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Possible Duplicates:
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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TheLearner
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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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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.

coneslayer
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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.

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

A Technical History of Apple's Operating Systems