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Derived from this question:

If opensolaris, freebsd, openbsd, netbsd are not UNIX, what is then?

What confuses me more, is the fact OSX1.5+ is UNIX, while prior version of OSX weren't

What is the difference between UNIX and UNIX-like?

And what is the diff between UNIX-Like and Linux?

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up vote 16 down vote accepted

UNIX is Unix and Unix is unix. But unix may not be Unix and Unix is not always UNIX.

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And GNU is definitely not Unix! – heavyd Sep 29 '09 at 22:00
Lets throw POSIX into the mix! – KFro Sep 30 '09 at 0:55
Yeah, what about POSIX? – OscarRyz Sep 30 '09 at 2:29
@heavyd, I wish I could double or triple up-vote that comment. – sal Sep 30 '09 at 2:40
The name that can be named is not the eternal name. Free from desire, you realize the mystery. Caught in desire, you see only the manifestations. --Lao-tzu – Richard Hoskins Sep 30 '09 at 4:11

Wikipedia on Unix:

Unix (officially trademarked as UNIX, sometimes also written as Unix with small caps) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna. Today the term Unix is used to describe any operating system that conforms to Unix standards, meaning the core operating system operates the same as the original Unix operating system. Today's Unix systems are split into various branches, developed over time by AT&T as well as various commercial vendors and non-profit organizations.

As of 2007, the owner of the trademark is The Open Group, an industry standards consortium. Only systems fully compliant with and certified according to the Single UNIX Specification are qualified to use the trademark; others are called "Unix system-like" or "Unix-like".

.. on Unix-like:

*A Unix-like (sometimes shortened to nix to circumvent trademark issues) operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification.

There is no standard for defining the term, and some difference of opinion is possible as to whether or not a certain OS is "Unix-like".

.. on Linux:

A Linux-based system is a modular Unix-like operating system. It derives much of its basic design from principles established in Unix during the 1970s and 1980s. Such a system uses a monolithic kernel, the Linux kernel, which handles process control, networking, and peripheral and file system access. [...]

Separate projects that interface with the kernel provide much of the system's higher-level functionality. The GNU userland is an important part of most Linux-based systems, [...]

.. on BSD (FreeBSD, NetBSD, OpenBSD):

Berkeley Software Distribution (BSD, sometimes called Berkeley Unix) is the UNIX operating system derivative developed and distributed by the Computer Systems Research Group of the University of California, Berkeley, from 1977 to 1995.

Historically, BSD has been considered a branch of UNIX — "BSD UNIX", because it shared the initial codebase and design with the original AT&T UNIX operating system. In the 1980s, BSD was widely adopted by vendors of workstation-class systems in the form of proprietary UNIX variants such as DEC ULTRIX and Sun Microsystems SunOS. This can be attributed to the ease with which it could be licensed, and the familiarity it found among the founders of many technology companies of this era. [...]

Today, the term of "BSD" is often non-specifically used to refer to any of these BSD descendants, e.g. FreeBSD, NetBSD or OpenBSD, which together form a branch of the family of Unix-like operating systems.

.. and on OS X 10.5:

Mac OS X v10.5 "Leopard" was released on October 26, 2007. It was called by Apple "the largest update of Mac OS X". [...] Leopard is an Open Brand UNIX 03 registered product on the Intel platform. It is also the first BSD-based OS to receive UNIX 03 certification.

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Unix is a trademark owned by The Open Group. "Only systems fully compliant with and certified according to the Single UNIX Specification are qualified to use the trademark." (1) (2)

That explains why version of OSX before 1.5 is not UNIX, because it wasn't fully compliant.

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OS X, Solaris, HP-UX, and AIX are the remaining UNIX distributions doing well in the market. UNIX-Like refers to an operating system that behaves like traditional UNIX (forking methods, same method of interprocess communication, Kernel features, etc) but does not conform to the Single UNIX specification. Examples of these are BSD variants, GNU/Linux distributions, and Minix. In the end it's more tied to It's Trademark and system behaviors.

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OSX is a UNIX, and conforms to the SUS. – Richard Hoskins Sep 30 '09 at 4:22
It's technically registered under the name, but it's also derived from nexstep and BSD, both of which are UNIX-like. It's a confusing one. – John T Sep 30 '09 at 5:30
OS X conforms to the Single UNIX specification, regardless of its heritage. Your answer states it does not. – Richard Hoskins Sep 30 '09 at 12:40
well if were going to get all huffy about it ill edit – John T Sep 30 '09 at 12:47
Didn't mean to come across as huffy. Just trying to correct a factual inaccuracy in your answer by commenting on it. It's the spirit of SU, no? Other people tell me I should just downvote without comment, which still other people tell me is rude. Downvoting with comment makes people think I'm too serious. What do you do? Ignore inaccurate answers? This is a serious question. I'm not trying to be argumentative. – Richard Hoskins Sep 30 '09 at 14:15

UNIX was an OS originally developed by AT&T back in the 60's. It was a closed-source OS, so many people cloned it's functionality to create UNIX-Like OS's like BSD and Linux. Others Licensed UNIX to create their OS's, like AIX.

UNIX has more recently become a specification for OS's licensed by the Open Group. They have to approve the OS before it can be called UNIX.

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"UNIX-Like OS's like BSD and Linux" - Linux is only a kernel. I normally wouldn't nitpick about it but this is a question completely about these operating systems. – John T Sep 29 '09 at 21:57

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