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 [1] (it ends in April if you want to go crawling for the entire thread).

Rgds,

Kevin

[1] https://listserv.nd.edu/cgi-bin/wa?A2=ind1103&L=CODE4LIB&T=0&F=&S=&P=112728

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Kevin Ford
Network Development and MARC Standards Office
Library of Congress
Washington, DC