What coreutils are you using? I get: ~$ man su NAME su - change user ID or become superuser OS X uses the FreeBSD coreutils and not the GNU ones, so

What coreutils are you using? I get: ~$ man su NAME su - change user ID or become superuser OS X uses the FreeBSD coreutils and not the GNU ones, so if you're on either OS X or FreeBSD, that would explain it.