Restricted shells

[Jonathan Sari]

Does anyone know of any public domain restricted-access shells that I
could get my hands on?  Barring that, does anyone know of a good, quick
way to impliment one?

What I mean is a shell that only allows execution of files in a certain
path, and only allows access to files in the user's own directory.

Preferably for BSD Unix, but I could port one if necessary.