid this, use the --atime-preserve[=METHOD] option, which can either reset the access time retroactively or avoid changing it in the first place.
--atime-preserve=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 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.
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).
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.
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 --old-archive (-o) 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.
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).
The --preserve option has no equivalent short option name. It is equivalent to --same-permissions plus --same-order.
Creating a tar archive on a particular system that is meant to be useful later on many other machines and with other versions of tar is more challenging than you might think. tar archive formats have been evolving since the first versions of Unix. Many such formats are around, and are not always compatible with each other. This section discusses a few problems, and gives some advice about making tar archives more portable.
One golden rule is simplicity. For example, limit your tar archives to contain only regular files and directories, avoiding other kind of special files. Do not attempt to save sparse files or contiguous files as such. Let's discuss a few more problems, in turn.
Use portable file and member names. A name is portable if it contains only ASCII letters and digits, ‘/’, ‘.’, ‘_’, and ‘-’; it cannot be empty, start with ‘-’ or ‘//’, or contain ‘/-’. Avoid deep directory nesting. For portability to old Unix hosts, limit your file name components to 14 characters or less.
If you intend to have your tar archives to be read under MSDOS, you should not rely on case distinction for file names, and you might use the GNU doschk program for helping you further diagnosing illegal MSDOS names, which are even more limited than System V's.
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. --dereference (-h) is used with --create (-c), and causes tar to archive the files symbolic links point to, instead of the links themselves. When this option is used, when tar encounters a symbolic link, it will archive the linked-to file, instead of simply recording the presence of a symbolic link.
The name under which the file is stored in the file system is not recorded in the archive. To record both the symbolic link name and the file name in the system, archive the file under both names. If all links were recorded automatically by tar, an extracted file might be linked to a file name that no longer exists in the file system.
If a linked-to file is encountered again by tar while creating the same archive, an entire second copy of it will be stored. (This might be considered a bug.)
So, for portable archives, do not archive symbolic links as such, and use --dereference (-h): many systems do not support symbolic links, and moreover, your distribution might be unusable if it contains unresolved symbolic links.
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.
Archive format defined by POSIX.1-1988 specification is called
ustar. Although it is more flexible than the V7 format, it
still has many restrictions (See 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 archive in ustar format, use --format=ustar
option in conjunction with the --create (-c).
GNU tar was based on an early draft of the
POSIX 1003.1 ustar standard. GNU extensions to
tar, such as the support for file names longer than 100
characters, use portions of the tar header record which were
specified in that POSIX draft as unused. Subsequent changes in
POSIX have allocated the same parts of the header record for
other purposes. As a result, GNU tar format is
incompatible with the current POSIX specification, and with
tar programs that follow it.
In the majority of cases, tar will be configured to create this format by default. This will change in the future releases, since we plan to make ‘POSIX’ format the default.
To force creation a GNU tar archive, use option --format=gnu.
Starting from version 1.14 GNU tar features full support for POSIX.1-2001 archives.
A POSIX conformant archive will be created if tar was given --format=posix (--format=pax) option. No special option is required to read and extract from a POSIX archive.
Keyword-list is a comma-separated list of keyword options, each keyword option taking one of the following forms:
delete=patternWhen 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 wildcards). For example:
--pax-option delete=security.*
would suppress security-related information.
exthdr.name=string| Meta-character | Replaced By
|
|---|---|
| %d | The directory name of the file, equivalent to the
result of the dirname utility on the translated pathname.
|
| %f | The filename of the file, equivalent to the result
of the basename utility on the translated pathname.
|
| %p | The process ID of the tar process.
|
| %% | A ‘%’ character.
|
Any other ‘%’ characters in string produce undefined results.
If no option ‘exthdr.name=string’ is specified, tar will use the following default value:
%d/PaxHeaders.%p/%f
globexthdr.name=string| Meta-character | Replaced By
|
|---|---|
| %n | An integer that represents the
sequence number of the global extended header record in the archive,
starting at 1.
|
| %p | The process ID of the tar process.
|
| %% | A ‘%’ character.
|
Any other ‘%’ characters in string produce undefined results.
If no option ‘globexthdr.name=string’ is specified, tar will use the following default value:
$TMPDIR/GlobalHead.%p.%n
where ‘$TMPDIR’ represents the value of the TMPDIR
environment variable. If TMPDIR is not set, tar
uses ‘/tmp’.
=value:=valueWhen 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.
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 eight 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 accept any. 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 compute checksums both ways, and accept any 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 that 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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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 Formats) will help you to do so.
In particular, when trying to archive files larger than 8GB or with
timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
12:56:31 utc, you will have to chose between GNU and
POSIX archive formats. When considering which format to
choose, bear in mind that the GNU format uses
two's-complement base-256 notation to store values that do not fit
into standard ustar range. Such archives can generally be
read only by a GNU tar implementation. Moreover, they sometimes
cannot be correctly restored on another hosts even by GNU tar. For
example, using two's complement representation for negative time
stamps that assumes a signed 32-bit time_t generates archives
that are not portable to hosts with differing time_t
representations.
On the other hand, POSIX archives, generally speaking, can be extracted by any tar implementation that understands older ustar format. The only exception are files larger than 8GB.
In previous sections you became acquainted with various quircks 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.
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 Tarcat). 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-2.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 many (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 as a different file by tar implementations that are not aware of GNU extensions. More specifically, the very first part retains its original name, and all subsequent parts are named using the pattern:
%d/GNUFileParts.%p/%f.%n
where symbols preceeded 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, a file var/longfile was split during archive creation between three volumes, and the creator tar process had process ID ‘27962’, then the member names will be:
var/longfile
var/GNUFileParts.27962/longfile.1
var/GNUFileParts.27962/longfile.2
When you extract your archive using a third-party tar, these files will be created on your disk, and the only thing you will need to do to restore your file in its original form is concatenate them in the proper order, for example:
$ cd var
$ cat GNUFileParts.27962/longfile.1 \
GNUFileParts.27962/longfile.2 >> longfile
$ rm -f GNUFileParts.27962
Notice, that if the tar implementation you use supports PAX format archives, it will probably emit warnings about unknown keywords during extraction. They will lool 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 warnigns and more files generated on your disk, e.g.:
$ tar xf vol-1.tar
var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
normal file
Unexpected EOF in archive
$ tar xf vol-2.tar
tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
'x', extracted as normal file
Ignore these warnings. The PaxHeaders.* directories created will contain files with extended header keywords describing the extracted files. You can delete them, unless they describe sparse members. Read further to learn more about them.
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 bloks (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 1.018, 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 number19.
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 suite your needs, you can explicitely specify output file name as a second argument to the command:
$ xsparse cond-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 simething 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, give it -v option:
$ xsparse -v /home/gray/GNUSparseFile.6058/sparsefile
Reading v.1.0 sparse map
Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
`/home/gray/sparsefile'
Done
Additionally, if your tar implementation has extracted the extended headers for this file, you can instruct xstar to use them in order to verify the integrity of the expanded file. The option -x sets the name of the extended header file to use. Continuing our example:
$ xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
/home/gray/GNUSparseFile.6058/sparsefile
Reading extended header file
Found variable GNU.sparse.major = 1
Found variable GNU.sparse.minor = 0
Found variable GNU.sparse.name = sparsefile
Found variable GNU.sparse.realsize = 217481216
Reading v.1.0 sparse map
Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
`/home/gray/sparsefile'
Done
An extended header is a special tar archive header
that precedes an archive member and contains a set of
variables, describing the member properties that cannot be
stored in the standard ustar header. While optional for
expanding sparse version 1.0 members, 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 see Sparse Formats). So, for this format, the question is: how to obtain
extended headers from the archive?
If you use a tar implementation that does not support PAX format, extended headers for each member will be extracted as a separate file. If we represent the member name as dir/name, then the extended header file will be named dir/PaxHeaders.n/name, where n is an integer number.
Things become more difficult if your tar implementation does support PAX headers, because in this case you will have to manually extract the headers. We recommend the following algorithm:
$ 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 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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The cpio archive formats, like tar, do have maximum pathname lengths. The binary and old ASCII formats have a max path length of 256, and the new ASCII and CRC ASCII formats have a max path length of 1024. GNU cpio can read and write archives with arbitrary pathname 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 inumbers (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 cpio was made when tar was present at the unix scene,
It wasn't. cpio first showed up in PWB/UNIX 1.0; no generally-available version of UNIX had tar at the time. I don't know whether any version that was generally available within AT&T had tar, or, if so, whether the people within AT&T who did cpio knew about it.
On restore, if there is a corruption on a tape tar will stop at that point, while cpio will skip over it and try to restore the rest of the files.
The main difference is just in the command syntax and header format.
tar is a little more tape-oriented in that everything is blocked to start on a record boundary.
Is there any differences between the ability to recover crashed archives between the two of them. (Is there any chance of recovering crashed archives at all.)
Theoretically it should be easier under tar since the blocking lets you find a header with some variation of ‘dd skip=nn’. However, modern cpio's and variations have an option to just search for the next file header after an error with a reasonable chance of resyncing. However, lots of tape driver software won't allow you to continue past a media error which should be the only reason for getting out of sync unless a file changed sizes while you were writing the archive.
If anyone knows why cpio was made when tar was present at the unix scene, please tell me about this too.
Probably because it is more media efficient (by not blocking everything and using only the space needed for the headers where tar always uses 512 bytes per file header) and it knows how to archive special files.
You might want to look at the freely available alternatives. The major ones are afio, GNU tar, and pax, each of which have their own extensions with some backwards compatibility.
Sparse files were 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.
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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 rmt-command=command option (See —rmt-command, for detailed description of this option. See 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>.
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.
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 Multi-Volume Archives.
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.
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. It is Copyright © 1983 by the Regents of the University of California, but 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.
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, includingF(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 tar was totally confused about this. When I wrote
PD TAR, I used the historically correct terminology (tar writes data records, which are grouped into blocks). It appears that the bogus terminology made it into POSIX (no surprise here), and now 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 Standard.) Each file written to the archive uses at least one full record. As a result, using a larger record size can result in more wasted space for small files. On the other hand, a larger record size can often be read and written much more efficiently.
Further complicating the problem is that some tape drives ignore the blocking entirely. For these, a larger record size can still improve performance (because the software layers above the tape drive still honor the blocking), but not as dramatically as on tape drives that honor blocking.
When reading an archive, tar can usually figure out the record size on itself. When this is the case, and a non-standard record size was used when the archive was created, tar will print a message about a non-standard blocking factor, and then operate normally. 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.
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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 Blocking Factor), you must specify that blocking-factor when operating on the archive. See Formats, 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 create, for information on writing archives.
Archives with blocking factors larger than 20 cannot be read by very old versions of tar, or by some newer versions of tar running on old machines with small address spaces. With GNU tar, the blocking factor of an archive is limited only by the maximum record size of the device containing the archive, or by the amount of available virtual memory.
Also, on some systems, not using adequate blocking factors, as sometimes imposed by the device drivers, may yield unexpected diagnostics. For example, this has been reported:
Cannot write to /dev/dlt: Invalid argument
In such cases, it sometimes happen that the tar bundled by the system is aware of block size idiosyncrasies, while GNU tar requires an explicit specification for the block size, which it cannot guess. This yields some people to consider GNU tar is misbehaving, because by comparison, the bundle tar works OK. Adding -b 256, for example, might resolve the problem.
If you use a non-default blocking factor when you create an archive, you must specify the same blocking factor when you modify that archive. Some archive devices will also require you to specify the blocking factor when reading that archive, however this is not typically the case. Usually, you can use --list (-t) without specifying a blocking factor—tar reports a non-default record size and then lists the archive members as it would normally. To extract files from an archive with a non-standard blocking factor (particularly if you're not sure what the blocking factor is), you can usually use the --read-full-records (-B) option while specifying a blocking factor larger then the blocking factor of the archive (i.e. ‘tar --extract --read-full-records --blocking-factor=300’. See list, for more information on the --list (-t) operation. See Reading, for a more detailed explanation of that option.
Device blocking
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:
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:
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.
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 loosing 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.
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 Blocking.
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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 mt). The restore script does
that automatically (see Scripted Restoration).
If you want to add new archive file entries to a tape, you should advance the tape to the end of the existing file entries, backspace over the last tape mark, and write the new archive file. If you were to add two archives to the example above, the tape might look like the following:
rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
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See Blocking Factor.
You can use the mt utility to advance or rewind a tape past a specified number of archive files on the tape. This will allow you to move to the beginning of an archive before extracting or reading it, or to the end of all the archives before writing a new one.
The syntax of the mt command is:
mt [-f tapename] operation [number]
where tapename is the name of the tape device, number is the number of times an operation is performed (with a default of one), and operation is one of the following:
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.
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 arvhive, 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 untill 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 Formats): ‘GNU’ and ‘POSIX’.
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 create). A multi-volume archive can be manipulated like any other archive (provided the --multi-volume option is specified), but is stored on more than one tape or disk.
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.
$ 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:
$ tar --create --tape-length=41943040 --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 is20:
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:
(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 unexisting 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 label, it only affects the number used in the prompt.)
If you want more elaborate behavior than this, you can write a special new volume script, that will be responsible for changing the volume, and instruct tar to use it instead of its normal prompting procedure:
The script-name is executed without any command line arguments. It inherits tar's shell environment. Additional data is passed to it via the following environment variables:
The volume script can instruct tar to use new archive name, by writing in to file descriptor 3 (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 cMff /dev/tape0 /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 #3. 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/sh
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 >&3
The same script cant 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 extracting archives, for more information about extracting archives.
Multi-volume archives can be modified like any other archive. To add files to a multi-volume archive, you need to only mount the last volume of the archive media (and new volumes, if needed). For all other operations, you need to use the entire archive.
If a multi-volume archive was labeled using --label=archive-label (see label) when it was created, tar will not automatically label volumes which are added later. To label subsequent volumes, specify --label=archive-label again in conjunction with the --append, --update or --concatenate operation.
Notice that multi-volume support is a GNU extension and the archives created in this mode should be read only using GNU tar. If you absolutely have to process such archives using a third-party tar implementation, read Split Recovery.
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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 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 give. See label.
When tar writes an archive to tape, it creates a single tape file. If multiple archives are written to the same tape, one after the other, they each get written as separate tape files. When extracting, it is necessary to position the tape at the right place before running tar. To do this, use the mt command. For more information on the mt command and on the organization of tapes into a sequence of tape files, see mt.
People seem to often do:
--label="some-prefix `date +some-format`"
or such, for pushing a common date in all volumes or an archive set.
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 in conjunction with the --create operation to include a label entry in the archive as it is being created.
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 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 explicitely 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 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 a single command line argument, tar compares the volume label with the argument. It exits with code 0 if the two strings match, and with code 2 otherwise. In this case no output is displayed. For example:
$ tar --test-label --file=iamanarchive 'iamalable'
=> 0
$ tar --test-label --file=iamanarchive 'iamalable' alabel
=> 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 labelled 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 labelled 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 exclude, for a precise description of how match is attempted22. 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.
The --label was once called --volume, but is not available under that name anymore.
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 cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`"
$ tar --create --file=/dev/tape --multi-volume \
--volume="Daily backup for `date +%Y-%m-%d`"
Also note that each label has its own date and time, which corresponds to when GNU tar initially attempted to write it, often soon after the operator launches tar or types the carriage return telling that the next tape is ready. Comparing date labels does give an idea of tape throughput only if the delays for rewinding tapes and the operator switching them were negligible, which is usually not the case.
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 compare.
Note that these two options have a slightly different intent. The --compare option checks how identical are the logical contents of some archive with what is on your disks, while the --verify option is really for checking if the physical contents agree and if the recording media itself is of dependable quality. So, for the --verify operation, tar tries to defeat all in-memory cache pertaining to the archive, while it lets the speed optimization undisturbed for the --compare option. If you nevertheless use --compare for media verification, you may have to defeat the in-memory cache yourself, maybe by opening and reclosing the door latch of your recording unit, forcing some doubt in your operating system about the fact this is really the same volume as the one just written or read.
The --verify option would not be necessary if drivers were indeed able to detect dependably all write failures. This sometimes require many magnetic heads, some able to read after the writes occurred. One would not say that drivers unable to detect all cases are necessarily flawed, as long as programming is concerned.
The --verify (-W) option will not work in conjunction with the --multi-volume (-M) option or the --append (-r), --update (-u) and --delete operations. See Operations, for more information on these operations.
Also, since tar normally strips leading ‘/’ from file names (see absolute), a command like ‘tar --verify -cf /tmp/foo.tar /etc’ will work as desired only if the working directory is /, as tar uses the archive's relative member names (e.g., etc/motd) when verifying the archive.
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.
This appendix lists some important user-visible changes between version GNU tar 1.15.92 and previous versions. An up-to-date version of this document is available at the GNU tar documentation page.
$ 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 –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 wildcards, for the detailed discussion of the use of globbing
patterns by GNU tar.
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 v7, for the detailed discussion of this issue and its implications.
. See tar-v7, 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.
Running tar --help displays the short tar option summary (see help). This summary is organised 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:
variable=value
where variable is the variable name, and value is a
numeric value to be assigned to the variable.
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 ARCHIVEIf 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 ARCHIVEand 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-noteinhibits this message. Normally, only one of variablesdup-argsordup-args-noteshould 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 formatthe 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 ARCHIVENotice, that the description starts on a separate line if
opt-doc-colvalue 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.
(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 update.
In addition to entries describing archive members, an archive may contain entries which tar itself uses to store information. See label, for an example of such an archive entry.
A tar archive file contains a series of blocks. Each block
contains BLOCKSIZE bytes. Although this format may be thought
of as being on magnetic tape, other media are often used.
Each file archived is represented by a header block which describes the file, followed by zero or more blocks which give the contents of the file. At the end of the archive file there are two 512-byte blocks filled with binary zeros as an end-of-file marker. A reasonable system should write such end-of-file marker at the end of an archive, but must not assume that such a block exists when reading an archive. In particular GNU tar always issues a warning if it does not encounter it.
The blocks may be blocked for physical I/O operations. Each record of n blocks (where n is set by the --blocking-factor=512-size (-b 512-size) option to tar) is written with a single ‘write ()’ operation. On magnetic tapes, the result of such a write is a single record. When writing an archive, the last record of blocks should be written at the full size, with blocks after the zero block containing all zeros. When reading an archive, a reasonable system should properly handle an archive whose last record is shorter than the rest, or which contains garbage records after a zero block.
The header block is defined in C as follows. In the GNU tar distribution, this is part of file src/tar.h:
/* tar Header Block, from POSIX 1003.1-1990. */
/* POSIX header. */
struct posix_header
{ /* byte offset */
char name[100]; /* 0 */
char mode[8]; /* 100 */
char uid[8]; /* 108 */
char gid[8]; /* 116 */
char size[12]; /* 124 */
char mtime[12]; /* 136 */
char chksum[8]; /* 148 */
char typeflag; /* 156 */
char linkname[100]; /* 157 */
char magic[6]; /* 257 */
char version[2]; /* 263 */
char uname[32]; /* 265 */
char gname[32]; /* 297 */
char devmajor[8]; /* 329 */
char devminor[8]; /* 337 */
char prefix[155]; /* 345 */
/* 500 */
};
#define TMAGIC "ustar" /* ustar and a null */
#define TMAGLEN 6
#define TVERSION "00" /* 00 and no null */
#define TVERSLEN 2
/* Values used in typeflag field. */
#define REGTYPE '0' /* regular file */
#define AREGTYPE '\0' /* regular file */
#define LNKTYPE '1' /* link */
#define SYMTYPE '2' /* reserved */
#define CHRTYPE '3' /* character special */
#define BLKTYPE '4' /* block special */
#define DIRTYPE '5' /* directory */
#define FIFOTYPE '6' /* FIFO special */
#define CONTTYPE '7' /* reserved */
#define XHDTYPE 'x' /* Extended header referring to the
next file in the archive */
#define XGLTYPE 'g' /* Global extended header */
/* Bits used in the mode field, values in octal. */
#define TSUID 04000 /* set UID on execution */
#define TSGID 02000 /* set GID on execution */
#define TSVTX 01000 /* reserved */
/* file permissions */
#define TUREAD 00400 /* read by owner */
#define TUWRITE 00200 /* write by owner */
#define TUEXEC 00100 /* execute/search by owner */
#define TGREAD 00040 /* read by group */
#define TGWRITE 00020 /* write by group */
#define TGEXEC 00010 /* execute/search by group */
#define TOREAD 00004 /* read by other */
#define TOWRITE 00002 /* write by other */
#define TOEXEC 00001 /* execute/search by other */
/* tar Header Block, GNU extensions. */
/* In GNU tar, SYMTYPE is for to symbolic links, and CONTTYPE is for
contiguous files, so maybe disobeying the `reserved' comment in POSIX
header description. I suspect these were meant to be used this way, and
should not have really been `reserved' in the published standards. */
/* *BEWARE* *BEWARE* *BEWARE* that the following information is still
boiling, and may change. Even if the OLDGNU format description should be
accurate, the so-called GNU format is not yet fully decided. It is
surely meant to use only extensions allowed by POSIX, but the sketch
below repeats some ugliness from the OLDGNU format, which should rather
go away. Sparse files should be saved in such a way that they do *not*
require two passes at archive creation time. Huge files get some POSIX
fields to overflow, alternate solutions have to be sought for this. */
/* Descriptor for a single file hole. */
struct sparse
{ /* byte offset */
char offset[12]; /* 0 */
char numbytes[12]; /* 12 */
/* 24 */
};
/* Sparse files are not supported in POSIX ustar format. For sparse files
with a POSIX header, a GNU extra header is provided which holds overall
sparse information and a few sparse descriptors. When an old GNU header
replaces both the POSIX header and the GNU extra header, it holds some
sparse descriptors too. Whether POSIX or not, if more sparse descriptors
are still needed, they are put into as many successive sparse headers as
necessary. The following constants tell how many sparse descriptors fit
in each kind of header able to hold them. */
#define SPARSES_IN_EXTRA_HEADER 16
#define SPARSES_IN_OLDGNU_HEADER 4
#define SPARSES_IN_SPARSE_HEADER 21
/* Extension header for sparse files, used immediately after the GNU extra
header, and used only if all sparse information cannot fit into that
extra header. There might even be many such extension headers, one after
the other, until all sparse information has been recorded. */
struct sparse_header
{ /* byte offset */
struct sparse sp[SPARSES_IN_SPARSE_HEADER];
/* 0 */
char isextended; /* 504 */
/* 505 */
};
/* The old GNU format header conflicts with POSIX format in such a way that
POSIX archives may fool old GNU tar's, and POSIX tar's might well be
fooled by old GNU tar archives. An old GNU format header uses the space
used by the prefix field in a POSIX header, and cumulates information
normally found in a GNU extra header. With an old GNU tar header, we
never see any POSIX header nor GNU extra header. Supplementary sparse
headers are allowed, however. */
struct oldgnu_header
{ /* byte offset */
char unused_pad1[345]; /* 0 */
char atime[12]; /* 345 Incr. archive: atime of the file */
char ctime[12]; /* 357 Incr. archive: ctime of the file */
char offset[12]; /* 369 Multivolume archive: the offset of
the start of this volume */
char longnames[4]; /* 381 Not used */
char unused_pad2; /* 385 */
struct sparse sp[SPARSES_IN_OLDGNU_HEADER];
/* 386 */
char isextended; /* 482 Sparse file: Extension sparse header
follows */
char realsize[12]; /* 483 Sparse file: Real size*/
/* 495 */
};
/* OLDGNU_MAGIC uses both magic and version fields, which are contiguous.
Found in an archive, it indicates an old GNU header format, which will be
hopefully become obsolescent. With OLDGNU_MAGIC, uname and gname are
valid, though the header is not truly POSIX conforming. */
#define OLDGNU_MAGIC "ustar " /* 7 chars and a null */
/* The standards committee allows only capital A through capital Z for
user-defined expansion. Other letters in use include:
'A' Solaris Access Control List
'E' Solaris Extended Attribute File
'I' Inode only, as in 'star'
'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'
/* For storing filenames that do not fit into the main header. */
#define GNUTYPE_NAMES 'N'
/* 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.
The name field is the file name of the file, with directory names
(if any) preceding the file name, separated by slashes.
The mode field provides nine bits specifying file permissions
and three bits to specify the Set UID, Set GID, and Save Text
(sticky) modes. Values for these bits are defined above.
When special permissions are required to create a file with a given
mode, and the user restoring files from the archive does not hold such
permissions, the mode bit(s) specifying those special permissions
are ignored. Modes which are not supported by the operating system
restoring files from the archive will be ignored. Unsupported modes
should be faked up when creating or updating an archive; e.g., the
group permission could be copied from the other permission.
The uid and gid fields are the numeric user and group
ID of the file owners, respectively. If the operating system does
not support numeric user or group IDs, these fields should be ignored.
The size field is the size of the file in bytes; linked files
are archived with this field specified as zero.
The mtime field is the data modification time of the file at
the time it was archived. It is the ASCII representation of the octal
value of the last time the file's contents were modified, represented
as an integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.
The chksum field is the ASCII representation of the octal value
of the simple sum of all bytes in the header block. Each 8-bit
byte in the header is added to an unsigned integer, initialized to
zero, the precision of which shall be no less than seventeen bits.
When calculating the checksum, the chksum field is treated as
if it were all blanks.
The typeflag field specifies the type of file archived. If a
particular implementation does not recognize or permit the specified
type, the file will be extracted as if it were a regular file. As this
action occurs, tar issues a warning to the standard error.
The atime and ctime fields are used in making incremental
backups; they store, respectively, the particular file's access and
status change times.
The offset is used by the --multi-volume (-M) option, when
making a multi-volume archive. The offset is number of bytes into
the file that we need to restart at to continue the file on the next
tape, i.e., where we store the location that a continued file is
continued at.
The following fields were added to deal with sparse files. A file is sparse if it takes in unallocated blocks which end up being represented as zeros, i.e., no useful data. A test to see if a file is sparse is to look at the number blocks allocated for it versus the number of characters in the file; if there are fewer blocks allocated for the file than would normally be allocated for a file of that size, then the file is sparse. This is the method tar uses to detect a sparse file, and once such a file is detected, it is treated differently from non-sparse files.
Sparse files are often dbm files, or other database-type files
which have data at some points and emptiness in the greater part of
the file. Such files can appear to be very large when an ‘ls
-l’ is done on them, when in truth, there may be a very small amount
of important data contained in the file. It is thus undesirable
to have tar think that it must back up this entire file, as
great quantities of room are wasted on empty blocks, which can lead
to running out of room on a tape far earlier than is necessary.
Thus, sparse files are dealt with so that these empty blocks are
not written to the tape. Instead, what is written to the tape is a
description, of sorts, of the sparse file: where the holes are, how
big the holes are, and how much data is found at the end of the hole.
This way, the file takes up potentially far less room on the tape,
and when the file is extracted later on, it will look exactly the way
it looked beforehand. The following is a description of the fields
used to handle a sparse file:
The sp is an array of struct sparse. Each struct
sparse contains two 12-character strings which represent an offset
into the file and a number of bytes to be written at that offset.
The offset is absolute, and not relative to the offset in preceding
array element.
The header can hold four of these struct sparse at the moment;
if more are needed, they are not stored in the header.
The isextended flag is set when an extended_header
is needed to deal with a file. Note that this means that this flag
can only be set when dealing with a sparse file, and it is only set
in the event that the description of the file will not fit in the
allotted room for sparse structures in the header. In other words,
an extended_header is needed.
The extended_header structure is used for sparse files which
need more sparse structures than can fit in the header. The header can
fit 4 such structures; if more are needed, the flag isextended
gets set and the next block is an extended_header.
Each extended_header structure contains an array of 21
sparse structures, along with a similar isextended flag
that the header had. There can be an indeterminate number of such
extended_headers to describe a sparse file.
REGTYPEAREGTYPEtypeflag 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.
LNKTYPElinkname field with a trailing null.
SYMTYPElinkname field with a trailing null.
CHRTYPEBLKTYPEdevmajor 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.
DIRTYPEname 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.
FIFOTYPECONTTYPEA ... ZOther 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.
(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'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'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'GNUTYPE_VOLHDR'V'name
field contains the name given after the --label=archive-label (-V archive-label) option.
The size field is zero. Only the first file in each volume
of an archive should have this type.
You may have trouble reading a GNU format archive on a non-GNU system if the options --incremental (-G), --multi-volume (-M), --sparse (-S), or --label=archive-label (-V archive-label) were used when writing the archive. In general, if tar does not use the GNU-added fields of the header, other versions of tar should be able to read the archive. Otherwise, the tar program will give an error, the most likely one being a checksum error.
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 sparse. This chapter describes the internal format GNU tar uses to store such files.
The support for sparse files in GNU tar has a long history. The earliest version featuring this support that I was able to find was 1.09, released in November, 1990. The format introduced back then is called old GNU sparse format and in spite of the fact that its design contained many flaws, it was the only format GNU tar supported until version 1.14 (May, 2004), which introduced initial support for sparse archives in PAX archives (see posix). This format was not free from design flows, 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.
The format introduced some time around 1990 (v. 1.09). It 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 entires) File map.
|
| 504 | 1 | isextended | Bool | 1 if an
extension sparse header follows, or 0 otherwise.
|
A header with isextended=0 ends the map.
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.sizeGNU.sparse.numblocksGNU.sparse.offsetGNU.sparse.numbytesThe 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 conficting with the POSIX specs.
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.mapTo 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.
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.majorGNU.sparse.minorThe name field in ustar header contains a special name,
constructed using the following pattern:
%d/GNUSparseFile.%p/%f
The real name of the sparse file is stored in the variable
GNU.sparse.name. The real size of the file is stored in the
variable GNU.sparse.realsize.
The sparse map itself is stored in the file data block, preceding the actual file data. It consists of a series of octal numbers of arbitrary length, delimited by newlines. The map is padded with nulls to the nearest block boundary.
The first number gives the number of entries in the map. Following are map entries, each one consisting of two numbers giving the offset and size of the data block it describes.
The format is designed in such a way that non-posix aware 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 Sparse Recovery, for the detailed information on how to extract
sparse members without GNU tar.
A snapshot file (or directory file) is created during incremental backups (see Incremental Dumps). It contains the status of the filesystem at the time of the dump and is used to determine which files were modified since the last backup.
GNU tar version 1.15.92 supports two snapshot file formats. The first format, called format 0, is the one used by GNU tar versions up to 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.
‘Format 0’ snapshot file begins with a line containing a decimal number that represents the UNIX timestamp of the beginning of the last archivation. This line is followed by directory metadata descriptions, one per line. Each description has the following format:
[nfs]dev inode name
where optional nfs is a single plus character (‘+’) if this directory is located on an NFS-mounted partition, dev and inode are the device and inode numbers of the directory, and name is the directory name.
‘Format 1’ snapshot file begins with a line specifying the format of the file. This line has the following structure:
‘GNU tar-’tar-version‘-’incr-format-version
where tar-version is the version of GNU tar implementation that created this snapshot, and incr-format-version is the version number of the snapshot format (in this case ‘1’).
The following 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.
Following lines contain directory metadate, one line per directory. The line format is:
[nfs]mtime-sec mtime-nsec dev inode name
where mtime-sec and mtime-nsec represent the last modification time of this directory with nanosecond precision; nfs, dev, inode and name have the same meaning as with ‘format 0’.
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:
Codes ‘Y’, ‘N’ and ‘D’ require filename argument to be a relative file name to the directory this dumpdir describes, whereas codes ‘R’, ‘T’ and ‘X’ require their argument to be an absolute file name.
The three codes ‘R’, ‘T’ and ‘X’ specify a renaming operation. In the simplest case it is:
Rsource\0Tdest\0
which means “rename file source to file dest”.
However, there are cases that require using a temporary directory. For example, consider the following scenario:
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 existing directory. To handle such case a temporary directory is required. GNU tar will create 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 Formats):
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 preceeds a data block containing the dumpdir.
This appendix describes genfile, an auxiliary program used in the GNU tar testsuite. If you are not interested in developing GNU tar, skip this appendix.
Initially, genfile was used to generate data files for the testsuite, hence its name. However, new operation modes were being implemented as the testsuite grew more sophisticated, and now genfile is a multi-purpose instrument.
There are three basic operation modes:
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.
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:
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, i.e. a string of characters, specifying the pattern to fill the fragment with. File offset can be suffixed with the following quantifiers:
For each letter in contents string genfile will generate
a block of data, filled with this letter and will write it to
the fragment. The size of block is given by --block-size
option. It defaults to 512. Thus, if the string consists of n
characters, the resulting file fragment will contain
n*block-size of data.
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.
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 --status command is 0
only if created file is actually sparse.
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 :
--stat
mode.777 will preserve lower nine bits of it. Notice, that you can
use any punctuation caracter in place of ‘.’.
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 *
This mode is designed for testing the behavior of paxutils
commands when some of the files change during archiving. It is an
experimental mode.
The ‘Exec Mode’ is toggled by --run command line option (or its alias -r). The argument to this option gives the command line to be executed. The actual command line is constructed by inserting --checkpoint option between the command name and its first argument (if any). Due to this, the argument to --run may not use traditional tar option syntax, i.e. the following is wrong:
# Wrong!
genfile --run 'tar cf foo bar'
Use the following syntax instead:
genfile --run 'tar -cf foo bar'
The rest of command line after --run or its equivalent specifies checkpoint values and actions to be executed upon reaching them. Checkpoint values are introduced with --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
Option --verbose instructs genfile to print on standard output notifications about checkpoints being executed and to verbosely describe exit status of the command.
While the command is being executed its standard output remains connected to descriptor 1. All messages it prints to file descriptor 2, except checkpoint notifications, are forwarded to standard error.
Genfile exits with the exit status of the executed command.
The biggest deficiency in the free software community today is not in the software—it is the lack of good free documentation that we can include with the free software. Many of our most important programs do not come with free reference manuals and free introductory texts. Documentation is an essential part of any software package; when an important free software package does not come with a free manual and a free tutorial, that is a major gap. We have many such gaps today.
Consider Perl, for instance. The tutorial manuals that people normally use are non-free. How did this come about? Because the authors of those manuals published them with restrictive terms—no copying, no modification, source files not available—which exclude them from the free software world.
That wasn't the first time this sort of thing happened, and it was far from the last. Many times we have heard a GNU user eagerly describe a manual that he is writing, his intended contribution to the community, only to learn that he had ruined everything by signing a publication contract to make it non-free.
Free documentation, like free software, is a matter of freedom, not price. The problem with the non-free manual is not that publishers charge a price for printed copies—that in itself is fine. (The Free Software Foundation sells printed copies of manuals, too.) The problem is the restrictions on the use of the manual. Free manuals are available in source code form, and give you permission to copy and modify. Non-free manuals do not allow this.
The criteria of freedom for a free manual are roughly the same as for free software. Redistribution (including the normal kinds of commercial redistribution) must be permitted, so that the manual can accompany every copy of the program, both on-line and on paper.
Permission for modification of the technical content is crucial too. When people modify the software, adding or changing features, if they are conscientious they will change the manual too—so they can provide accurate and clear documentation for the modified program. A manual that leaves you no choice but to write a new manual to document a changed version of the program is not really available to our community.
Some kinds of limits on the way modification is handled are acceptable. For example, requirements to preserve the original author's copyright notice, the distribution terms, or the list of authors, are ok. It is also no problem to require modified versions to include notice that they were modified. Even entire sections that may not be deleted or changed are acceptable, as long as they deal with nontechnical topics (like this one). These kinds of restrictions are acceptable because they don't obstruct the community's normal use of the manual.
However, it must be possible to modify all the technical content of the manual, and then distribute the result in all the usual media, through all the usual channels. Otherwise, the restrictions obstruct the use of the manual, it is not free, and we need another manual to replace it.
Please spread the word about this issue. Our community continues to lose manuals to proprietary publishing. If we spread the word that free software needs free reference manuals and free tutorials, perhaps the next person who wants to contribute by writing documentation will realize, before it is too late, that only free manuals contribute to the free software community.
If you are writing documentation, please insist on publishing it under the GNU Free Documentation License or another free documentation license. Remember that this decision requires your approval—you don't have to let the publisher decide. Some commercial publishers will use a free license if you insist, but they will not propose the option; it is up to you to raise the issue and say firmly that this is what you want. If the publisher you are dealing with refuses, please try other publishers. If you're not sure whether a proposed license is free, write to licensing@gnu.org.
You can encourage commercial publishers to sell more free, copylefted manuals and tutorials by buying them, and particularly by buying copies from the publishers that paid for their writing or for major improvements. Meanwhile, try to avoid buying non-free documentation at all. Check the distribution terms of a manual before you buy it, and insist that whoever seeks your business must respect your freedom. Check the history of the book, and try reward the publishers that have paid or pay the authors to work on it.
The Free Software Foundation maintains a list of free documentation published by other publishers, at http://www.fsf.org/doc/other-free-books.html.
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This appendix contains an index of all GNU tar long command line options. The options are listed without the preceeding double-dash. For a cross-reference of short command line options, Short Option Summary.
absolute-names: absoluteabsolute-names, summary: Option Summaryadd-file: filesafter-date: afterafter-date, summary: Option Summaryanchored: controlling pattern-matchinganchored, summary: Option Summaryappend: appendappend, summary: Operation Summaryatime-preserve: Attributesatime-preserve, summary: Option Summarybackup: backupbackup, summary: Option Summaryblock-number: verboseblock-number, summary: Option Summaryblocking-factor: Blocking Factorblocking-factor, summary: Option Summarybzip2: gzipbzip2, summary: Option Summarycatenate: concatenatecatenate, summary: Operation Summarycheck-links, summary: Option Summarycheckpoint: verbosecheckpoint, summary: Option Summarycompare: comparecompare, summary: Operation Summarycompress: gzipcompress, summary: Option Summaryconcatenate: concatenateconcatenate, summary: Operation Summaryconfirmation, summary: Option Summarycreate, additional options: create optionscreate, complementary notes: Basic tarcreate, introduced: Creating the archivecreate, summary: Operation Summarycreate, using with --verbose: create verbosecreate, using with --verify: verifydelay-directory-restore: Directory Modification Times and Permissionsdelay-directory-restore, summary: Option Summarydelete: deletedelete, summary: Operation Summarydereference: dereferencedereference, summary: Option Summarydiff, summary: Operation Summarydirectory: directorydirectory, summary: Option Summarydirectory, using in --files-from argument: filesexclude: excludeexclude, potential problems with: problems with excludeexclude, summary: Option Summaryexclude-caches: excludeexclude-caches, summary: Option Summaryexclude-from: excludeexclude-from, summary: Option Summaryextract: extractextract, additional options: extract optionsextract, complementary notes: Basic tarextract, summary: Operation Summaryextract, using with --listed-incremental: Incremental Dumpsfile, short description: filefile, summary: Option Summaryfile, tutorial: file tutorialfiles-from: filesfiles-from, summary: Option Summaryforce-local, short description: Deviceforce-local, summary: Option Summaryformat, summary: Option Summaryget, summary: Operation Summarygroup: overridegroup, summary: Option Summarygunzip, summary: Option Summarygzip: gzipgzip, summary: Option Summaryhelp: help tutorialhelp, introduction: helphelp, summary: Option Summaryignore-case: controlling pattern-matchingignore-case, summary: Option Summaryignore-command-error: Writing to an External Programignore-command-error, summary: Option Summaryignore-failed-read: Ignore Failed Readignore-failed-read, summary: Option Summaryignore-zeros: Ignore Zerosignore-zeros, short description: Blocking Factorignore-zeros, summary: Option Summaryincremental, summary: Option Summaryincremental, using with --list: Footnotesindex-file, summary: Option Summaryinfo-script: Multi-Volume Archivesinfo-script, short description: Deviceinfo-script, summary: Option Summaryinteractive: interactiveinteractive, summary: Option Summarykeep-newer-files: Keep Newer Fileskeep-newer-files, summary: Option Summarykeep-old-files: Keep Old Fileskeep-old-files, introduced: Dealing with Old Fileskeep-old-files, summary: Option Summarylabel: labellabel, summary: Option Summarylist: listlist, summary: Operation Summarylist, using with --incremental: Footnoteslist, using with --listed-incremental: Footnoteslist, using with --verbose: listlist, using with file name arguments: listlisted-incremental: Incremental Dumpslisted-incremental, summary: Option Summarylisted-incremental, using with --extract: Incremental Dumpslisted-incremental, using with --list: Footnotesmode: overridemode, summary: Option Summarymtime: overridemtime, summary: Option Summarymulti-volume: Multi-Volume Archivesmulti-volume, short description: Devicemulti-volume, summary: Option Summarynew-volume-script: Multi-Volume Archivesnew-volume-script, short description: Devicenew-volume-script, summary: Option Summarynewer: afternewer, summary: Option Summarynewer-mtime: afternewer-mtime, summary: Option Summaryno-anchored: controlling pattern-matchingno-anchored, summary: Option Summaryno-delay-directory-restore: Directory Modification Times and Permissionsno-delay-directory-restore, summary: Option Summaryno-ignore-case: controlling pattern-matchingno-ignore-case, summary: Option Summaryno-ignore-command-error: Writing to an External Programno-ignore-command-error, summary: Option Summaryno-overwrite-dir, summary: Option Summaryno-quote-chars, summary: Option Summaryno-recursion: recurseno-recursion, summary: Option Summaryno-same-owner: Attributesno-same-owner, summary: Option Summaryno-same-permissions, summary: Option Summaryno-unquote: Selecting Archive Membersno-unquote, summary: Option Summaryno-wildcards: controlling pattern-matchingno-wildcards, summary: Option Summaryno-wildcards-match-slash: controlling pattern-matchingno-wildcards-match-slash, summary: Option Summarynull: nulnull, summary: Option Summarynumeric-owner: Attributesnumeric-owner, summary: Option Summaryoccurrence, summary: Option Summaryold-archive, summary: Option Summaryone-file-system: oneone-file-system, summary: Option Summaryoverwrite: Overwrite Old Filesoverwrite, introduced: Dealing with Old Filesoverwrite, summary: Option Summaryoverwrite-dir: Overwrite Old Filesoverwrite-dir, introduced: Dealing with Old Filesoverwrite-dir, summary: Option Summaryowner: overrideowner, summary: Option Summarypax-option: PAX keywordspax-option, summary: Option Summaryportability, summary: Option Summaryposix, summary: Option Summarypreserve: Attributespreserve, summary: Option Summarypreserve-order: Same Orderpreserve-order, summary: Option Summarypreserve-permissions: Setting Access Permissionspreserve-permissions, short description: Attributespreserve-permissions, summary: Option Summaryquote-chars, summary: Option Summaryquoting-style: quoting stylesquoting-style, summary: Option Summaryread-full-records: read full recordsread-full-records: Readingread-full-records, short description: Blocking Factorread-full-records, summary: Option Summaryrecord-size, summary: Option Summaryrecursion: recurserecursion, summary: Option Summaryrecursive-unlink: Recursive Unlinkrecursive-unlink, summary: Option Summaryremove-files: remove filesremove-files, summary: Option Summaryrestrict, summary: Option Summaryrmt-command, summary: Option Summaryrsh-command: Devicersh-command, summary: Option Summarysame-order: Same Ordersame-order, summary: Option Summarysame-owner: Attributessame-owner, summary: Option Summarysame-permissions: Setting Access Permissionssame-permissions, short description: Attributessame-permissions, summary: Option Summaryseek, summary: Option Summaryshow-defaults: defaultsshow-defaults, summary: Option Summaryshow-omitted-dirs: verboseshow-omitted-dirs, summary: Option Summaryshow-stored-names: listshow-stored-names, summary: Option Summaryshow-transformed-names: transformshow-transformed-names, summary: Option Summarysparse: sparsesparse, summary: Option Summarysparse-version: sparsesparse-version, summary: Option Summarystarting-file: Starting Filestarting-file, summary: Option Summarystrip-components: transformstrip-components, summary: Option Summarysuffix: backupsuffix, summary: Option Summarytape-length: Multi-Volume Archivestape-length, short description: Devicetape-length, summary: Option Summarytest-label: labeltest-label, summary: Option Summaryto-command: Writing to an External Programto-command, summary: Option Summaryto-stdout: Writing to Standard Outputto-stdout, summary: Option Summarytotals: verbosetotals, summary: Option Summarytouch: Attributestouch: Data Modification Timestouch, summary: Option Summarytransform: transformtransform, summary: Option Summaryuncompress: gzipuncompress, summary: Option Summaryungzip: gzipungzip, summary: Option Summaryunlink-first: Unlink Firstunlink-first, introduced: Dealing with Old Filesunlink-first, summary: Option Summaryunquote: Selecting Archive Membersunquote, summary: Option Summaryupdate: updateupdate, summary: Operation Summaryusage: helpuse-compress-program: gzipuse-compress-program, summary: Option Summaryutc, summary: Option Summaryverbose: verboseverbose, introduced: verbose tutorialverbose, summary: Option Summaryverbose, using with --create: create verboseverbose, using with --list: listverify, short description: verifyverify, summary: Option Summaryverify, using with --create: verifyversion: helpversion, summary: Option Summaryvolno-file: Multi-Volume Archivesvolno-file, summary: Option Summarywildcards: controlling pattern-matchingwildcards, summary: Option Summarywildcards-match-slash: controlling pattern-matchingwildcards-match-slash, summary: Option SummaryADMINISTRATOR: General-Purpose Variablesago in date strings: Relative items in date stringsam in date strings: Time of day itemsARGP_HELP_FMT, environment variable: Configuring Help Summaryget_date: Authors of get_dateBACKUP_DIRS: General-Purpose VariablesBACKUP_FILES: General-Purpose VariablesBACKUP_HOUR: General-Purpose VariablesBLOCKING: General-Purpose Variablesconcatenate: concatenateconcatenate vs cat: concatenateday in date strings: Relative items in date stringsDIRLIST: General-Purpose Variablesdoc-opt-col: Configuring Help SummaryDUMP_BEGIN: User HooksDUMP_END: User HooksDUMP_REMIND_SCRIPT: General-Purpose Variablesdup-args: Configuring Help Summarydup-args-note: Configuring Help Summaryexclude: excludeexclude-caches: excludeexclude-from: excludeNUL: nulFILELIST: General-Purpose Variablesfirst in date strings: General date syntaxfortnight in date strings: Relative items in date stringsget_date: Date input formatsGNU.sparse.major, extended header variable: PAX 1GNU.sparse.map, extended header variable: PAX 0GNU.sparse.minor, extended header variable: PAX 1GNU.sparse.name, extended header variable: PAX 0GNU.sparse.name, extended header variable, in v.1.0: PAX 1GNU.sparse.numblocks, extended header variable: PAX 0GNU.sparse.numbytes, extended header variable: PAX 0GNU.sparse.offset, extended header variable: PAX 0GNU.sparse.realsize, extended header variable: PAX 1GNU.sparse.size, extended header variable: PAX 0header-col: Configuring Help Summaryhook: User Hookshour in date strings: Relative items in date stringslast day: Day of week itemslast in date strings: General date syntaxlong-opt-col: Configuring Help Summarymidnight in date strings: Time of day itemsminute in date strings: Relative items in date stringsmonth in date strings: Relative items in date stringsMT: General-Purpose VariablesMT_BEGIN: Magnetic Tape ControlMT_OFFLINE: Magnetic Tape ControlMT_REWIND: Magnetic Tape ControlMT_STATUS: Magnetic Tape Controlnext day: Day of week itemsnext in date strings: General date syntaxnoon in date strings: Time of day itemsnow in date strings: Relative items in date stringsntape device: ManyNUL terminated file names: nulopt-doc-col: Configuring Help Summarypm in date strings: Time of day itemsRESTORE_BEGIN: User HooksRESTORE_END: User Hooksrmargin: Configuring Help Summaryrmt: Remote Tape ServerRSH: General-Purpose VariablesRSH_COMMAND: General-Purpose Variablesshort-opt-col: Configuring Help SummarySIMPLE_BACKUP_SUFFIX: backupSLEEP_MESSAGE: General-Purpose VariablesSLEEP_TIME: General-Purpose VariablesTAPE: file tutorialTAPE_FILE: General-Purpose VariablesTAR: General-Purpose VariablesTAR_ARCHIVE, info script environment variable: Multi-Volume ArchivesTAR_ATIME, to-command environment: Writing to an External ProgramTAR_CTIME, to-command environment: Writing to an External ProgramTAR_FILENAME, to-command environment: Writing to an External ProgramTAR_FILETYPE, to-command environment: Writing to an External ProgramTAR_FORMAT, info script environment variable: Multi-Volume ArchivesTAR_GID, to-command environment: Writing to an External ProgramTAR_GNAME, to-command environment: Writing to an External ProgramTAR_MODE, to-command environment: Writing to an External ProgramTAR_MTIME, to-command environment: Writing to an External ProgramTAR_OPTIONS, environment variable: using tar optionsTAR_REALNAME, to-command environment: Writing to an External ProgramTAR_SIZE, to-command environment: Writing to an External ProgramTAR_SUBCOMMAND, info script environment variable: Multi-Volume ArchivesTAR_UID, to-command environment: Writing to an External ProgramTAR_UNAME, to-command environment: Writing to an External ProgramTAR_VERSION, info script environment variable: Multi-Volume ArchivesTAR_VOLUME, info script environment variable: Multi-Volume Archivestarcat: Tarcatthis in date strings: Relative items in date stringstoday in date strings: Relative items in date stringstomorrow in date strings: Relative items in date stringsTZ: Specifying time zone rulesusage-indent: Configuring Help Summaryuuencode: Applicationsversion-control Emacs variable: backupVERSION_CONTROL: backupVOLNO_FILE: General-Purpose Variablesweek in date strings: Relative items in date stringsXLIST: General-Purpose Variablesxsparse: Sparse Recoveryyear in date strings: Relative items in date stringsyesterday in date strings: Relative items in date strings[1] This is only accidentally
true, but not in general. Whereas modification times are always
restored, in most cases, one has to be root for restoring the owner,
and use a special option for restoring permissions. Here, it just
happens that the restoring user is also the owner of the archived
members, and that the current umask is compatible with original
permissions.
[2] Clustering many
options, the last of which has an argument, is a rather opaque way to
write options. Some wonder if GNU getopt should not
even be made helpful enough for considering such usages as invalid.
[3] Beware that if you precede options with a dash, you are announcing the short option style instead of the old option style; short options are decoded differently.
[4] Before GNU tar version 1.11.6, a bug prevented intermixing old style options with long options in some cases.
[5] 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 Changes, for more information.
[6] Earlier versions of GNU tar understood -l as a synonym for --one-file-system. This has changed in version 1.15.91. See Changes, for more information.
[7] This option was called --strip-path in version 1.14.
[8] There are plans to merge the cpio and
tar packages into a single one which would be called
paxutils. So, who knows if, one of this days, the
--version would not output ‘tar (GNU paxutils) 3.2’
[9] This is well described in Unix-haters Handbook, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG Books, ISBN 1-56884-203-1.
[10] Unless you give it --keep-old-files option, or the disk copy is newer than the the one in the archive and you invoke tar with --keep-newer-files option
[11] This can cause multiple members to have the same name, for information on how this affects reading the archive, multiple.
[12] Notice, that since both archives were created withouth -P option (see absolute), these commands should be run from the root file system.
[13] 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 incovenient and were changed in version 1.16
[14] 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.
[15] 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.
[16] Notice that earlier GNU tar versions used globbing for inclusion members, which contradicted to UNIX98 specification and was not documented. See Changes, for more information on this and other changes.
[17] 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.
[19] technically speaking, n is a process ID of the tar process which created the archive (see PAX keywords).
[20] If you run GNU tar under a different locale, the translation to the locale's language will be used.
[21] See –restrict, for more information about this option
[22] 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.