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(tar.info)update

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Updating an Archive
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     _(This message will disappear, once this node revised.)_

   In the previous section, you learned how to use `--append' (`-r') to
add a file to an existing archive.  A related operation is `--update'
(`-u').  The `--update' operation updates a `tar' archive by comparing
the date of the specified archive members against the date of the file
with the same name.  If the file has been modified more recently than
the archive member, then the newer version of the file is added to the
archive (as with `--append' (`-r')).

   Unfortunately, you cannot use `--update' with magnetic tape drives.
The operation will fail.

   Both `--update' and `--append' work by adding to the end of the
archive.  When you extract a file from the archive, only the version
stored last will wind up in the file system, unless you use the
`--backup' option.

how to update