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Restricted mode provides hardly any protection against untrusted code.

Also, it's a bashism. It's not implemented by the OpenBSD's /bin/sh.

openBSD uses mksh which includes a restricted mode (http://www.mirbsd.org/htman/i386/man1/mksh.htm)

OpenBSD started out as pdksh I believe, not mksh. But it still has a restricted mode.