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  -u   Update files

  This option only affects PKZIP operations involving an already existing
  .ZIP file.  When -u is used in conjunction with a PKZIP command, the files
  specified for archiving will be compared against the files already present
  in the .ZIP file.  If the file to be added into the .ZIP file is already
  present and is not NEWER, PKZIP will not bother to re-compress the file.

  By using this option, you may save yourself time when archiving files that
  are backed up repeatedly.  This option differs from the behavior of the -f
  option in that files which are not already present in the .ZIP file will
  be added.

     PKZIP p:\backup\sdsource.zip -u d:\c700\work\sd*.*

  In this example a .ZIP file called "sdsource.zip" is created in a "backup"
  directory on the P: drive.  All files in the source directory matching the
  file specification will be added or updated.