AUnix-like (sometimes referred to asUN*X,*nix or*NIX)operating system is one that behaves in a manner similar to aUnix system, although not necessarily conforming to or being certified to any version of theSingle UNIX Specification. A Unix-likeapplication is one that behaves like the correspondingUnix command orshell. Although there are generalphilosophies for Unix design, there is notechnical standard defining the term, and opinions can differ about the degree to which a particular operating system or application is Unix-like. Some well-known examples of Unix-like operating systems includeLinux,FreeBSD andOpenBSD. These systems are often used on servers as well as on personal computers and other devices. Many popular applications, such as theApache web server and theBash shell, are also designed to be used on Unix-like systems.
The Open Group owns theUNIXtrademark and administers the Single UNIX Specification, with the "UNIX" name being used as acertification mark. They do not approve of the construction "Unix-like", and consider it a 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.[1]
Other parties frequently treat "Unix" as agenericized trademark. Some add awildcard character to the name to make an abbreviation like "Un*x"[2] or "*nix", since Unix-like systems often have Unix-like names such asAIX,A/UX,HP-UX,IRIX,Linux,Minix,Ultrix,Xenix, andXNU. These patterns do not literally match many system names, but are still generally recognized to refer to any UNIX system, descendant, or work-alike, even those with completely dissimilar names such asDarwin/macOS,illumos/Solaris orFreeBSD.
In 2007, Wayne R. Gray sued to dispute the status of UNIX as a trademark, but lost his case, and lost again on appeal, with the court upholding the trademark and its ownership.[3][4]
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"Unix-like" systems started to appear in the late 1970s and early 1980s. Manyproprietary versions, such asIdris (1978),UNOS (1982),Coherent (1983), andUniFlex (1985), aimed to provide businesses with the functionality available to academic users of UNIX.
WhenAT&T allowed relatively inexpensive commercial binary sublicensing of UNIX in 1979, a variety of proprietary systems were developed based on it, includingAIX,HP-UX,IRIX,SunOS,Tru64,Ultrix, andXenix. These largely displaced the proprietary clones. Growing incompatibility among these systems led to the creation of interoperability standards, includingPOSIX and theSingle UNIX Specification.
Various free, low-cost, and unrestricted substitutes for UNIX emerged in the 1980s and 1990s, including4.4BSD,Linux, andMinix. Some of these have in turn been the basis for commercial "Unix-like" systems, such asBSD/OS andmacOS. Several versions of (Mac) OS X/macOS running on Intel-based and Apple silicon-based Mac computers have been certified under theSingle UNIX Specification.[5][6][7][8][9][10][11][12] The BSD variants are descendants of UNIX developed by the University of California at Berkeley, with UNIX source code fromBell Labs. However, the BSD code base has evolved since then, replacing all the AT&T code. Since theBSD variants are not certified as compliant with the Single UNIX Specification, they are referred to as "UNIX-like" rather than "UNIX".
Eric S. Raymond and Rob Landley have suggested that there are three kinds of Unix-like systems: "genetic UNIX" derived from the original Bell Labs UNIX, "trademark UNIX" (or "branded UNIX") for which theOpen Group have licensed the UNIX trademark, and a third category of operating systems that are not in either of those categories but that are patterned after the original Bell Labs UNIX.[13]
Those systems with a historical connection to theAT&T codebase. Most commercial UNIX systems fall into this category. So do theBSD systems, which are descendants of work done at theUniversity 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.
These systems—largely commercial in nature—have been determined by theOpen Group to meet theSingle 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, although Apple macOS 10.5 and later is a BSD variant that has been certified, andEulerOS[14][15] andInspur K-UX[16] are Linux distributions that have been certified. A few other systems (such as IBM z/OS) earned the trademark through a POSIX compatibility layer and are not otherwise inherently Unix systems. Manyancient UNIX systems no longer meet this definition.
Broadly, any Unix-like system that behaves in a manner roughly consistent with the UNIX specification, including having a "program which manages your login andcommand line sessions";[17] more specifically, this can refer to systems such asLinux orMinix 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.[18]
Dennis Ritchie, one of the original creators of Unix, mentioned Linux as one of the healthiest of the direct Unix derivatives due to its strong adherence to Unix principles.[19]
Around 2001 Linux was given the opportunity to get a certification including free help from the POSIX chair Andrew Josey for the symbolic price of one dollar.[citation needed] There have been some activities to make Linux POSIX-compliant, with Josey having prepared a list of differences between the POSIX standard and theLinux Standard Base specification,[20] but in August 2005, this project was shut down because of missing interest at the LSB work group.[citation needed] As per§ Trademark or branded UNIX, some Linux distributions have been certified.
Some non-Unix-like operating systems provide a Unix-likecompatibility layer, with varying degrees of Unix-like functionality.
Other means of Windows-Unix interoperability include:
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