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The simplest way to add a file to an already existing archive is the --append (-r) operation, which writes specified files into the archive whether or not they are already among the archived files.
When you use --append, you must specify file name arguments, as there is no default. If you specify a file that already exists in the archive, another copy of the file will be added to the end of the archive. As with other operations, the member names of the newly added files will be exactly the same as their names given on the command line. The --verbose (-v) option will print out the names of the files as they are written into the archive.
--append cannot be performed on some tape drives, unfortunately, due to deficiencies in the formats those tape drives use. The archive must be a valid tar archive, or else the results of using this operation will be unpredictable. See Media.
To demonstrate using --append to add a file to an archive, create a file called rock in the practice directory. Make sure you are in the practice directory. Then, run the following tar command to add rock to collection.tar:
$ tar --append --file=collection.tar rock
If you now use the --list (-t) operation, you will see that rock has been added to the archive:
$ tar --list --file=collection.tar
-rw-r--r-- me user 28 1996-10-18 16:31 jazz
-rw-r--r-- me user 21 1996-09-23 16:44 blues
-rw-r--r-- me user 20 1996-09-23 16:44 folk
-rw-r--r-- me user 20 1996-09-23 16:44 rock