Originally Posted by gsa700
Quite true, it's definitely UNIX though which was the OP's question.
kinda. It is unix, but not in the context of this discussion.
As many people know, Linux is just a kernel and all the stuff on top is just user space stuff. However Unix isn't the same; Unix is a certification and not a kernel. So for an OS to be "Unix", it takes into account user space components (Unix certification is much like POSIX actually). While there was once a "UNIX OS", this was back in the 60s and the platform has forked and mutated since then. So technically (and I mean real nitpicking levels of technicality here) BSD is neither UNIX nor Unix Certified.
Going back to the original question, OS X is
Unix Certified and does have a shared heritage with UNIX (which Linux does not) due to it being based on a couple of BSD systems which in turn are the grandchildren of UNIX forks. However I wouldn't say XNU (OS X's kernel) was itself UNIX (in fact, XNU actually stands for X
NIX - but that might have been a humorous poke at Linux's user base name: GNU - G
So anyway, OS X is
Unix Certified, but neither Darwin nor XNU is UNIX.