Posix requires a bourne shell of some sort, not (t)csh, with a binary name of sh. Freebsd provides this through Ash. BSDs have traditionally provided a C-shell,

Posix requires a bourne shell of some sort, not (t)csh, with a binary name of sh. Freebsd provides this through Ash. BSDs have traditionally provided a C-shell, with FreeBSD being one of the few that's kept csh as the default. OpenBSD uses pdksh, and NetBSD asks.