Why GNU tools are sometimes not my favorite programs

Presented in the traditional illustrated form:

; cat a
root
cks
; cat b
root
cks
abc
; comm -13 a b >/dev/null
comm: file 2 is not in sorted order

If your comm doesn't do this, don't be surprised; this behavior is new in the very latest version of comm, from coreutils 7.2 (as installed on Fedora 11; Fedora 10 didn't have it). This behavior is turned off by the new --nocheck-order option, although the manpage contains scary warnings about this not being supported.

Congratulations, GNU coreutils maintainers. You have just broken any number of scripts that were using comm to obtain differences between ordered files; all of these scripts now produce extra output, which is bad. Worse, fixing this will make the scripts unportable, since not even previous versions of GNU comm understand the new --nocheck-order option.

(Yes, yes, technically this behavior is allowed by the Single Unix Specification. But in real life, this is false; the true specification is not whatever is allowed by the letter of standards, it is what everything does and what people write to.)

Also, this is utterly the wrong way to change behavior like this. The correct way is to first introduce the necessary command line switches but not default to emitting a warning, with a note that in X amount of time the default will change. Then several versions later you can start to think about changing the default, since people have had a chance to add the new options to their scripts. (You will still fail, because people don't even look at perfectly working scripts, much less update them, but at least you will have made vague motions towards doing the right thing instead of being an asshole.)