In (info "(coreutils) comm invocation") mention that even with --check-order there can still be some erroneous output emitted before the error is detected. Therefore check first with sort -c to be sure...
- bug#15040: comm --check-order doc boost jidanni
- bug#15040: comm --check-order doc boost Pádraig Brady
- bug#15040: closed (Re: bug#15040: comm --check-order d... jidanni