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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
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
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-action=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
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