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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
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4 Answers

up vote 41 down vote accepted

Most versions of Mac OS X since 10.5 have been certified as Unix by The Open Group:

OS X 10.7 (Lion) never was certified. I don't know why.

(I checked Apple's page on The Open Group site periodically while OS X 10.7 was current, expecting to need to update this answer. Using the obvious pattern in the URLs above, you can examine all of the URLs intermediate between 10.6's certification page and 10.8's page to see that none belong to Apple.)

Apple only keeps the current and prior major versions of OS X certified at any given time.

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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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 '13 at 21:50
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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 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
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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 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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