File command

The File command
The  command has been a part of unix distributions for many years.

The Debian manpage gives an overview of its functionality

The program source includes a database of heuristics for determining file types, which includes a variety of 'magic numbers' which characterise files. This database is not portable, but is potentially adaptable.

For the source, see:
 * FreeBSD port tree (referring to the porting of the original distribution into FreeBSD
 * XXX Need more sources!