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

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

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

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does it really matter? – hop Sep 2 '10 at 8:28
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A very similar question has been answered on superuser: superuser.com/questions/49434/how-unix-is-mac-os-x – mouviciel Sep 2 '10 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♦ Sep 2 '10 at 17:55

4 Answers

up vote 38 down vote accepted

OS X is certified as UNIX by The Open Group. Currently OS X 10.8 (Mountain Lion) and 10.6 (Snow Leopard) are so certified.

OS X 10.7 (Lion) never was certified, that I saw. (I checked the Apple page on the Open Group's web site periodically, expecting to need to update this answer.) Since Apple chose to get 10.8 certified, I assume Apple also chose to skip certification for the 10.7.x release series. Perhaps this is part of the tick-tock strategy Apple has been favoring in the past few years.

OS X 10.5 (Leopard) was certified but dropped off the list when 10.8 came out.

OS X'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.

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up vote 4 down vote

Well, given that it's fully POSIX compliant I would say yes.

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Then the Microsoft Windows Services make Windows an UNIX, too ... ;) – maxschlepzig Sep 2 '10 at 8:21
But legally, it can only be called "UNIX" if it's certified by The Open Group. – bahamat Aug 21 '12 at 19:26
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. – Stephane Chazelas Mar 26 at 21:50
up vote 0 down vote

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 Sep 2 '10 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 Sep 2 '10 at 21:27
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Re Linux-like userland: No, not really. OS X gets a lot of its userland from FreeBSD, and it shows. One biggie I miss on OS X from Linux is GNU find, which assumes "-print" if given no other action, and "." if not given a directory. So, you can just say "find" on Linux to get a recursive file list from the CWD, or "find /somedir" to do the same elsewhere. Traditional Unix find, including BSD find, makes you say "find . -print" or "find /somedir -print". Yes, I know you can install GNU findutils, coreutils, etc. via Fink or MacPorts, but you can't count on having them. – Warren Young Sep 2 '10 at 21:29
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 Aug 21 '12 at 19:29
up vote -1 down vote

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 man 5 compat 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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