C 4 pass_diff.chk - Checks passwords of accounts that have changed their passwords since the last run. pass_diff.chk [ options ] pass_diff.chk is a front end for the pass.chk program. All it does is run a diff on the last password file checked, and pass the accounts with changed passwords to pass.chk, along with any options it is called with. It will not run pass.chk at all if no difference was found.

old_passwd
passwd.diff
pass.chk

pass.chk(1) It calls pass.chk with the -P option in order to pass the difference from the last run. So calling pass_diff.chk with the -P option is pointless.