It is painful to see the unconditional complaint (grep 3.8) about [ef]grep usage when those programs have been present since the beginning of Unix. I have hundreds of scripts that use [ef]grep since for many years they were either the only, or then later the most portable, way to get the behavior.
So, I suggest giving that warning only if POSIXLY_CORRECT is set. Seems to me this is a case for which that is intended. -k
