In the previous section, you learned how to use --append 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).
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.
