Browse Amazon Books: UNIX
UNIX© is a registered trademark which has been passed around so much I'm not sure who owns it now, possibly Caldera. This fact leads to the use of such handy phrases as "POSIX-compliant UNIX-like operating system," or *NIX (usually pronounced "NIKS") to refer to the many implementations of the *NIX standard which are not the annointed "UNIX©." There are many "religious" viewpoints regarding the proliferation of *NIX flavors -- here's one: "Forget the other stuff, BSD rules!"