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.

  • 2
    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, 2011 at 3:25
  • 2
    It's "duck typed" Unix.
    – delete
    Feb 17, 2011 at 10:26

5 Answers 5


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.

  • Is it normal that Illumos (fork of OpenSolaris) can say on their website that it is a Unix operating system (illumos.org) but we can only say FreeBSD is Unix-like instead of Unix?
    – baptx
    May 3, 2019 at 18:08
  • To legally carry the name "Unix" it has to pass an expensive certification process. Not worth it for the free Unix-like systes.
    – vonbrand
    Sep 8, 2020 at 2:35

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

  • 1
    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, 2011 at 4:00
  • 1
    @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. Feb 17, 2011 at 7:37
  • it and ATT shares a common descendant then, Gilles :)
    – gnud
    Feb 17, 2011 at 9:41
  • 2
    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, 2011 at 15:16
  • 1
    en.wikipedia.org/wiki/File:Unix_history-simple.svg has a fairly simplified tree showing a lot of the common inheritance of the BSDs. It's based on the rather complex graph here: levenez.com/unix
    – jsbillings
    Feb 17, 2011 at 16:21

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? ;-)


As they all say, legally it is not UNIX. Think of it this way; if you had a boat with 100 pieces, every year you would switch one old piece for a new piece and store that old piece in a shed. In 99 years, when you only have one piece of wood from the original boat, it technically 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.


Every comment keep saying that is just a legal matter, but is a little more complicated than that.

DARPA wanted to standardize all the UNIX flavors they were using, they hired Berkeley to do so. So they got access to the source code of AT&T. One guy in the team thought "hey we can share this if we remove all the AT&T files" And they did in 2 years.

So the code is not AT&T's or decendent, it is a complete rewrite cloning the original, with it's own innovations. So I think clone is more accurate than descendent. SunOS is a descendent, because they do have AT&T code.

  • ... except they didn't need to rewrite the extensive parts of BSD code that were already in AT&T SVR4
    – Grump
    Nov 19, 2021 at 7:01

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