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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).


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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).


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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.


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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.


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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’.


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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.


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3.3.3 Old Option Style

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

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

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

 
$ tar cvbf 20 /dev/rmt0

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

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

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

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

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

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

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

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

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

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

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

On the other hand, the following commands are not equivalent to the previous set:

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

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


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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.


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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.


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3.4.2 tar Options

--absolute-names
-P

Normally when creating an archive, tar strips an initial ‘/’ from member names, and when extracting from an archive tar treats names specially if they have initial ‘/’ or internal ‘..’. This option disables that behavior. 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 (5).

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 can specify a symbolic name, or a numeric ID, or both as name: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 Ignore Fail Read.

--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=command
--new-volume-script=command
-F command

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

--interactive
--confirmation
-w

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

--keep-directory-symlink

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

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

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

--keep-newer-files

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

--keep-old-files
-k

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

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 1.27, 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 can specify a symbolic name, or a numeric ID, or both as name:id. See section Overriding File Metadata.

This option does not affect extraction from archives.

--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. See section Obtaining GNU tar default values.

--show-omitted-dirs

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

--show-snapshot-field-ranges

Displays the range of values allowed by this version of tar for each field in the snapshot file, then exits successfully. See section Format of the Incremental Snapshot Files.

--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.

--skip-old-files

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

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

The ‘--warning=existing-file’ option can be used together with this option to produce warning messages about existing old files (see section Controlling Warning Messages).

--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.


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3.4.3 Short Options Cross Reference

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

Short Option

Reference

-A

–concatenate.

-B

–read-full-records.

-C

–directory.

-F

–info-script.

-G

–incremental.

-J

–xz.

-K

–starting-file.

-L

–tape-length.

-M

–multi-volume.

-N

–newer.

-O

–to-stdout.

-P

–absolute-names.

-R

–block-number.

-S

–sparse.

-T

–files-from.

-U

–unlink-first.

-V

–label.

-W

–verify.

-X

–exclude-from.

-Z

–compress.

-b

–blocking-factor.

-c

–create.

-d

–compare.

-f

–file.

-g

–listed-incremental.

-h

–dereference.

-i

–ignore-zeros.

-j

–bzip2.

-k

–keep-old-files.

-l

–check-links.

-m

–touch.

-o

When creating, –no-same-owner, when extracting — –portability.

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

-p

–preserve-permissions.

-r

–append.

-s

–same-order.

-t

–list.

-u

–update.

-v

–verbose.

-w

–interactive.

-x

–extract.

-z

–gzip.


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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) 1.27
Copyright (C) 2013 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(6).

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.


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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.


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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 cvf -’, or even ‘tar cv’—if the installer let standard output be the default archive). In that case tar writes verbose output to the standard error stream.

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

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

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

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

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

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

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

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

--totals=signo

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

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

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

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

This example shows the default checkpoint message used by tar. If you place a dot immediately after the equal sign, it will print a ‘.’ at each checkpoint(7). For example:

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

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

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

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

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


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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 command. For example:

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

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

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

TAR_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.

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

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

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

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

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

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


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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.

record-size

Record size = %lu blocks

Keywords controlling incremental extraction:

rename-directory

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

new-directory

%s: Directory is new

xdev

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

bad-dumpdir

Malformed dumpdir: 'X' never used


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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.


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3.11 Running External Commands

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

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

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

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

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

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


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