oblio is right that is a shell not an os. a shell is an aplication. and realies on another os to function (like win 3.1 relied on dos)
an os would be able to interface at the machine level ( with 101010101 10101 and all that fun stuff) and at the user level ( with a GUI) i suggest you find a tutorial about that if you want to know more.

you could probably program your own shell/os if you wanted one and then of course it would be open source