tar: external

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1 3.11 Running External Commands
1 ==============================
1 
1 Certain GNU 'tar' operations imply running external commands that you
1 supply on the command line.  One of such operations is checkpointing,
1 described above (⇒checkpoint exec).  Another example of this
1 feature is the '-I' option, which allows you to supply the program to
11 use for compressing or decompressing the archive (⇒
 use-compress-program).
1 
1    Whenever such operation is requested, 'tar' first splits the supplied
1 command into words much like the shell does.  It then treats the first
1 word as the name of the program or the shell script to execute and the
1 rest of words as its command line arguments.  The program, unless given
1 as an absolute file name, is searched in the shell's 'PATH'.
1 
1    Any additional information is normally supplied to external commands
1 in environment variables, specific to each particular operation.  For
1 example, the '--checkpoint-action=exec' option, defines the
1 'TAR_ARCHIVE' variable to the name of the archive being worked upon.
1 You can, should the need be, use these variables in the command line of
1 the external command.  For example:
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1      $ tar -x -f archive.tar \
1          --checkpoint-action=exec='printf "%04d in %32s\r" $TAR_CHECKPOINT $TAR_ARCHIVE'
1 
1 This command prints for each checkpoint its number and the name of the
1 archive, using the same output line on the screen.
1 
1    Notice the use of single quotes to prevent variable names from being
1 expanded by the shell when invoking 'tar'.
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