Is Mac OS X developed on a licensed Unix or is it a Unix-like clone that, unlike Linux, conforms to Unix specification well enough to be registered as a Unix OS. Not until Leopard, Mac OS X did not gain the Unix certification. But in Leopard, Terminal still print:
GNU bash, version 3.2.48(1)-release (x86_64-apple-darwin10.0)
But GNU is GNU's not Unix, and Mac OS X is registered as Unix. That gets me confused whether OS X is unix or unix-like.
In other words, is OS X written on top of Unix or a re-write of Unix that is as Unix as it can possible be. May be along the answer someone can provide lineage or other background information.
I would also recommend reading How Unix is Mac OS X.
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