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Writing to an External Program

You can instruct tar to send the contents of each extracted file to the standard input of an external program:

--to-command=command

Extract files and pipe their contents to the standard input of command. When this option is used, instead of creating the files specified, tar invokes command and pipes the contents of the files to its standard output. The command may contain command line arguments (see Running External Commands, for more detail).

Notice, that command is executed once for each regular file extracted. Non-regular files (directories, etc.) are ignored when this option is used.

The command can obtain the information about the file it processes from the following environment variables:

TAR_FILETYPE

Type of the file. It is a single letter with the following meaning:

f

Regular file

d

Directory

l

Symbolic link

h

Hard link

b

Block device

c

Character device

Currently only regular files are supported.

TAR_MODE

File mode, an octal number.

TAR_FILENAME

The name of the file.

TAR_REALNAME

Name of the file as stored in the archive.

TAR_UNAME

Name of the file owner.

TAR_GNAME

Name of the file owner group.

TAR_ATIME

Time of last access. It is a decimal number, representing seconds since the Epoch. If the archive provides times with nanosecond precision, the nanoseconds are appended to the timestamp after a decimal point.

TAR_MTIME

Time of last modification.

TAR_CTIME

Time of last status change.

TAR_SIZE

Size of the file.

TAR_UID

UID of the file owner.

TAR_GID

GID of the file owner.

Additionally, the following variables contain information about tar mode and the archive being processed:

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_VOLUME

Ordinal number of the volume tar is processing.

TAR_FORMAT

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

These variables are defined prior to executing the command, so you can pass them as arguments, if you prefer. For example, if the command proc takes the member name and size as its arguments, then you could do:

 
$ tar -x -f archive.tar \
       --to-command='proc $TAR_FILENAME $TAR_SIZE'

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

If command exits with a non-0 status, tar will print an error message similar to the following:

 
tar: 2345: Child returned status 1

Here, ‘2345’ is the PID of the finished process.

If this behavior is not wanted, use ‘--ignore-command-error’:

--ignore-command-error

Ignore exit codes of subprocesses. Notice that if the program exits on signal or otherwise terminates abnormally, the error message will be printed even if this option is used.

--no-ignore-command-error

Cancel the effect of any previous ‘--ignore-command-error’ option. This option is useful if you have set ‘--ignore-command-error’ in TAR_OPTIONS (see TAR_OPTIONS) and wish to temporarily cancel it.


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