I think you\'re confusing two things. A mac as an underlying Un*x-system (BSD-based), kinda like Linux, and you should be able to get to the terminal program somehow (try system menus - I never had a Mac, and haven\'t even tried OSX, so I can\'t tell you where it\'s hidden). Commands here include \"ls\", \"cat\", \"man\".
The thing you can get to by holding a key when booting, is the Mac \"BIOS\", called OpenFirmware. It\'s more like the sun PROM Monitor than a PC BIOS though, I believe. Don\'t play around with that unless you know what you\'re doing, you might end up with a system that\'s unable to boot - or the other way around if you know what you\'re doing: Fixing a system that\'s unable to boot.