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Mac OS X = UNIX?
How Unix is Mac OS X?

I am quite new to Mac OS X. Is it built using Unix?

Forgive me for my ignorance but if it's built using Unix then it should be open source, right?

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Yes, it is UNIX. There's no requirement for UNIX operating systems to be open source (indeed many, like IBM's AIX and HP's HP-UX, aren't), they must conform to a specification called the Single UNIX Specification (formerly POSIX). However, it happens that much of Mac OS X is open source, with most of the UNIX layer being part of the Darwin project. The source code is available from Apple.

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  • POSIX is still POSIX. How do you mean "formerly"?
    – Dennis Williamson
    Commented Jul 15, 2010 at 16:03
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    I mean that formerly the systems had to be POSIX-compliant, now they have to be SUS-compliant (the POSIX is just the same as the SUS after it has been ratified by ISO, it's exactly the same Austin Group document)
    – user135
    Commented Jul 15, 2010 at 16:47
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    In fact almost no UNIX (according to this definition) is open source. There are exactly 4 OS-es that satisfy the Single UNIX specification: Mac OS X (from Leopard onwards), Solaris, HP-UX and AIX. None of these is (in their full commercial form) open source. Of course there are projects like Darwin and OpenSolaris, which are open source. Not sure whether Darwin by itself is already SUS-compliant. Note that Linux and the (Net|Free|Open)BSD's do not follow the SUS standard completely and thus are not officially UNIX, but maybe more "UNIX-like". They follow their own standards..
    – Henno
    Commented Jul 15, 2010 at 17:18
  • z/OS is Unix 95 compliant. I believe they're working on a more modern compliance. It is closed source in all modern versions.
    – dsolimano
    Commented Jul 15, 2010 at 18:18
  • @Henno: Darwin and OpenSolaris are not tested to be SUS-compliant, so neither can claim to be UNIX (nor is it possible to assert that they're compatible, though that's highly likely). Oh, and to pick a nit, Mac OS X Leopard on Intel hardware is SUS compliant.
    – user135
    Commented Jul 16, 2010 at 7:25
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Yes, the Darwin core of OS X is based on UNIX operating systems, including FreeBSD.

The second part of your question labors under a couple of misconceptions. First, UNIX has not traditionally been an open-source operating system. Linux, NetBSD, and FreeBSD were notable for being open-source implementations of UNIX, created well after UNIX had been around for a long time as a closed-source system.

Second, unlike GPL-licensed operating systems like Linux, BSD-licensed code can be modified and used commercially, without the resulting modified code being open-source.

All that said, as Graham pointed out, Darwin is open-source. But there's no reason it had to be that way.

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    FreeBSD isn't Unix, it's Unix-like just as Linux is Unix-like.
    – MDMarra
    Commented Jul 15, 2010 at 19:41
  • It's UNIX-like in a different way than Linux is UNIX-like. FreeBSD is a descendant of what used to be considered a UNIX implementation (BSD), before the SUS existed. The Design and Implementation of the 4.4BSD Operating System consistently refers to BSD as a UNIX system, for example. Linux does not have this heritage.
    – coneslayer
    Commented Jul 15, 2010 at 19:54
  • I meant is in the sense that neither meet the SUS.
    – MDMarra
    Commented Jul 18, 2010 at 20:36
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The only requirement to be called Unix is to have the Single Unix Specification certification. Open source is not a requirement to SUS.

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Suggest this book, Amit Singh's Mac OS X internals, chapter 1

A Technical History of Apple's Operating Systems

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