As suggested by jilles in https://reviews.freebsd.org/D17146: > This is not wrong, but perhaps some details about different types > of errors may be helpful, such as that nonexistent usernames in -U > are immediately fatal while an empty result (such as from only > nonexistent process IDs in -p or only existing users that do not > currently run any process in -U) still writes the header line (if > not disabled) and also returns a non-zero exit status.