An fstyp module for each filesystem type to be checked is executed; each of these modules applies some appropriate heuristic to determine whether the supplied special file is of the type for which it checks. If it is, the program prints on standard output the usual filesystem identifier for that type and exits with a return code of 0; if none of the modules succeed, the error message ``unknown_fstyp (no matches)'' is returned and the exit status is 1. If more than one module succeeds the error message ``unknown_fstyp (multiple matches)'' is returned and the exit status is 2.