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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, which owns the UNIX® trademark and administers the Single UNIX Specification, considers "Unix-like" to be a misuse of the name, and believes that a better alternative term would be "POSIX-conforming system", referring to the original standard established to facilitate compatibility between Unix-like systems. However, the term "POSIX-conforming" has its own problems. For example, there are systems which conform to at least the key POSIX standards (e.g., Microsoft Windows) and therefore can claim to be POSIX-conforming, but that are nevertheless not considered Unix-like and many applications written to compile and run on Unix will not work on them. Furthermore, operating systems that are generally considered "Unix-like" may not comply with the full POSIX specification.
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.
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:
See also
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