For the Linux version of TrID, you currently require the .def file to be in the same directory as the executable. For Windows, this is pretty standard, but for Linux this is highly unusual (and usually considered a bad idea).
Might I suggest a using list of places for TrID to look for the defs? TrID could stop when it finds the first def file, or it could combine them all in memory somehow, or each one found overrides the previous ones found, or whatever.
Typically, programs will look in places like /etc, /usr/etc, /usr/share, etc. I'd suggest something like this:
/etc
/usr/etc
/usr/share/trid
/usr/local/etc
/usr/local/share/trid
$HOME (user's home dir)
