Plan 9, from Bell Labs, is an attempt to work on the concept of an operating system (OS) from the ground up. Initially developed by the people who devised Unix at Bell Labs, Plan 9 took OS development back into the realm of research. Plan 9 is not Unix. If you think of it as Unix, you may become frustrated when something doesn't exist or when it works differently.
www.cs.Bell-Labs.com is apparently the same machine as Plan9.Bell-Labs.com, the machine that hosts this wiki. The command set of Plan 9 are found at the UNIX to Plan 9 command translation page, and the Plan 9 Documentation Task Force "intends to fix, update and extend the various sets of Plan 9 documentation".