A Unix-like (sometimes shortened to *nix) 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. The term can include free software / open source operating systems inspired by Bell Labs’ Unix or designed to emulate its features, commercial and proprietary work-alikes, and even versions based on the licensed UNIX source code (which may be deemed so “Unix-like” that they are certified to bear the “UNIX” trademark). There is no standard for defining the term, and some difference of opinion is possible as to whether a certain OS is “Unix-like” or not.
The Open Group owns the UNIX trademark and administers the Single UNIX Specification, with the “UNIX” name being used as a certification mark. They do not approve of the construction “Unix-like”, and consider it misuse of their trademark. Their guidelines require “UNIX” to be presented in uppercase or otherwise distinguished from the surrounding text, strongly encourage using it as a branding adjective for a generic word such as “system”, and discourage its use in hyphenated phrases. The closest phrase they consider correct is “UNIX system-like”.
Other parties frequently treat “Unix” as a genericized trademark, in much the same way that in the United States of America “Band-Aid” is used in reference to any adhesive bandage. Some add a wildcard character to the name to make a euphemistic abbreviation like “Un*x” or “*nix” (the latter pronounced as "asternix", a play on the asterisk used for the wildcard), since Unix-like systems often have Unix-like names such as AIX, HP-UX, IRIX, Linux, Minix, Ultrix, and Xenix. These patterns do not literally match many system names, but are still generally recognized to refer to any UNIX descendant system, even those with completely dissimilar names such as Solaris, FreeBSD or Mac OS X. This is still contrary to Open Group guidelines.
As of 2007, there is an active legal battle between Wayne R. Gray and Open Group that centers on the use of UNIX as a trademark. Trademark Trial and Appeal Board court documents indicate that Gray’s legal team is seeking for the Open Group to provide documentation for their trademark claim.
Also in 2007, the Open Group forced the German University of Kassel not to use "UNIK" as their short form.
Dennis Ritchie, one of the original creators of UNIX, has expressed his opinion that Unix-like systems such as Linux are de facto UNIX systems. Eric S. Raymond has suggested that there are three kinds of Unix-like systems:Genetic UNIX: Those systems with a historical connection to the AT&T codebase. Most but not all commercial UNIX systems fall into this category. So do the BSD systems, which are descendants of work done at the University of California, Berkeley in the late 1970s and early 1980s. Some of these systems have no original AT&T code but can still trace their ancestry to AT&T designs.Trademark or Branded UNIX: These systems—largely commercial in nature—have been determined by the Open Group to meet the Single UNIX Specification and are allowed to carry the UNIX name. Most such systems are commercial derivatives of the System V code base in one form or another, though a few (such as IBM's z/OS) earned the trademark through a POSIX compatibility layer and are not otherwise inherently Unix systems. Many Ancient UNIX Systems no longer meet this definition.Functional UNIX: Broadly, any Unix-like system that behaves in a manner roughly consistent with the UNIX specification; more specifically, this can refer to systems such as Linux or Minix that behave similarly to a UNIX system but have no genetic or trademark connection to the AT&T code base. Most free/open-source implementations of the UNIX design, whether Genetic Unix or not, fall into the restricted definition of this third category due to the expense of obtaining Open Group certification which costs thousands of dollars.
Cygwin, while not being an operating system, does provide a Unix-like environment on top of Microsoft Windows.
“Unix-like” systems started to appear in the late ’70s and early ’80s. Many proprietary versions, such as Idris (1978), Coherent (1983), and UniFlex (1985), aimed to provide businesses with the functionality available to academic users of UNIX.
When AT&T later allowed commercial licensing of UNIX in the 1980s, a variety of proprietary systems were developed based on it, including AIX, HP-UX, IRIX, SunOS, Tru64, Ultrix, and Xenix. These largely displaced the proprietary clones. Growing incompatibility between these systems led to the creation of interoperability standards, including POSIX and the Single UNIX Specification.
Meanwhile, the GNU Project was launched in 1983 with the goal of making GNU, an operating system which all computer users could freely use, study, modify, and redistribute. Various “Unix-like” operating systems developed alongside GNU, frequently sharing substantial components with it (leading to some disagreement about whether they should be called “GNU” or not). These primarily served as low-cost and unrestricted substitutes for UNIX, and include 4.4BSD, Linux, and Minix. Some of these have in turn been the basis for commercial “Unix-like” systems, such as BSD/OS and Mac OS X. Notably, Mac OS X 10.5, Leopard, is certified under the Single UNIX Specification.
The various BSD variants are notable in that they are in fact descendants of UNIX, developed by the University of California at Berkeley with UNIX source code from Bell Labs. However, the BSD code base has evolved since then, replacing all of the AT&T code. Since the BSD variants are not certified as compliant with the Single UNIX Specification (except for Mac OS X 10.5 Leopard), they are referred to as "UNIX-'like'".
Most vendors of open source Unix-like systems do not seek UNIX branding for their product even if conformant: the costs for certification are generally considered prohibitive. The term Freenix is sometimes used to refer to such systems. Current examples include GNU, Linux, Minix, OpenSolaris, Plan 9, and BSD and its descendents. Notable BSD operating systems include FreeBSD, NetBSD, and OpenBSD.
There are a number of current proprietary Unix and Unix-likes, including AIX; HP-UX; IRIX; Mac OS X 10.0 (Cheetah) -10.5 (Leopard); LynxOS; QNX; SCO OpenServer; Solaris; Tru64 UNIX (based on OSF/1); UnixWare and Xenix.