Unique spool filenames?

Is there a way within SQLPlus to create unique filenames for spool files. In unix one can use 'filename.$$' which will append the process ID. Can I do something similar from SQLPlus, running under unix?

Thanks

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   |  \/  |(_)| |__ ___                                      __o
   | |\/| || || / // -_)      Mike Moritz                  _ \<._
   |_|  |_||_||_\_\\___|      mmoritz_at_netcom.com      ____(_)/ (_)____