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.
@FIXME{other examples of media on which --update will fail? need to ask charles and/or mib/thomas/dave shevett..}
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 (@FIXME-ref{Multiple Members with the Same Name}).
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