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?


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

  • 15
    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
  • 2
    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.


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.


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.


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.

  • "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

"Unix" may refer to a few different terms:

  • UNIX is a trademark belonging to The Open Group. Systems branded as UNIX undergo lots of (expensive) compliance testing, conform to the Single UNIX Specification and POSIX standards, and are officially certified as UNIX by The Open Group. UNIX systems are historically commercial. Notable examples include AIX, Solaris, and HP-UX.

  • Unix may refer to a family of operating systems derived from the original Research Unix, or any operating system from this family. Most commercial Unixes are based on AT&T's UNIX System V, which hoped to be the standard for all coming Unixes. On the other hand are the BSDs. The original BSD (Berkeley Software Distribution) was essentially 6th Edition Research Unix with "Berkeley additions", and Berkeley extensions were, after some time, used in the original Unix. However, after a decade BSD had to be rewritten so that they didn't include any proprietary code from Bell Labs. Notable examples of the BSDs include FreeBSD, OpenBSD, and NetBSD, which you can install on your computer for free. (Fun fact: Mac OS is based on FreeBSD, so is also Unix).

  • Research Unix, usually referred to as Unix, is the original Unix system made by Dennis Ritchie, Ken Thompson, et al. It was the first portable operating system and was written almost entirely in C. Research Unix was made with what is now called the Unix philosophy, a philosophy of simplicity and modularity. The design of Research Unix proved to be extremely good, hence the enormous amount of Unix and Unix-like systems in use even today. Unfortunately, work on Research Unix ended in 1989.

  • Unix-like systems are systems that use the general design of Unix, but are not descended from it. Operating systems like GNU/Linux and Minix have inherited the same general design of Unix, such as the hierarchical filesystem, the shell, and basic user commands (ls, cat, grep), but do not descend from Research Unix. Some Unix-like systems fully conform to POSIX and sometimes even to all of The Open Group's standards (EulerOS, for example, conforms to the UNIX 03 standard), whereas some systems don't even comply with the Filesystem Hierarchy Standard. Regardless, they are still Unix-like.

Some people also believe that an operating system is not Unix unless it follows the Unix philosophy. In that sense, there really is no real Unix anymore, and to get a feel of real Unix you would need to get a running Research Unix system (V7/x86 in a virtual machine would be a good start).

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