GNU tar: an archiver tool

This manual is for GNU tar (version 1.26, 12 March 2011), which creates and extracts files from archives.

Copyright © 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with 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 have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it 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

Using Less Space through Compression

Creating and Reading Compressed Archives

Making tar Archives More Portable

GNU tar and POSIX tar

How to Extract GNU-Specific Data Using Other tar Implementations

Tapes and Other Archive Media

Blocking

Many Archives on One Tape

Using Multiple Tapes

Tar Internals

Storing Sparse Files

Genfile

Copying This Manual

1. Introduction

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

1.1 What this Book Contains

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

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

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

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

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

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

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

1.2 Some Definitions

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

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

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

1.3 What tar Does

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

Initially, tar archives were used to store files conveniently on magnetic tape. The name tar comes from this use; it stands for 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.

Storage

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

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

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

Backup

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

Transportation

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

1.4 How tar Archives are Named

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

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

1.5 GNU tar Authors

GNU tar was originally written by John Gilmore, and modified by many people. The GNU enhancements were written by Jay Fenlason, then Joy Kendall, and the whole package has been further maintained by Thomas Bushnell, n/BSG, 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 1.26 were edited by the current maintainer, Sergey Poznyakoff.

For version 1.12, Daniel Hagerty contributed a great deal of technical consulting. In particular, he is the primary author of Performing Backups and Restoring Files.

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

Support for POSIX archives was added by Sergey Poznyakoff.

1.6 Reporting bugs or suggestions

If you find problems or have suggestions about this program or manual, please report them to ‘[email protected]’.

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

2. Tutorial Introduction to tar

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

2.1 Assumptions this Tutorial Makes

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

• Before you start to work through this tutorial, you should understand what the terms “archive” and “archive member” mean (see section Some Definitions). In addition, you should understand something about how Unix-type operating systems work, and you should know how to use some basic utilities. For example, you should know how to create, list, copy, rename, edit, and delete files and directories; how to change between directories; and how to figure out where you are in the file system. You should have some basic understanding of directory structure and how files are named according to which directory they are in. You should understand concepts such as standard output and standard input, what various definitions of the term ‘argument’ mean, and the differences between relative and absolute file names.
• This manual assumes that you are working from your own home directory (unless we state otherwise). In this tutorial, you will create a directory to practice tar commands in. When we show file names, we will assume that those names are relative to your home directory. For example, my home directory is ‘/home/fsf/melissa’. All of my examples are in a subdirectory of the directory named by that file name; the subdirectory is called ‘practice’.
• In general, we show examples of archives which exist on (or can be written to, or worked with from) a directory on a hard disk. In most cases, you could write those archives to, or work with them on any other device, such as a tape drive. However, some of the later examples in the tutorial and next chapter will not work on tape drives. Additionally, working with tapes is much more complicated than working with hard disks. For these reasons, the tutorial does not cover working with tape drives. See section Tapes and Other Archive Media, for complete information on using tar archives with tape drives.

2.2 Stylistic Conventions

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

2.3 Basic tar Operations and Options

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

Some arguments fall into a class called operations; exactly one of these is both allowed and required for any instance of using tar; you may not specify more than one. People sometimes speak of operating modes. You are in a particular operating mode when you have specified the operation which specifies it; there are eight operations in total, and thus there are eight operating modes.

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

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

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

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

you can type

tar -c -v -f afiles.tar apple angst aspic

or even

tar -cvf afiles.tar apple angst aspic

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

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

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

2.4 The Three Most Frequently Used Operations

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

‘--create’

‘-c’

Create a new tar archive.

‘--list’

‘-t’

List the contents of an archive.

‘--extract’

‘-x’

Extract one or more members from an archive.

2.5 Two Frequently Used Options

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

The ‘--file’ Option

‘--file=archive-name’

‘-f archive-name’

Specify the name of an archive file.

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

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

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

To avoid confusion, we recommend that you always specify an archive file name by using ‘--file=archive-name’ (‘-f archive-name’) when writing your tar commands. For more information on using the ‘--file=archive-name’ (‘-f archive-name’) option, see Choosing and Naming Archive Files.

The ‘--verbose’ Option

‘--verbose’

‘-v’

Show the files being worked on as tar is running.

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

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

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

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

For example, to create an archive in verbose mode:

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

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

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

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

$ tar --create --verbose --verbose …

Note that you must double the hyphens properly each time.

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

The full output consists of six fields:

• File type and permissions in symbolic form. These are displayed in the same format as the first column of ls -l output (see format=verbose: (fileutils)What information is listed section `Verbose listing' in GNU file utilities).
• Owner name and group separated by a slash character. If these data are not available (for example, when listing a ‘v7’ format archive), numeric ID values are printed instead.
• Size of the file, in bytes.
• File modification date in ISO 8601 format.
• File modification time.
• File name. If the name contains any special characters (white space, newlines, etc.) these are displayed in an unambiguous form using so called quoting style. For the detailed discussion of available styles and on how to use them, see Quoting Member Names.

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

  ‘-> link-name’

  The file or archive member is a symbolic link and link-name is the name of file it links to.

  ‘link to link-name’

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

  ‘--Long Link--’

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

  ‘--Long Name--’

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

  ‘--Volume Header--’

  The archive member is a GNU volume header (see section Tape Files).

  ‘--Continued at byte n--’

  Encountered only at the beginning of a multi-volume archive (see section Using Multiple Tapes). This archive member is a continuation from the previous volume. The number n gives the offset where the original file was split.

  ‘unknown file type c’

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

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

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

Getting Help: Using the ‘--help’ Option

‘--help’

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

2.6 How to Create Archives

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

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

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

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

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

2.6.1 Preparing a Practice Directory for Examples

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

Now cd to the directory named ‘practice’; ‘practice’ is now your working directory. (Please note: Although the full file name of this directory is ‘/homedir/practice’, in our examples we will refer to this directory as ‘practice’; the homedir is presumed.)

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

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

2.6.2 Creating the Archive

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

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

The order of the arguments is not very important, when using long option forms. 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 section Short Forms with ‘create’, for more information on this.

In this example, you type the command as shown above: ‘--create’ is the operation which creates the new archive (‘collection.tar’), and ‘--file’ is the option which lets you give it the name you chose. The files, ‘blues’, ‘folk’, and ‘jazz’, are now members of the archive, ‘collection.tar’ (they are file name arguments to the ‘--create’ operation. See section Choosing Files and Names for tar, for the detailed discussion on these.) Now that they are in the archive, they are called archive members, not files. (see section members).

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

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

blues   folk   jazz   collection.tar

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

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

Caution: Do not attempt to use ‘--create’ (‘-c’) to add files to an existing archive; it will delete the archive and write a new one. Use ‘--append’ (‘-r’) instead. See section How to Add Files to Existing Archives: ‘--append’.

2.6.3 Running ‘--create’ with ‘--verbose’

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

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

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

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

2.6.4 Short Forms with ‘create’

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

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

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

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

$ tar -cfv collection.tar blues folk jazz

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

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

This example,

$ tar 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.

2.6.5 Archiving Directories

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

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

$ cd ..
$

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

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

tar should output:

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

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

If you give tar a command such as

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

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

2.7 How to List Archives

Frequently, you will find yourself wanting to determine exactly what a particular archive contains. You can use the ‘--list’ (‘-t’) operation to get the member names as they currently appear in the archive, as well as various attributes of the files at the time they were archived. For example, 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 section Absolute File Names, for more information). In other words, in verbose mode GNU tar shows file names when creating an archive and member names when listing it. Consider this example:

$ tar --create --verbose --file archive /etc/mail
tar: Removing leading `/' from member names
/etc/mail/
/etc/mail/sendmail.cf
/etc/mail/aliases
$ tar --test --file 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.

‘--show-stored-names’

Print member (as opposed to file) names when creating the archive.

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

Because tar preserves file names, these 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 section Wildcards Patterns and Matching, for a detailed discussion of globbing patterns and related tar command line options.

Listing the Contents of a Stored Directory

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

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

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

tar responds:

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

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

2.8 How to Extract Members from an Archive

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

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

2.8.1 Extracting an Entire Archive

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

$ tar -xvf collection.tar

produces this:

-rw-r--r-- 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

2.8.2 Extracting Specific Files

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

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

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

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

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

Remember that as with other operations, specifying the exact member name is important. 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 section How to List Archives). 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 section Wildcards Patterns and Matching.

You can extract a file to standard output by combining the above options with the ‘--to-stdout’ (‘-O’) option (see section Writing to Standard Output).

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

2.8.3 Extracting Files that are Directories

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

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

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

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

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

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

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

2.8.4 Extracting Archives from Untrusted Sources

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

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

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

2.8.5 Commands That Will Fail

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

If you try to use this command,

$ tar -xvf music.tar folk jazz

you will get the following response:

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

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

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

Likewise, if you try to use this command,

$ tar -tvf music.tar folk jazz

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

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

2.9 Going Further Ahead in this Manual

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

3. Invoking GNU tar

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

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

Some options are so special they are fully described right in this chapter. They have the effect of inhibiting the normal operation of tar or else, they globally alter the amount of feedback the user receives about what is going on. These are the ‘--help’ and ‘--version’ (see section GNU tar documentation), ‘--verbose’ (see section Checking tar progress) and ‘--interactive’ options (see section Asking for Confirmation During Operations).

3.1 General Synopsis of tar

The GNU tar program is invoked as either one of:

tar option… [name]…
tar letter… [argument]… [option]… [name]…

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

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

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

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

tar interprets relative file names as being relative to the working directory. tar will make all file names relative (by removing leading slashes when archiving or restoring files), unless you specify otherwise (using the ‘--absolute-names’ option). See section Absolute File Names, for more information about ‘--absolute-names’.

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

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

Even if names are often specified on the command line, they can also be read from a text file in the file system, using the ‘--files-from=file-of-names’ (‘-T file-of-names’) option.

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

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

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

0

‘Successful termination’.

1

‘Some files differ’. If tar was invoked with ‘--compare’ (‘--diff’, ‘-d’) command line option, this means that some files in the archive differ from their disk counterparts (see section Comparing Archive Members with the File System). If tar was given ‘--create’, ‘--append’ or ‘--update’ option, this exit code means that some files were changed while being archived and so the resulting archive does not contain the exact copy of the file set.

2

‘Fatal error’. This means that some fatal, unrecoverable error occurred.

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

3.2 Using tar Options

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

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

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

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

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

3.3 The Three Option Styles

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

Some options must take an argument(2). Where you place the arguments generally depends on which style of options you choose. We will detail specific information relevant to each option style in the sections on the different option styles, below. The differences are subtle, yet can often be very important; incorrect option placement can cause you to overwrite a number of important files. We urge you to note these differences, and only use the option style(s) which makes the most sense to you until you feel comfortable with the others.

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

3.3.1 Long Option Style

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

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

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

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

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

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

3.3.2 Short Option Style

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

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

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

Short options which take optional arguments take their arguments immediately following the option letter, without any intervening white space characters.

Short options' letters may be clumped together, but you are not required to do this (as compared to old options; see below). When short options are clumped as a set, use one (single) dash for them all, e.g., ‘tar -cvf’. Only the last option in such a set is allowed to have an argument(3).

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

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

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

3.3.3 Old Option Style

Like short options, old options are single letters. However, old options must be written together as a single clumped set, without spaces separating them or dashes preceding them(4). 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.

3.3.4 Mixing Option Styles

All three styles may be intermixed in a single tar command, so long as the rules for each style are fully respected(5). 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.

3.4 All tar Options

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

3.4.1 Operations

 

‘--append’

‘-r’

Appends files to the end of the archive. See section How to Add Files to Existing Archives: ‘--append’.

 

‘--catenate’

‘-A’

Same as ‘--concatenate’. See section Combining Archives with ‘--concatenate’.

 

‘--compare’

‘-d’

Compares archive members with their counterparts in the file system, and reports differences in file size, mode, owner, modification date and contents. See section Comparing Archive Members with the File System.

 

‘--concatenate’

‘-A’

Appends other tar archives to the end of the archive. See section Combining Archives with ‘--concatenate’.

 

‘--create’

‘-c’

Creates a new tar archive. See section How to Create Archives.

 

‘--delete’

Deletes members from the archive. Don't try this on an archive on a tape! See section Removing Archive Members Using ‘--delete’.

 

‘--diff’

‘-d’

Same ‘--compare’. See section Comparing Archive Members with the File System.

 

‘--extract’

‘-x’

Extracts members from the archive into the file system. See section How to Extract Members from an Archive.

 

‘--get’

‘-x’

Same as ‘--extract’. See section How to Extract Members from an Archive.

 

‘--list’

‘-t’

Lists the members in an archive. See section How to List Archives.

 

‘--update’

‘-u’

Adds files to the end of the archive, but only if they are newer than their counterparts already in the archive, or if they do not already exist in the archive. See section Updating an Archive.

3.4.2 tar Options

 

‘--absolute-names’

‘-P’

Normally when creating an archive, tar strips an initial ‘/’ from member names. This option disables that behavior. See section Absolute File Names.

 

‘--after-date’

(See ‘--newer’, see section Operating Only on New Files)

 

‘--anchored’

A pattern must match an initial subsequence of the name's components. See section Controlling Pattern-Matching.

 

‘--atime-preserve’

‘--atime-preserve=replace’

‘--atime-preserve=system’

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

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

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

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

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

 

‘--auto-compress’

‘-a’

During a ‘--create’ operation, enables automatic compressed format recognition based on the archive suffix. The effect of this option is cancelled by ‘--no-auto-compress’. See section Creating and Reading Compressed Archives.

 

‘--backup=backup-type’

Rather than deleting files from the file system, tar will back them up using simple or numbered backups, depending upon backup-type. See section Backup options.

 

‘--block-number’

‘-R’

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

 

‘--blocking-factor=blocking’

‘-b blocking’

Sets the blocking factor tar uses to blocking x 512 bytes per record. See section The Blocking Factor of an Archive.

 

‘--bzip2’

‘-j’

This option tells tar to read or write archives through bzip2. See section Creating and Reading Compressed Archives.

 

‘--check-device’

Check device numbers when creating a list of modified files for incremental archiving. This is the default. See device numbers, for a detailed description.

 

‘--checkpoint[=number]’

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

 

‘--checkpoint-action=action’

Instruct tar to execute an action upon hitting a breakpoint. Here we give only a brief outline. See section Checkpoints, for a complete description.

The action argument can be one of the following:

bell

Produce an audible bell on the console.

dot

.

Print a single dot on the standard listing stream.

echo

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

echo=string

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

exec=command

Execute the given command.

sleep=time

Wait for time seconds.

ttyout=string

Output string on the current console (‘/dev/tty’).

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

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

 

‘--check-links’

‘-l’

If this option was given, tar will check the number of links dumped for each processed file. If this number does not match the total number of hard links for the file, a warning message will be output (6).

See section Hard Links.

 

‘--compress’

‘--uncompress’

‘-Z’

tar will use the compress program when reading or writing the archive. This allows you to directly act on archives while saving space. See section Creating and Reading Compressed Archives.

 

‘--confirmation’

(See ‘--interactive’.) See section Asking for Confirmation During Operations.

 

‘--delay-directory-restore’

Delay setting modification times and permissions of extracted directories until the end of extraction. See section Directory Modification Times and Permissions.

 

‘--dereference’

‘-h’

When reading or writing a file to be archived, tar accesses the file that a symbolic link points to, rather than the symlink itself. See section Symbolic Links.

 

‘--directory=dir’

‘-C dir’

When this option is specified, tar will change its current directory to dir before performing any operations. When this option is used during archive creation, it is order sensitive. See section Changing the Working Directory.

 

‘--exclude=pattern’

When performing operations, tar will skip files that match pattern. See section Excluding Some Files.

 

‘--exclude-backups’

Exclude backup and lock files. See section exclude-backups.

 

‘--exclude-from=file’

‘-X file’

Similar to ‘--exclude’, except tar will use the list of patterns in the file file. See section Excluding Some Files.

 

‘--exclude-caches’

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

See section exclude-caches.

 

‘--exclude-caches-under’

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

See section Excluding Some Files.

 

‘--exclude-caches-all’

Exclude from dump any directory containing a valid cache directory tag file. See section Excluding Some Files.

 

‘--exclude-tag=file’

Exclude from dump any directory containing file named file, but dump the directory node and file itself. See section exclude-tag.

 

‘--exclude-tag-under=file’

Exclude from dump the contents of any directory containing file named file, but dump the directory node itself. See section exclude-tag-under.

 

‘--exclude-tag-all=file’

Exclude from dump any directory containing file named file. See section exclude-tag-all.

 

‘--exclude-vcs’

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

See section exclude-vcs.

 

‘--file=archive’

‘-f archive’

tar will use the file archive as the tar archive it performs operations on, rather than tar's compilation dependent default. See section The ‘--file’ Option.

 

‘--files-from=file’

‘-T file’

tar will use the contents of file as a list of archive members or files to operate on, in addition to those specified on the command-line. See section Reading Names from a File.

 

‘--force-local’

Forces tar to interpret the file name given to ‘--file’ as a local file, even if it looks like a remote tape drive name. See local and remote archives.

 

‘--format=format’

‘-H format’

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

‘v7’

Creates an archive that is compatible with Unix V7 tar.

‘oldgnu’

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

‘gnu’

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

‘ustar’

Creates a POSIX.1-1988 compatible archive.

‘posix’

Creates a POSIX.1-2001 archive.

See section Controlling the Archive Format, for a detailed discussion of these formats.

 

‘--full-time’

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

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

or, when creating an archive:

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

Notice, thar when creating the archive you need to specify ‘--verbose’ twice to get a detailed output (see section The ‘--verbose’ Option).

 

‘--group=group’

Files added to the tar archive will have a group ID of group, rather than the group from the source file. group is first decoded as a group symbolic name, but if this interpretation fails, it has to be a decimal numeric group ID. See section Overriding File Metadata.

Also see the comments for the ‘--owner=user’ option.

 

‘--gzip’

‘--gunzip’

‘--ungzip’

‘-z’

This option tells tar to read or write archives through gzip, allowing tar to directly operate on several kinds of compressed archives transparently. See section Creating and Reading Compressed Archives.

 

‘--hard-dereference’

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

See section Hard Links.

 

‘--help’

‘-?’

tar will print out a short message summarizing the operations and options to tar and exit. See section GNU tar documentation.

 

‘--ignore-case’

Ignore case when matching member or file names with patterns. See section Controlling Pattern-Matching.

 

‘--ignore-command-error’

Ignore exit codes of subprocesses. See section Writing to an External Program.

 

‘--ignore-failed-read’

Do not exit unsuccessfully merely because an unreadable file was encountered. See section Options to Help Read Archives.

 

‘--ignore-zeros’

‘-i’

With this option, tar will ignore zeroed blocks in the archive, which normally signals EOF. See section Options to Help Read Archives.

 

‘--incremental’

‘-G’

Informs tar that it is working with an old GNU-format incremental backup archive. It is intended primarily for backwards compatibility only. See section Using tar to Perform Incremental Dumps, for a detailed discussion of incremental archives.

 

‘--index-file=file’

Send verbose output to file instead of to standard output.

 

‘--info-script=script-file’

‘--new-volume-script=script-file’

‘-F script-file’

When tar is performing multi-tape backups, script-file is run at the end of each tape. If script-file exits with nonzero status, tar fails immediately. See info-script, for a detailed discussion of script-file.

 

‘--interactive’

‘--confirmation’

‘-w’

Specifies that tar should ask the user for confirmation before performing potentially destructive options, such as overwriting files. See section Asking for Confirmation During Operations.

 

‘--keep-newer-files’

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

 

‘--keep-old-files’

‘-k’

Do not overwrite existing files when extracting files from an archive. See section Keep Old Files.

 

‘--label=name’

‘-V name’

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

 

‘--level=n’

Force incremental backup of level n. As of GNU tar version 1.26, the option ‘--level=0’ truncates the snapshot file, thereby forcing the level 0 dump. Other values of n are effectively ignored. See –level=0, for details and examples.

The use of this option is valid only in conjunction with the ‘--listed-incremental’ option. See section Using tar to Perform Incremental Dumps, for a detailed description.

 

‘--listed-incremental=snapshot-file’

‘-g snapshot-file’

During a ‘--create’ operation, specifies that the archive that tar creates is a new GNU-format incremental backup, using snapshot-file to determine which files to backup. With other operations, informs tar that the archive is in incremental format. See section Using tar to Perform Incremental Dumps.

 

‘--lzip’

This option tells tar to read or write archives through lzip. See section Creating and Reading Compressed Archives.

 

‘--lzma’

This option tells tar to read or write archives through lzma. See section Creating and Reading Compressed Archives.

‘--lzop’

This option tells tar to read or write archives through lzop. See section Creating and Reading Compressed Archives.

 

‘--mode=permissions’

When adding files to an archive, tar will use permissions for the archive members, rather than the permissions from the files. permissions can be specified either as an octal number or as symbolic permissions, like with chmod. See section Overriding File Metadata.

 

‘--mtime=date’

When adding files to an archive, tar will use date as the modification time of members when creating archives, instead of their actual modification times. The value of date can be either a textual date representation (see section Date input formats) or a name of the existing file, starting with ‘/’ or ‘.’. In the latter case, the modification time of that file is used. See section Overriding File Metadata.

 

‘--multi-volume’

‘-M’

Informs tar that it should create or otherwise operate on a multi-volume tar archive. See section Using Multiple Tapes.

‘--new-volume-script’

(see ‘--info-script’)

 

‘--newer=date’

‘--after-date=date’

‘-N’

When creating an archive, tar will only add files that have changed since date. If date begins with ‘/’ or ‘.’, it is taken to be the name of a file whose data modification time specifies the date. See section Operating Only on New Files.

 

‘--newer-mtime=date’

Like ‘--newer’, but add only files whose contents have changed (as opposed to just ‘--newer’, which will also back up files for which any status information has changed). See section Operating Only on New Files.

 

‘--no-anchored’

An exclude pattern can match any subsequence of the name's components. See section Controlling Pattern-Matching.

 

‘--no-auto-compress’

Disables automatic compressed format recognition based on the archive suffix. See –auto-compress. See section Creating and Reading Compressed Archives.

 

‘--no-check-device’

Do not check device numbers when creating a list of modified files for incremental archiving. See device numbers, for a detailed description.

 

‘--no-delay-directory-restore’

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

 

‘--no-ignore-case’

Use case-sensitive matching. See section Controlling Pattern-Matching.

 

‘--no-ignore-command-error’

Print warnings about subprocesses that terminated with a nonzero exit code. See section Writing to an External Program.

 

‘--no-null’

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

 

‘--no-overwrite-dir’

Preserve metadata of existing directories when extracting files from an archive. See section Overwrite Old Files.

 

‘--no-quote-chars=string’

Remove characters listed in string from the list of quoted characters set by the previous ‘--quote-chars’ option (see section Quoting Member Names).

 

‘--no-recursion’

With this option, tar will not recurse into directories. See section Descending into Directories.

 

‘--no-same-owner’

‘-o’

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

 

‘--no-same-permissions’

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

 

‘--no-seek’

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

 

‘--no-unquote’

Treat all input file or member names literally, do not interpret escape sequences. See input name quoting.

 

‘--no-wildcards’

Do not use wildcards. See section Controlling Pattern-Matching.

 

‘--no-wildcards-match-slash’

Wildcards do not match ‘/’. See section Controlling Pattern-Matching.

 

‘--null’

When tar is using the ‘--files-from’ option, this option instructs tar to expect file names terminated with NUL, so tar can correctly work with file names that contain newlines. See section NUL-Terminated File Names.

 

‘--numeric-owner’

This option will notify tar that it should use numeric user and group IDs when creating a tar file, rather than names. See section Handling File Attributes.

‘-o’

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

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

See section Changes, for more information.

 

‘--occurrence[=number]’

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

This option instructs tar to process only the numberth occurrence of each named file. Number defaults to 1, so

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

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

 

‘--old-archive’

Synonym for ‘--format=v7’.

 

‘--one-file-system’

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

 

‘--overwrite’

Overwrite existing files and directory metadata when extracting files from an archive. See section Overwrite Old Files.

 

‘--overwrite-dir’

Overwrite the metadata of existing directories when extracting files from an archive. See section Overwrite Old Files.

 

‘--owner=user’

Specifies that tar should use user as the owner of members when creating archives, instead of the user associated with the source file. user is first decoded as a user symbolic name, but if this interpretation fails, it has to be a decimal numeric user ID. See section Overriding File Metadata.

This option does not affect extraction from archives.

 

‘--pax-option=keyword-list’

This option enables creation of the archive in POSIX.1-2001 format (see section GNU tar and POSIX tar) and modifies the way tar handles the extended header keywords. Keyword-list is a comma-separated list of keyword options. See section Controlling Extended Header Keywords, for a detailed discussion.

 

‘--portability’

‘--old-archive’

Synonym for ‘--format=v7’.

 

‘--posix’

Same as ‘--format=posix’.

 

‘--preserve’

Synonymous with specifying both ‘--preserve-permissions’ and ‘--same-order’. See section Setting Access Permissions.

 

‘--preserve-order’

(See ‘--same-order’; see section Options to Help Read Archives.)

 

‘--preserve-permissions’

‘--same-permissions’

‘-p’

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

 

‘--quote-chars=string’

Always quote characters from string, even if the selected quoting style would not quote them (see section Quoting Member Names).

 

‘--quoting-style=style’

Set quoting style to use when printing member and file names (see section Quoting Member Names). Valid style values are: literal, shell, shell-always, c, escape, locale, and clocale. Default quoting style is escape, unless overridden while configuring the package.

 

‘--read-full-records’

‘-B’

Specifies that tar should reblock its input, for reading from pipes on systems with buggy implementations. See section Options to Help Read Archives.

 

‘--record-size=size[suf]’

Instructs tar to use size bytes per record when accessing the archive. The argument can be suffixed with a size suffix, e.g. ‘--record-size=10K’ for 10 Kilobytes. See size-suffixes, for a list of valid suffixes. See section The Blocking Factor of an Archive, for a detailed description of this option.

 

‘--recursion’

With this option, tar recurses into directories (default). See section Descending into Directories.

 

‘--recursive-unlink’

Remove existing directory hierarchies before extracting directories of the same name from the archive. See section Recursive Unlink.

 

‘--remove-files’

Directs tar to remove the source file from the file system after appending it to an archive. See section Removing Files.

 

‘--restrict’

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

 

‘--rmt-command=cmd’

Notifies tar that it should use cmd instead of the default ‘/usr/libexec/rmt’ (see section Remote Tape Server).

 

‘--rsh-command=cmd’

Notifies tar that is should use cmd to communicate with remote devices. See section Device Selection and Switching.

 

‘--same-order’

‘--preserve-order’

‘-s’

This option is an optimization for tar when running on machines with small amounts of memory. It informs tar that the list of file arguments has already been sorted to match the order of files in the archive. See section Options to Help Read Archives.

 

‘--same-owner’

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

‘--same-permissions’

(See ‘--preserve-permissions’; see section Setting Access Permissions.)

 

‘--seek’

‘-n’

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

 

‘--show-defaults’

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

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

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

 

‘--show-omitted-dirs’

Instructs tar to mention the directories it is skipping when operating on a tar archive. See show-omitted-dirs.

 

‘--show-transformed-names’

‘--show-stored-names’

Display file or member names after applying any transformations (see section Modifying File and Member Names). In particular, when used in conjunction with one of the archive creation operations it instructs tar to list the member names stored in the archive, as opposed to the actual file names. See listing member and file names.

 

‘--sparse’

‘-S’

Invokes a GNU extension when adding files to an archive that handles sparse files efficiently. See section Archiving Sparse Files.

 

‘--sparse-version=version’

Specifies the format version to use when archiving sparse files. Implies ‘--sparse’. See section Archiving Sparse Files. For the description of the supported sparse formats, See section Storing Sparse Files.

 

‘--starting-file=name’

‘-K name’

This option affects extraction only; tar will skip extracting files in the archive until it finds one that matches name. See section Coping with Scarce Resources.

 

‘--strip-components=number’

Strip given number of leading components from file names before extraction. For example, if archive ‘archive.tar’ contained ‘/some/file/name’, then running

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

would extract this file to file ‘name’.

 

‘--suffix=suffix’

Alters the suffix tar uses when backing up files from the default ‘~’. See section Backup options.

 

‘--tape-length=num[suf]’

‘-L num[suf]’

Specifies the length of tapes that tar is writing as being num x 1024 bytes long. If optional suf is given, it specifies a multiplicative factor to be used instead of 1024. For example, ‘-L2M’ means 2 megabytes. See size-suffixes, for a list of allowed suffixes. See section Using Multiple Tapes, for a detailed discussion of this option.

 

‘--test-label’

Reads the volume label. If an argument is specified, test whether it matches the volume label. See –test-label option.

 

‘--to-command=command’

During extraction tar will pipe extracted files to the standard input of command. See section Writing to an External Program.

 

‘--to-stdout’

‘-O’

During extraction, tar will extract files to stdout rather than to the file system. See section Writing to Standard Output.

 

‘--totals[=signo]’

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

 

‘--touch’

‘-m’

Sets the data modification time of extracted files to the extraction time, rather than the data modification time stored in the archive. See section Setting Data Modification Times.

 

‘--transform=sed-expr’

‘--xform=sed-expr’

Transform file or member names using sed replacement expression sed-expr. For example,

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

will add to ‘archive’ files from the current working directory, replacing initial ‘./’ prefix with ‘usr/’. For the detailed discussion, See section Modifying File and Member Names.

To see transformed member names in verbose listings, use ‘--show-transformed-names’ option (see show-transformed-names).

‘--uncompress’

(See ‘--compress’, see section Creating and Reading Compressed Archives)

‘--ungzip’

(See ‘--gzip’, see section Creating and Reading Compressed Archives)

 

‘--unlink-first’

‘-U’

Directs tar to remove the corresponding file from the file system before extracting it from the archive. See section Unlink First.

 

‘--unquote’

Enable unquoting input file or member names (default). See input name quoting.

 

‘--use-compress-program=prog’

‘-I=prog’

Instructs tar to access the archive through prog, which is presumed to be a compression program of some sort. See section Creating and Reading Compressed Archives.

 

‘--utc’

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

 

‘--verbose’

‘-v’

Specifies that tar should be more verbose about the operations it is performing. This option can be specified multiple times for some operations to increase the amount of information displayed. See section Checking tar progress.

 

‘--verify’

‘-W’

Verifies that the archive was correctly written when creating an archive. See section Verifying Data as It is Stored.

 

‘--version’

Print information about the program's name, version, origin and legal status, all on standard output, and then exit successfully. See section GNU tar documentation.

 

‘--volno-file=file’

Used in conjunction with ‘--multi-volume’. tar will keep track of which volume of a multi-volume archive it is working in file. See volno-file.

 

‘--warning=keyword’

Enable or disable warning messages identified by keyword. The messages are suppressed if keyword is prefixed with ‘no-’. See section Controlling Warning Messages.

 

‘--wildcards’

Use wildcards when matching member names with patterns. See section Controlling Pattern-Matching.

 

‘--wildcards-match-slash’

Wildcards match ‘/’. See section Controlling Pattern-Matching.

 

‘--xz’

‘-J’

Use xz for compressing or decompressing the archives. See section Creating and Reading Compressed Archives.

3.4.3 Short Options Cross Reference

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

3.5 GNU tar documentation

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

tar (GNU tar) 1.26
Copyright (C) 2010 Free Software Foundation, Inc.
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by John Gilmore and Jay Fenlason.

The first occurrence of ‘tar’ in the result above is the program name in the package (for example, rmt is another program), while the second occurrence of ‘tar’ is the name of the package itself, containing possibly many programs. The package is currently named ‘tar’, after the name of the main program it contains(7).

Another thing you might want to do is 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 section Configuring Help Summary, for a detailed description.

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

The short help output is quite succinct, and you might have to get back to the full documentation for precise points. If you are reading this paragraph, you already have the tar manual in some form. This manual is available in a variety of forms from 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.

3.6 Obtaining GNU tar default values

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

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

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

The above output shows that this version of GNU tar defaults to using ‘gnu’ archive format (see section Controlling the Archive Format), it uses standard output as the archive, if no ‘--file’ option has been given (see section The ‘--file’ Option), the default blocking factor is 20 (see section The Blocking Factor of an Archive). It also shows the default locations where tar will look for rmt and rsh binaries.

3.7 Checking tar progress

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

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

With ‘--create’ or ‘--extract’, ‘--verbose’ used once just prints the names of the files or members as they are processed. Using it twice causes tar to print a longer listing (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:

‘--totals=signo’

Print statistics upon delivery of signal signo. Valid arguments are: SIGHUP, SIGQUIT, SIGINT, SIGUSR1 and SIGUSR2. Shortened names without ‘SIG’ prefix are also accepted.

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

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

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

This example shows the default checkpoint message used by tar. If you place a dot immediately after the equal sign, it will print a ‘.’ at each checkpoint(8). For example:

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

The ‘--checkpoint’ option provides a flexible mechanism for executing arbitrary actions upon hitting checkpoints, see the next section (see section Checkpoints), for more information on it.

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

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

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

3.8 Checkpoints

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

The checkpoint facility is enabled using the following option:

‘--checkpoint[=n]’

Schedule checkpoints before writing or reading each nth record. The default value for n is 10.

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

‘--checkpoint-action=action’

Execute an action at each checkpoint.

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

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

This is the default action, so running:

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

is equivalent to:

$ tar -c --checkpoint=1000 /var

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

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

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

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

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

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

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

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

--checkpoint-action="ttyout=\rHit %s checkpoint #%u"

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

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

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

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

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

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

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

This program is executed using /bin/sh -c, with no additional arguments. Its exit code is ignored. It gets a copy of tar's environment plus the following variables:

TAR_VERSION

GNU tar version number.

TAR_ARCHIVE

The name of the archive tar is processing.

TAR_BLOCKING_FACTOR

Current blocking factor (see section Blocking).

TAR_CHECKPOINT

Number of the checkpoint.

TAR_SUBCOMMAND

A short option describing the operation tar is executing. See section The Five Advanced tar Operations, for a complete list of subcommand options.

TAR_FORMAT

Format of the archive being processed. See section Controlling the Archive Format, for a complete list of archive format names.

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

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

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

3.9 Controlling Warning Messages

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

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

‘--warning=keyword’

Control display of the warning messages identified by keyword. If keyword starts with the prefix ‘no-’, such messages are suppressed. Otherwise, they are enabled.

Multiple ‘--warning’ messages accumulate.

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

Keywords controlling tar operation

all

Enable all warning messages. This is the default.

none

Disable all warning messages.

filename-with-nuls

‘%s: file name read contains nul character’

alone-zero-block

‘A lone zero block at %s’

Keywords applicable for tar --create

cachedir

‘%s: contains a cache directory tag %s; %s’

file-shrank

‘%s: File shrank by %s bytes; padding with zeros’

xdev

‘%s: file is on a different filesystem; not dumped’

file-ignored

‘%s: Unknown file type; file ignored’
‘%s: socket ignored’
‘%s: door ignored’

file-unchanged

‘%s: file is unchanged; not dumped’

ignore-archive

‘%s: file is the archive; not dumped’

file-removed

‘%s: File removed before we read it’

file-changed

‘%s: file changed as we read it’

Keywords applicable for tar --extract

timestamp

‘%s: implausibly old time stamp %s’
‘%s: time stamp %s is %s s in the future’

contiguous-cast

‘Extracting contiguous files as regular files’

symlink-cast

‘Attempting extraction of symbolic links as hard links’

unknown-cast

‘%s: Unknown file type `%c', extracted as normal file’

ignore-newer

‘Current %s is newer or same age’

unknown-keyword

‘Ignoring unknown extended header keyword `%s'’

decompress-program

Controls verbose description of failures occurring when trying to run alternative decompressor programs (see alternative decompression programs). This warning is disabled by default (unless ‘--verbose’ is used). A common example of what you can get when using this warning is:

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

This means that tar first tried to decompress ‘archive.Z’ using compress, and, when that failed, switched to gzip.

Keywords controlling incremental extraction:

rename-directory

‘%s: Directory has been renamed from %s’
‘%s: Directory has been renamed’

new-directory

‘%s: Directory is new’

xdev

‘%s: directory is on a different device: not purging’

bad-dumpdir

‘Malformed dumpdir: 'X' never used’

3.10 Asking for Confirmation During Operations

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

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

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

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

4. GNU tar Operations

4.1 Basic GNU tar Operations

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

‘--create’

‘-c’

Creating an empty archive would have some kind of elegance. One can initialize an empty archive and later use ‘--append’ (‘-r’) for adding all members. Some applications would not welcome making an exception in the way of adding the first archive member. On the other hand, many people reported that it is dangerously too easy for tar to destroy a magnetic tape with an empty archive(9). The two most common errors are:

1. Mistakingly using create instead of extract, when the intent was to extract the full contents of an archive. This error is likely: keys c and x are right next to each other on the QWERTY keyboard. Instead of being unpacked, the archive then gets wholly destroyed. When users speak about exploding an archive, they usually mean something else :-).
2. Forgetting the argument to file, when the intent was to create an archive with a single file in it. This error is likely because a tired user can easily add the f key to the cluster of option letters, by the mere force of habit, without realizing the full consequence of doing so. The usual consequence is that the single file, which was meant to be saved, is rather destroyed.

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

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

‘--extract’

‘--get’

‘-x’

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

‘‘--list’ (‘-t’)’

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

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

4.2 Advanced GNU tar Operations

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

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

4.2.1 The Five Advanced tar Operations

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

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

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

Unless we state otherwise, all practicing you do and examples you follow in this chapter will take place in the ‘practice’ directory that you created in the previous chapter; see Preparing a Practice Directory for Examples. (Below in this section, we will remind you of the state of the examples where the last chapter left them.)

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

‘--append’

‘-r’

Add new entries to an archive that already exists.

‘--update’

‘-u’

Add more recent copies of archive members to the end of an archive, if they exist.

‘--concatenate’

‘--catenate’

‘-A’

Add one or more pre-existing archives to the end of another archive.

‘--delete’

Delete items from an archive (does not work on tapes).

‘--compare’

‘--diff’

‘-d’

Compare archive members to their counterparts in the file system.

4.2.2 How to Add Files to Existing Archives: ‘--append’

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

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

Other operations don't deal with these members as perfectly as you might prefer; if you were to use ‘--extract’ to extract the archive, only the most recently added copy of a member with the same name as other members would end up in the working directory. This is because ‘--extract’ extracts an archive in the order the members appeared in the archive; the most recently archived members will be extracted last. Additionally, an extracted member will replace a file of the same name which existed in the directory already, and tar will not prompt you about this(10). Thus, only the most recently archived member will end up being extracted, as it will replace the one extracted before it, and so on.

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

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

would extract only the second copy. See section —occurrence, for the description of ‘--occurrence’ option.

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

4.2.2.1 Appending Files to an Archive

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

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

‘--append’ cannot be performed on some tape drives, unfortunately, due to deficiencies in the formats those tape drives use. The archive must be a valid tar archive, or else the results of using this operation will be unpredictable. See section Tapes and Other Archive Media.

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

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

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

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

4.2.2.2 Multiple Members with the Same Name

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

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

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

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

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

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

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

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

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

See section Changing How tar Writes Files, for more information on ‘--extract’ and see –occurrence, for a description of ‘--occurrence’ option.

4.2.3 Updating an Archive

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

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

Both ‘--update’ and ‘--append’ work by adding to the end of the archive. When you extract a file from the archive, only the version stored last will wind up in the file system, unless you use the ‘--backup’ option. See section Multiple Members with the Same Name, for a detailed discussion.

4.2.3.1 How to Update an Archive Using ‘--update’

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

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

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

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

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

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

4.2.4 Combining Archives with ‘--concatenate’

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

To use ‘--concatenate’, give the first archive with ‘--file’ option and name the rest of archives to be concatenated on the command line. The members, and their member names, will be copied verbatim from those archives to the first one(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 section Ignoring Blocks of Zeros, for further information on dealing with archives improperly combined using the cat shell utility.

4.2.5 Removing Archive Members Using ‘--delete’

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

Unlike other operations, ‘--delete’ has no short form.

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

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

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

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

4.2.6 Comparing Archive Members with the File System

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

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

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

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

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

The spirit behind the ‘--compare’ (‘--diff’, ‘-d’) option is to check whether the archive represents the current state of files on disk, more than validating the integrity of the archive media. For this latter goal, see Verifying Data as It is Stored.

4.3 Options Used by ‘--create’

The previous chapter described the basics of how to use ‘--create’ (‘-c’) to create an archive from a set of files. See section How to Create Archives. This section described advanced options to be used with ‘--create’.

4.3.1 Overriding File Metadata

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

‘--mode=permissions’

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

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

‘--mtime=date’

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

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

$ tar -c -f archive.tar --mtime='1970-01-01' .

When used with ‘--verbose’ (see section The ‘--verbose’ Option) 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 …

‘--owner=user’

Specifies that tar should use user as the owner of members when creating archives, instead of the user associated with the source file. The argument user can be either an existing user symbolic name, or a decimal numeric user ID.

There is no value indicating a missing number, and ‘0’ usually means root. Some people like to force ‘0’ as the value to offer in their distributions for the owner of files, because the root user is anonymous anyway, so that might as well be the owner of anonymous archives. For example:

$ tar -c -f archive.tar --owner=0 .

or:

$ tar -c -f archive.tar --owner=root .

‘--group=group’

Files added to the tar archive will have a group ID of group, rather than the group from the source file. The argument group can be either an existing group symbolic name, or a decimal numeric group ID.

4.3.2 Ignore Fail Read

‘--ignore-failed-read’

Do not exit with nonzero on unreadable files or directories.

4.4 Options Used by ‘--extract’

The previous chapter showed how to use ‘--extract’ to extract an archive into the file system. Various options cause tar to extract more information than just file contents, such as the owner, the permissions, the modification date, and so forth. This section presents options to be used with ‘--extract’ when certain special considerations arise. You may review the information presented in How to Extract Members from an Archive for more basic information about the ‘--extract’ operation.

4.4.1 Options to Help Read Archives

Normally, tar will request data in full record increments from an archive storage device. If the device cannot return a full record, tar will report an error. However, some devices do not always return full records, or do not require the last record of an archive to be padded out to the next record boundary. To keep reading until you obtain a full record, or to accept an incomplete record if it contains an end-of-archive marker, specify the ‘--read-full-records’ (‘-B’) option in conjunction with the ‘--extract’ or ‘--list’ operations. See section Blocking.

The ‘--read-full-records’ (‘-B’) option is turned on by default when tar reads an archive from standard input, or from a remote machine. This is because on BSD Unix systems, attempting to read a pipe returns however much happens to be in the pipe, even if it is less than was requested. If this option were not enabled, tar would fail as soon as it read an incomplete record from the pipe.

If you're not sure of the blocking factor of an archive, you can read the archive by specifying ‘--read-full-records’ (‘-B’) and ‘--blocking-factor=512-size’ (‘-b 512-size’), using a blocking factor larger than what the archive uses. This lets you avoid having to determine the blocking factor of an archive. See section The Blocking Factor of an Archive.

Reading Full Records

‘--read-full-records’

‘-B’

Use in conjunction with ‘--extract’ (‘--get’, ‘-x’) to read an archive which contains incomplete records, or one which has a blocking factor less than the one specified.

Ignoring Blocks of Zeros

Normally, tar stops reading when it encounters a block of zeros between file entries (which usually indicates the end of the archive). ‘--ignore-zeros’ (‘-i’) allows tar to completely read an archive which contains a block of zeros before the end (i.e., a damaged archive, or one that was created by concatenating several archives together).

The ‘--ignore-zeros’ (‘-i’) option is turned off by default because many versions of tar write garbage after the end-of-archive entry, since that part of the media is never supposed to be read. GNU tar does not write after the end of an archive, but seeks to maintain compatibility among archiving utilities.

‘--ignore-zeros’

‘-i’

To ignore blocks of zeros (i.e., end-of-archive entries) which may be encountered while reading an archive. Use in conjunction with ‘--extract’ or ‘--list’.

4.4.2 Changing How tar Writes Files

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

Options Controlling the Overwriting of Existing Files

When extracting files, if tar discovers that the extracted file already exists, it normally replaces the file by removing it before extracting it, to prevent confusion in the presence of hard or symbolic links. (If the existing file is a symbolic link, it is removed, not followed.) However, if a directory cannot be removed because it is nonempty, tar normally overwrites its metadata (ownership, permission, etc.). The ‘--overwrite-dir’ option enables this default behavior. To be more cautious and preserve the metadata of such a directory, use the ‘--no-overwrite-dir’ option.

To be even more cautious and prevent existing files from being replaced, use the ‘--keep-old-files’ (‘-k’) option. It causes tar to refuse to replace or update a file that already exists, i.e., a file with the same name as an archive member prevents extraction of that archive member. Instead, it reports an error.

To be more aggressive about altering existing files, use the ‘--overwrite’ option. It causes tar to overwrite existing files and to follow existing symbolic links when extracting.

Some people argue that GNU tar should not hesitate to overwrite files with other files when extracting. When extracting a tar archive, they expect to see a faithful copy of the state of the file system when the archive was created. It is debatable that this would always be a proper behavior. For example, suppose one has an archive in which ‘usr/local’ is a link to ‘usr/local2’. Since then, maybe the site removed the link and renamed the whole hierarchy from ‘/usr/local2’ to ‘/usr/local’. Such things happen all the time. I guess it would not be welcome at all that GNU tar removes the whole hierarchy just to make room for the link to be reinstated (unless it also simultaneously restores the full ‘/usr/local2’, of course!) GNU tar is indeed able to remove a whole hierarchy to reestablish a symbolic link, for example, but only if ‘--recursive-unlink’ is specified to allow this behavior. In any case, single files are silently removed.

Finally, the ‘--unlink-first’ (‘-U’) option can improve performance in some cases by causing tar to remove files unconditionally before extracting them.

Overwrite Old Files

‘--overwrite’

Overwrite existing files and directory metadata when extracting files from an archive.

This causes tar to write extracted files into the file system without regard to the files already on the system; i.e., files with the same names as archive members are overwritten when the archive is extracted. It also causes tar to extract the ownership, permissions, and time stamps onto any preexisting files or directories. If the name of a corresponding file name is a symbolic link, the file pointed to by the symbolic link will be overwritten instead of the symbolic link itself (if this is possible). Moreover, special devices, empty directories and even symbolic links are automatically removed if they are in the way of extraction.

Be careful when using the ‘--overwrite’ option, particularly when combined with the ‘--absolute-names’ (‘-P’) option, as this combination can change the contents, ownership or permissions of any file on your system. Also, many systems do not take kindly to overwriting files that are currently being executed.

‘--overwrite-dir’

Overwrite the metadata of directories when extracting files from an archive, but remove other files before extracting.

Keep Old Files

‘--keep-old-files’

‘-k’

Do not replace existing files from archive. The ‘--keep-old-files’ (‘-k’) option prevents tar from replacing existing files with files with the same name from the archive. The ‘--keep-old-files’ option is meaningless with ‘--list’ (‘-t’). Prevents tar from replacing files in the file system during extraction.

Keep Newer Files

‘--keep-newer-files’

Do not replace existing files that are newer than their archive copies. This option is meaningless with ‘--list’ (‘-t’).

Unlink First

‘--unlink-first’

‘-U’

Remove files before extracting over them. This can make tar run a bit faster if you know in advance that the extracted files all need to be removed. Normally this option slows tar down slightly, so it is disabled by default.

Recursive Unlink

‘--recursive-unlink’

When this option is specified, try removing files and directory hierarchies before extracting over them. This is a dangerous option!

If you specify the ‘--recursive-unlink’ option, tar removes anything that keeps you from extracting a file as far as current permissions will allow it. This could include removal of the contents of a full directory hierarchy.

Setting Data Modification Times

Normally, tar sets the data modification times of extracted files to the corresponding times recorded for the files in the archive, but limits the permissions of extracted files by the current umask setting.

To set the data modification times of extracted files to the time when the files were extracted, use the ‘--touch’ (‘-m’) option in conjunction with ‘--extract’ (‘--get’, ‘-x’).

‘--touch’

‘-m’

Sets the data modification time of extracted archive members to the time they were extracted, not the time recorded for them in the archive. Use in conjunction with ‘--extract’ (‘--get’, ‘-x’).

Setting Access Permissions

To set the modes (access permissions) of extracted files to those recorded for those files in the archive, use ‘--same-permissions’ in conjunction with the ‘--extract’ (‘--get’, ‘-x’) operation.

‘--preserve-permissions’

‘--same-permissions’

‘-p’

Set modes of extracted archive members to those recorded in the archive, instead of current umask settings. Use in conjunction with ‘--extract’ (‘--get’, ‘-x’).