I finally had occasion today (read: remembered) to see if the *nix "file" command would recognize a MARC record file. I haven't tested extensively, but it did identify the file as MARC21 Bibliographic record. It also correctly identified a MARC21 Authority Record. I'm running the most recent version of Ubuntu (12.04 - precise pangolin).
I write because the inclusion of a "file" MARC21 specification rule in the magic.db stems from a Code4lib exchange that started in March 2011  (it ends in April if you want to go crawling for the entire thread).
Network Development and MARC Standards Office
Library of Congress