An older article in Linux Magazine about the use of ssh. Contains a whole series of tips and further links. Among other things, it also points to nosh ( Source and Debian Binary), a shell that very restrictively allows the definition of commands that users can execute. Since the article contains old links, the current ones are provided above. I developed nosh at some point from osh (I can't find a homepage for it) because I needed a shell that really only allowed users to do what was absolutely necessary. In critical environments, this is often much simpler than blocking inappropriate areas through Unix permissions (or alternatively building chroot jails). Here is the original article.