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A "Unix-like" 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 formal standard for defining the term, and some difference of opinion is possible as to whether a certain OS is "Unix-like" or not.
The term "Unix-like" and the UNIX trademark
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 generic noun or descriptor for operating systems that are not necessarily covered by the "UNIX" trademark, in much the same way that "Band-Aid" is used in reference to any bandage or "Xerox" to any photocopier. Some abbreviate or "wildcard" the name as "Un*x", "*nix", or some similar construction, which is also contrary to Open Group guidelines. These euphemistic spellings were derived as a way of intimating UNIX without formally saying it. They were inspired in part by a tendency for Unix-like systems to be given names resembling "UNIX", particularly ending in "x", such as AIX, IRIX, Linux, Minix, Ultrix, and Xenix. Few of these names actually match "*nix". However, wildcards like "*nix" are often meant to match any UNIX descendant system, even those with completely dissimilar names, such as Solaris or FreeBSD.
There is an active legal battle between Wayne R. Gray and Open Group that is centered around the use of the UNIX "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.
Categories
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, as do the BSD systems, descendants of work done at the University of California at 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, can be used to refer to systems such as GNU/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.
Development of Unix-like systems
"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, Solaris, 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 BSD, Linux, and Minix. Some of these have in turn been the basis for commercial "Unix-like" systems, such as BSD/OS, NEXTSTEP, and Mac OS X.
The various BSD systems 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, and these operating systems are not certified as compliant with the Single UNIX Specification, so they are merely "UNIX-like".
Current examples
Free Software / Open Source
Most free/open-source Unix-like systems do not seek UNIX branding for their product even if fully or mostly conformant; the costs for certification (especially given frequent release cycles) are generally considered prohibitive. The term Freenix is occasionally but rarely used to refer to such systems. Components are often shared between systems, with hybrids possible.
- Descendants of Berkeley Software Distribution 386BSD:
- FreeBSD and descendants:
- NetBSD and descendants:
- GNU/kNetBSD
- OpenBSD and descendants:
- GNU – Other systems based on GNU are often identified by the kernel they use:
- LUnix
- Minix and descendants:
- OpenSolaris — based on System V
- Plan 9 from Bell Labs – UNIX's successor, shares much of its design philosophy but for the whole system
- Inferno – The sister to Plan 9 from Bell Labs, now owned by Vita Nuova,
- Phoenix-RTOS
- VxWorks
Proprietary
- IBM AIX* — based on System V Release 3
- HP HP-UX*
- SGI IRIX*
- Apple Mac OS X — based on Darwin
- LynxOS RTOS
- QNX — total rewrite, no UNIX-related code
- Sun
- Compaq Tru64* — based on OSF/1
- Microsoft Xenix
* UNIX® branded systems
See also
External links
- UNIX history – a history time line graph of most UNIX and Unix-like systems by Éric Lévénez
- Grokline's UNIX Ownership History Project – a project to map out the technical history of UNIX and Unix-like systems
"Unix-like"