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Some people told me FreeBSD is NOT Unix, is that right? I'm confused. I checked some articles, but the expressions are pretty vague, and I need some clarification.

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It's obviously not a duplicate question, but I think all you need to know can be found at the "Is Linux a Unix?" question –  frabjous Feb 17 '11 at 3:25
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It's "duck typed" Unix. –  delete Feb 17 '11 at 10:26

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up vote 15 down vote accepted

It all come down to whether you are speaking legally, or from a technology viewpoint. Legally, FreeBSD, like Linux, cannot use the trademarked term Unix. From a technology point of view, FreeBSD is as much Unix as Solaris, HP-UX, or any of the other commercial versions that have paid to be able to be legally called Unix.

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The word Unix is actually a trademark name that refers to System V Unix. So "legally" it is not Unix. But it is related and "looks" like Unix, so it is often called Unix, or Unix-like. http://en.wikipedia.org/wiki/Unix

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In addition, FreeBSD is a direct decendent of BSD4.4 which is derived from ATT Unix. The only reason FreeBSD cannot use the term Unix is because the certification costs money the organization can spend better elsewhere. –  Rob Feb 17 '11 at 4:00
    
@Rob: Actually BSD could only become FreeBSD after it had replaced all ATT code. So it's no more derived from ATT Unix than Linux is. –  Gilles Feb 17 '11 at 7:37
    
it and ATT shares a common descendant then, Gilles :) –  gnud Feb 17 '11 at 9:41
    
@Gilles - Linux is not based on or arrive directly from ATT Unix but Minix which also didn't come directly from ATT. FreeBSD was ATT Unix but had to be rewritten for legal reasons as you correctly identify. –  Rob Feb 17 '11 at 13:27
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And Linux isn't based on Minix in any meaningful way. (It never shared any code, and the design isn't similar.) –  mattdm Feb 17 '11 at 15:16

Legally, something is allowed to call itself "Unix" if it goes through an expensive certification process by X/Open. So no, FreeBSD is not Unix, they wouldn't dream of squandering the little cash they have to go through a multi-year certification that is obsolete the moment they get it (one change, start over). On the other hand, all the BSDs are direct decendents of the original Unix source code from AT&T, hacked beyond recognition by a horde of enthusiasts from all over the world. So yes, FreeBSD is a (genetic) Unix. In the same way, Linux is not Unix, neither legally nor genetically. But Linux was conceived as a faithful workalike of what Unix is all about (it closely follows POSIX), so, yes, Linux is also Unix.

Confused enough? ;-)

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As they all say, legally it is not UNIX. Think of it this way; if you had a boat with 100 peices, every year you would switch one old piece for a new peice and store that old piece in a shed. In 99 years, when you only have one peice of wood from the original boat, it techincally still is the same boat, right? But when you remove that last piece of original wood, would it still be the same ship? The BSD's right now are that ship.

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