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I had this argument recently saying Mac OS X was not UNIX, but Unix-like.

I know there is a Single Unix Specification and those spec compliant could use the UNIX trade mark.

Is Mac OS X a UNIX operating system or is it a Unix-like?

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    A very similar question has been answered on superuser: superuser.com/questions/49434/how-unix-is-mac-os-x
    – mouviciel
    Commented Sep 2, 2010 at 9:16
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    In the APUE2e I seem to recall it saying something like "if it walks like a duck, and quacks like a duck, it's probably a duck" it was referring to Linux which is not Certified UNIX but regardless I think this applies here too.
    – xenoterracide
    Commented Sep 2, 2010 at 17:55
  • This article has the whole detailed history
    – artu-hnrq
    Commented Mar 18, 2021 at 2:18

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All but one release of Mac OS X (now macOS) has been certified as Unix by The Open Group, starting with 10.5:

At any given time, Apple's page on The Open Group site only lists the current version of macOS and sometimes the previous version, but all of the links above were at one point found via that page.

macOS's status as a certified Unix is called out in Apple's Unix technology brief, which also has other good technical bits in it that will help you compare it to other UNIX® and Unix-like systems.

Andrew Josey, VP Standards & Certification of the Open Group confirms that 10.7 Lion was never registered as a UNIX 03 product.

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Yes, OS X is UNIX.

"UNIX" is really just a trademarked name, applied by The Open Group, upon completion of a certification. Many different - not at all compatible - OSes are certified as a UNIX. OS X among them. Here is the current certification page for OS X 10.9 "Mavericks" as "UNIX 03" certified: http://www.opengroup.org/openbrand/register/brand3602.htm

Apple has submitted OS X for certification (and received it,) every version since 10.5. However, versions prior to 10.5 (as with many 'UNIX-like' OSes such as many distributions of Linux,) could probably have passed certification had they applied for it.

So it really depends on if you define "UNIX" as "the trademarked name by The Open Group, as applied to operating systems that have certification from The Open Group as a UNIX system" or if you define "unix" as "an operating system that functions like the original AT&T Unix operating system, and meets the standards set forward in any version of the Single Unix Specification, even if it was never submitted to The Open Group for testing and certification," then every OS X back to the original one would likely qualify. (As would most Linux distributions, even though none have undergone The Open Group certification.)

Oh, and I can't add a comment yet, but as an update to Warren Young's post - Apple did get UNIX certification for 10.7 (or at least they claim to have:) https://ssl.apple.com/media/us/osx/2012/docs/OSX_for_UNIX_Users_TB_July2011.pdf

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    I'm not going to believe Apple's marketing document just because the independent, trustworthy sources of information come up dry. (Open Group, Google, Wikipedia, etc.) I checked Apple's page on The Open Group's web site many times while 10.7 was current, and never saw a certification link appear. Google searches for site:opengroup.org "10.6" and ..."10.8" find the surrounding certs, but searching for 10.7 fails. Poking around the opengroup.org site by handwriting URLs turns up bupkis. Absence of evidence isn't evidence of absence, but my case is fairly solid.
    – Warren Young
    Commented Nov 22, 2014 at 12:11
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    @WarrenYoung, see also the web archive. 10.8 conformance was announced on the austingroup ml on 2012-07-27, and until then only 10.5 and 10.6 were listed on the certification register (then changed to 10.6 and 10.8)
    – Stéphane Chazelas
    Commented Mar 10, 2018 at 18:42
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Well, given that it's fully POSIX compliant I would say yes.

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    But legally, it can only be called "UNIX" if it's certified by The Open Group.
    – bahamat
    Commented Aug 21, 2012 at 19:26
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    It's not enough to be POSIX compliant to be Unix compliant (the Unix spec is a superset of the POSIX spec). It's enough to be Unix compliant enough in the eyes of the certifiers to be certified though.
    – Stéphane Chazelas
    Commented Mar 26, 2013 at 21:50
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MacOS uses a Unix kernel at it's core. The graphics layer is simply (well, maybe not "simply") layered on top of the Unix underpinnings. All the pointy-clicky stuff is just a construct for those that don't know the terminal. :)

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One big difference is that X11 integration is a little different. X11 app will not looks good on a Mac, you have to manually start an XServer that's not given too much love. Other than that I think it's a Mach micro-kernel with the FreeBSD network stack, and the userland is like Linux.

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    Re X11 apps not looking good: In my experience, they look pretty much like on any other *ix. What they don't do is take on an Aqua-like theme, which Apple could do for some apps by customizing Gnome and KDE, but I guess they figure it's more work than it's worth.
    – Warren Young
    Commented Sep 2, 2010 at 21:26
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    Re manually starting X: The need for that seems to have gone down over the years. I'm guessing they're getting more clever about detecting when X is needed.
    – Warren Young
    Commented Sep 2, 2010 at 21:27
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    OS X by default sets a DISPLAY to a FIFO with launchd at the other end. launchd automatically starts X11.app when anything tries to use the display. It's been like this at least as far back as 10.6.
    – bahamat
    Commented Aug 21, 2012 at 19:29
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    Re Linux-like userland: OS X's userland is closer to FreeBSD. Examples: OS X uses BSD find which requires the directory (it doesn't default to . as GNU find does); BSD commands lack --long-options; GNU commands have more options than in BSD; OS X lacks /proc; OS X's primary GUI is almost entirely different from Linux's (X11 is a mere sidecar on OS X); OS X's dynamic linkage system is entirely different from Linux's; dtruss vs strace; etc., etc. Homebrew can fix some of this, but defaults matter, and parallel command sets can be a problem.
    – Warren Young
    Commented May 16, 2014 at 13:03
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    X11 is not part of a Unix system.
    – Thorbjørn Ravn Andersen
    Commented Mar 21, 2018 at 16:48
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Mac OS X 10.6 can compile the source code of the (already mentioned) book Advanced Programming in the UNIX® Environment, Second Edition if the _POSIX_C_SOURCE and _DARWIN_C_SOURCE preprocessor flags are enabled (see compat(5) man page and here). So my answer would be 'Yes!'.

Can the source code of this book (somehow) be compiled on Windows as well?

(Re Linux-like userland: precompiled Mac OS X binaries of the GNU findutils package and other GNU tools are available at rudix.org).

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    This doesn't answer the question.
    – Wildcard
    Commented Jul 12, 2016 at 19:56
  • @Wildcard In fact, the book itself, referring to FreeBSD 5.2.1, Linux 2.4.22, Mac OS X 10.3, and Solaris 9, says "only Solaris can call itself a UNIX system." (This has, of course, changed with later releases of OS X/macOS.)
    – Dominick Pastore
    Commented Oct 31, 2019 at 16:03
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In a legal sense yes. UNIX is a trade mark owned by The Open Group, which Apple has bought rights to use. Ditto for HP, IBM and Oracle regarding the UNIXes they sell.

However, unix (not capitalised) is also a specification which prior to 1986 was also copyrighted. It is no accident that commercial unix exploded in the late 80s and free unix followed along e.g. Linux in 1991.

Linux and FreeBSD no less than OS X (Darwin) technically qualify as unix but haven't paid the Open Group for UNIX certification and therefore cannot use the name.

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