How to execute UNIX command from PL/SQL??

Hi,

    I had use PL/SQL to write some files     into UNIX directory (definition in UTL_FILE_DIR).

    And I wish when user press a button,     it can use UNIX command "tar" and "gzip"     to compress those files to .tar.gz file automatic.     (and better let UNIX know the file name which should be compressed)

    Is there any solution to do that interface?     Thanks for help~~~ ^_^

                                                  Ivan