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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. See multiple, for a detailed discussion.