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3 Invoking GNU tar

@UNREVISED

This chapter is about how one invokes the GNU tar command, from the command synopsis (see section 3.1 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 3.4.1 Operations), other options are meant to detail how this operation should be performed (see section 3.4.2 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 3.3 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 3.5 GNU tar documentation), --verbose (-v) (see section 3.6 Checking tar progress) and --interactive (-w) options (see section 3.7 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 (-t). 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 (-P) option). See section 6.8.2 Absolute File Names, for more information about --absolute-names (-P).

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 filesystem 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 6.5 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 (-t), --extract (--get, -x), --compare (--diff, -d), and --update (-u)) will act on the entire contents of the archive.

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

GNU tar returns only a few exit statuses. I'm really aiming simplicity in that area, for now. If you are not using the --compare (--diff, -d) option, zero means that everything went well, besides maybe innocuous warnings. Nonzero means that something went wrong. Right now, as of today, "nonzero" is almost always 2, except for remote operations, where it may be 128.

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 (two lists of four operations each may be found at section 2.4 The Three Most Frequently Used Operations and section 4.2.1 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 3.4 All tar Options.)

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. (For example, --file=archive-name (-f archive-name) takes the name of an archive file as an argument. If you do not supply an archive file name, tar will use a default, but this can be confusing; thus, we recommend that you always supply a specific archive file name.) Where you place the arguments generally depends on which style of options you choose. We will detail specific information relevant to each option style in the sections on the different option styles, below. The differences are subtle, yet can often be very important; incorrect option placement can cause you to overwrite a number of important files. We urge you to note these differences, and only use the option style(s) which makes the most sense to you until you feel comfortable with the others.

3.3.1 Mnemonic 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 mnemonic option has many different different names which are synonymous, such as `--compare' and `--diff'. In addition, long option names can be given unique abbreviations. For example, `--cre' can be used in place of `--create' because there is no other mnemonic 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.)

Mnemonic 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.

Mnemonic options which require arguments take those arguments immediately following the option name; they are introduced by an equal sign. 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 the notation `--file=archive.tar' for the mnemonic option.

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

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

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

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

3.3.3 Old Option Style

@UNREVISED

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(2). 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 mnemonic 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(3) version 1.11.6, a bug prevented intermixing old style options with mnemonic options in some cases.}. 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 4.2.3 How to Add Files to Existing Archives: --append.
--catenate
-A
Same as `--concatenate'. See section 4.2.5 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 4.2.7 Comparing Archive Members with the File System.
--concatenate
-A
Appends other tar archives to the end of the archive. See section 4.2.5 Combining Archives with --concatenate.
--create
-c
Creates a new tar archive. See section 2.6 How to Create Archives.
--delete
Deletes members from the archive. Don't try this on a archive on a tape! See section 4.2.6 Removing Archive Members Using `--delete'.
--diff
-d
Same `--compare'. See section 4.2.7 Comparing Archive Members with the File System.
--extract
-x
Extracts members from the archive into the file system. See section 2.8 How to Extract Members from an Archive.
--get
-x
Same as `--extract'. See section 2.8 How to Extract Members from an Archive.
--list
-t
Lists the members in an archive. See section 2.7 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 4.2.4 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.
--after-date
(See `--newer'.)
--atime-preserve
Tells tar to preserve the access time field in a file's inode when dumping it. Due to limitations in the utimes system call, the modification time field is also preserved, which may cause problems if the file is simultaneously being modified by another program.
--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.
--block-number
-R
With this option present, tar prints error messages for read errors with the block number in the archive file.
--blocking-factor=blocking
-b blocking
Sets the blocking factor tar uses to blocking x 512 bytes per record.
--bzip2
-I
This option tells tar to read or write archives through bzip2.
--checkpoint
This option directs tar to print periodic checkpoint messages as it reads through the archive. Its intended for when you want a visual indication that tar is still running, but don't want to see `--verbose' output.
--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.
--confirmation
(See `--interactive'.)
--dereference
-h
When creating a tar archive, tar will archive the file that a symbolic link points to, rather than archiving the symlink.
--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.
--exclude=pattern
When performing operations, tar will skip files that match pattern.
--exclude-from=file
-X file
Similar to `--exclude', except tar will use the list of patterns in the file file.
--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.
--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.
--force-local
Forces tar to interpret the filename given to `--file' as a local file, even if it looks like a remote tape drive name.
--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. Also see the comments for the --owner=user option.
--gunzip
(See `--gzip'.)
--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.
--help
tar will print out a short message summarizing the operations and options to tar and exit.
--ignore-failed-read
Do not exit unsuccessfully merely because an unreadable file was encountered. See section 4.4.1 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 4.4.1 Options to Help Read Archives.
--incremental
-G
Used to inform tar that it is working with an old GNU-format incremental backup archive. It is intended primarily for backwards compatibility only.
--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.
--interactive
--confirmation
-w
Specifies that tar should ask the user for confirmation before performing potentially destructive options, such as overwriting files.
--keep-old-files
-k
Do not overwrite existing files when extracting files from an archive. See section 4.4.2 Changing How tar Writes 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.
--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.
--mode=permissions
When adding files to an archive, tar will use permissions for the archive members, rather than the permissions from the files. The program chmod and this tar option share the same syntax for what permissions might be. See section `File permissions' in GNU file utilities. This reference also has useful information for those not being overly familiar with the Unix permission system. Of course, permissions might be plainly specified as an octal number. However, by using generic symbolic modifications to mode bits, this allows 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.
--multi-volume
-M
Informs tar that it should create or otherwise operate on a multi-volume tar archive.
--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.
--newer-mtime
In conjunction with `--newer', tar will only add files whose contents have changed (as opposed to just `--newer', which will also back up files for which any status information has changed).
--no-recursion
With this option, tar will not recurse into directories unless a directory is explicitly named as an argument to tar.
--no-same-owner
When extracting an archive, do not attempt to preserve the owner specified in the tar archive. This the default behavior for ordinary users; this option has an effect only for the superuser.
--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; this option has an effect only for the superuser.
--null
When tar is using the `--files-from' option, this option instructs tar to expect filenames terminated with NUL, so tar can correctly work with file names that contain newlines.
--numeric-owner
This option will notify tar that it should use numeric user and group IDs when creating a tar file, rather than names.
--old-archive
(See `--portability'.)
--one-file-system
-l
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 4.4.2 Changing How tar Writes 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. 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. This option does not affect extraction from archives.
--portability
--old-archive
-o
Tells tar to create an archive that is compatible with Unix V7 tar.
--posix
Instructs tar to create a POSIX compliant tar archive.
--preserve
Synonymous with specifying both `--preserve-permissions' and `--same-order'.
--preserve-order
(See `--same-order'; see section 4.4.1 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 4.4.2 Changing How tar Writes Files.
--read-full-records
-B
Specifies that tar should reblock its input, for reading from pipes on systems with buggy implementations. See section 4.4.1 Options to Help Read Archives.
--record-size=size
Instructs tar to use size bytes per record when accessing the archive.
--recursive-unlink
Remove existing directory hierarchies before extracting directories of the same name from the archive. See section 4.4.2 Changing How tar Writes Files.
--remove-files
Directs tar to remove the source file from the file system after appending it to an archive.
--rsh-command=cmd
Notifies tar that is should use cmd to communicate with remote devices.
--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 4.4.1 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.
--same-permissions
(See `--preserve-permissions'; see section 4.4.2 Changing How tar Writes Files.)
--show-omitted-dirs
Instructs tar to mention directories its skipping over when operating on a tar archive.
--sparse
-S
Invokes a GNU extension when adding files to an archive that handles sparse files efficiently.
--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 4.4.3 Coping with Scarce Resources.
--suffix=suffix
Alters the suffix tar uses when backing up files from the default `~'.
--tape-length=num
-L num
Specifies the length of tapes that tar is writing as being num x 1024 bytes long.
--to-stdout
-O
During extraction, tar will extract files to stdout rather than to the file system. See section 4.4.2 Changing How tar Writes Files.
--totals
Displays the total number of bytes written after creating an archive.
--touch
-m
Sets the modification time of extracted files to the extraction time, rather than the modification time stored in the archive. See section 4.4.2 Changing How tar Writes Files.
--uncompress
(See `--compress'.)
--ungzip
(See `--gzip'.)
--unlink-first
-U
Directs tar to remove the corresponding file from the file system before extracting it from the archive. See section 4.4.2 Changing How tar Writes Files.
--use-compress-program=prog
Instructs tar to access the archive through prog, which is presumed to be a compression program of some sort.
--verbose
-v
Specifies that tar should be more verbose about the operations its performing. This option can be specified multiple times for some operations to increase the amount of information displayed.
--verify
-W
Verifies that the archive was correctly written when creating an archive.
--version
tar will print an informational message about what version it is and a copyright message, some credits, and then exit.
--volno-file=file
Used in conjunction with `--multi-volume'. tar will keep track of which volume of a multi-volume archive its working in file.

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.

-A
`--concatenate'
-B
`--read-full-records'
-C
`--directory'
-F
`--info-script'
-G
`--incremental'
-I
`--bzip2'
-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'
-k
`--keep-old-files'
-l
`--one-file-system'
-m
`--touch'
-o
`--portability'
-p
`--preserve-permissions'
-r
`--append'
-s
`--same-order'
-t
`--list'
-u
`--update'
-v
`--verbose'
-w
`--interactive'
-x
`--extract'
-z
`--gzip'

3.5 GNU tar documentation

Being careful, the first thing is really checking that you are using GNU tar, indeed. The --version option will generate a message giving confirmation that you are using GNU tar, with the precise version of GNU tar you are using. tar identifies itself and prints the version number to the standard output, then immediately exits successfully, without doing anything else, ignoring all other options. For example, `tar --version' might return:

tar (GNU tar) 1.13.17

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(4) and tar packages into a single one which would be called paxutils. So, who knows if, one of this days, the --version would not yield `tar (GNU paxutils) 3.2'}.

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.

The perceptive reader would have noticed some contradiction in the previous paragraphs. It is written that both --version and --help print something, and have all other options ignored. In fact, they cannot ignore each other, and one of them has to win. We do not specify which is stronger, here; experiment if you really wonder!

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 printed form, as a kind of small book. It may 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 long to GNU tar, or it has not been produced by GNU. Currently, GNU tar documentation is provided in Texinfo format only, if we except, of course, the short result of tar --help.

3.6 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 (-c) or --extract (--get, -x), --verbose (-v) 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 (reminiscent of `ls -l') for each member. Since --list (-t) already prints the names of the members, --verbose (-v) used once with --list (-t) 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 xvv 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.

The --totals option--which is only meaningful when used with --create (-c)---causes tar to print the total amount written to the archive, after it has been fully created.

The --checkpoint option prints an occasional message as tar reads or writes the archive. In fact, it print directory names while reading 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.

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

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

This option is especially useful when reading damaged archives, since it helps pinpoint the damaged sections. It can also be used with --list (-t) when listing a file-system backup tape, allowing you to choose among several backup tapes when retrieving a file later, in favor of the tape where the file appears earliest (closest to the front of the tape).

3.7 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 6 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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