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The term Unix-like is widely used to describe operating systems that share many of the characteristics of the original UNIX, which was written in 1969 by Ken Thompson at Bell Labs, and its early successors.

The popularity of this term is mainly due to the fact that it is a very convenient and logical way of describing the numerous and diverse operating systems that fit into this category. It is also due in part to the controversy about what really is UNIX and how this term should be used. This controversy, in turn, has arisen largely as a result of the complex history of UNIX and the operating systems that have been based on it.

The category of Unix-like operating systems clearly includes those systems that have all three of the following characteristics: a direct source code lineage from the original UNIX, obvious UNIX characteristics and officially describe themselves as being UNIX. Source code is the version of software as it is originally written (i.e., typed into a computer) by a human in plain text (i.e., human readable alphanumeric characters). Such systems include AIX (developed by IBM), HP-UX (developed by HP), IRIX (developed by Silicon Graphics), Solaris (developed by Sun Microsystems) and True64 (developed by Compaq for alpha processors and now owned by HP).

The category of Unix-like operating systems also includes UNIX clones. A clone is a program (i.e., an operating system or an application program) that has functions and behavior similar to another program but which does not contain source code from that program.

The functions and behavior of the UNIX clones are so similar to those of the early (and later) versions of UNIX, in fact, that many knowledgeable users consider them to be UNIX. However, they do not have a direct lineage to the original UNIX source code, and most of them do not officially describe themselves as being UNIX. This category includes the BSDs (i.e., FreeBSD, NetBSD, OpenBSD and Darwin), Linux, MINIX, QNX and Cygwin.

Unix-like operating systems also generally contain most or all of the enhancements and new features that were subsequently added at the University of California at Berkeley (UCB) and which are known as the Berkeley extensions. The Berkeley extensions include such now nearly universal innovations as the C shell, also referred to as csh, TCP/IP (transmission control protocol/Internet protocol), which forms the basis for both the Internet and most local networks, virtual memory, which allows a hard disk drive (HDD) to simulate additional main memory, and the vi text editor. Thus, it has been suggested that operating systems that refer to themselves as UNIX might more appropriately call themselves something such as UNIX/Berkeley.

BSD (Berkeley Software Distribution), the version of UNIX developed at UCB in the 1970s and early 1980s, was widely considered to be a type of UNIX, as are its modern day descendants. In fact, BSD even included UNIX in its name, such as BSD UNIX 4.0, which was released in October 1980.

The original name for the operating system developed at Bell Labs was UNIX (all upper case). This term became a trademark that was eventually acquired by The Open Group, an industry consortium that was formed in 1996. The Open Group attempts to permit operating systems to call themselves UNIX only if they both conform to its Single UNIX Specification and pay a substantial fee (which is one of its main sources of revenue). Thus, at least theoretically, an operating system would not have to contain any of the original UNIX source code or bear much superficial resemblance to the original UNIX to be permitted to call itself UNIX.

Adding to the complexity of the situation is the fact that there is some controversy as to whether the term UNIX is actually a valid trademark. This is because trademark law states that when a term becomes very widely used as a generic term (i.e., a term that describes a class of products rather than just a single brand) it can lose its legally protected trademark status.

Apple Computer, which uses Darwin (a Unix-like operating system based on BSD) as the core of its increasingly popular Mac OS X operating system, claims that the term UNIX has become generic and thus that it is not a legally valid trademark. Apple has vastly greater legal and financial resources than the BSDs or most of the Linux distributions (i.e., versions), and thus it is not afraid to call its operating system UNIX. In fact, it makes a number of statements on its website explicitly referring to itself as UNIX (and not as Unix-like), including the following: "Don't let its elegant and easy-to-use interface fool you. Beneath the surface of Mac OS X lies an industrial-strength UNIX foundation hard at work to ensure that your computing experience remains free of system crashes and compromised performance."

As a result, the Open Group is suing Apple Computer for alleged trademark violation. This is a necessary move for the Open Group, as attempting to protect a trademark from abuse can be an important factor in legal decisions about the validity of the trademark. However, some industry observers expect that this case will be settled quietly out of court because neither side wants to risk a negative judgment¹.

The BSD operating systems are widely regarded as free (both in a monetary sense and with regard to use) implementations of UNIX, and thus as UNIX. However, in contrast to Apple, they make a point of not referring to themselves as being UNIX on their websites or other materials in order to avoid potentially costly legal problems. For example, FreeBSD, the most popular of the BSD systems (at least until the recent rise of Darwin), describes itself as being " . . . a very economical alternative to commercial UNIX® workstations." NetBSD describes itself on its homepage as " . . . a free, secure, and highly portable Unix-like Open Source operating system available for many platforms, . . . " OpenBSD describes itself as " . . . a FREE, multi-platform 4.4BSD-based UNIX-like operating system."

Some operating systems do not seek UNIX branding because the royalties would be prohibitively expensive for them due to the facts that they are updated frequently (which would require costly recertification) and that they are made available freely over the Internet. Moreover, developers of such systems generally feel that such branding would not provide any significant advantage, as their users are generally well informed and are interested in the intrinsic values of the systems rather than in their names.

The term Unix-like does not have any negative implication about the validity of the UNIX trademark. In fact, it could even be argued that it supports the trademark by providing a convenient term for describing operating systems that resemble the original UNIX and its early successors so that they, and their users, will not be tempted to inappropriately use the term UNIX.

Other terminology is also sometimes used to refer to Unix-like operating systems, particularly UN*X, Un*x, *NIX and *nix.

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¹An excellent summary of this dispute is provided by the article Open Group, Tech Attorney Talk of Apple's Unix Trademark Dispute, OSViews, April 2005.

Created April 19, 2005. Updated June 18, 2006.
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