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A Unix-like operating system is one that behaves in a manner similar to the UNIX system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification. The term can include open source operating systems inspired by UNIX or designed to emulate its features, commercial and proprietary work-alikes, and even licensed versions of UNIX (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. 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 an 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 disregard these guidelines, willfully treating "unix" as a generic term or descriptor for operating systems that are not necessarily covered by the "UNIX" trademark. Some abbreviate or "wildcard" the name as "*nix" (or some similar construction), which is also contrary to Open Group guidelines.
Development of Unix-like systems
The first "Unix-like" operating systems were developed because of AT&T's licensing of Unix, which prevented the sale of Unix for commercial purposes. These systems were intended to provide businesses with the features available to academic users of UNIX. The proprietary Unix-like operating systems that were available in the 1980s and early 1990s included Idris, Coherent, and UniFlex .
When AT&T later allowed commercial licensing of UNIX in the 1980s, a variety of proprietary systems developed, including AIX, HP-UX, IRIX, Solaris, Ultrix, and Xenix; these largely displaced the clones.
Meanwhile, non-commercial Unix-like operating systems were developed to serve as inexpensive or free substitutes for UNIX. These include BSD, GNU, Minix, and Linux. The BSDs are notable in that they are in fact descendents of Unix that were developed by the University of California at Berkeley with Unix source code that AT&T Bell Labs provided them. However, the BSDs are still UNIX-like operating systems.
Unix-like open source operating systems
The term is most often used as a simple way of referring to the Unix-like open source operating systems:
- FreeBSD and descendants:
- OpenBSD and descendants:
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