Evolution of Unix and Unix-like systems
|Company / developer||Many|
|Source model||Closed source, open source|
|Marketing target||Personal computers, embedded devices, mobile devices, servers|
|Available programming languages(s)||Many|
|Kernel type||Monolithic, Microkernel|
|Default user interface||Many|
A Unix-like (sometimes referred to as UN*X or *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.
There is no standard for defining the term, and some difference of opinion is possible as to the degree to which a given operating system is "Unix-like".
The term can include free and 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 sufficiently "Unix-like" to pass certification and bear 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 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.
Other parties frequently treat "Unix" as a genericized trademark. Some add a wildcard character to the name to make an abbreviation like "Un*x" or "*nix", since Unix-like systems often have Unix-like names such as AIX, A/UX, 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 or work-alike system, even those with completely dissimilar names such as Solaris or FreeBSD.
||This section needs additional citations for verification. (April 2010)|
"Unix-like" systems started to appear in the late 1970s and early 1980s. Many proprietary versions, such as Idris (1978), UNOS (1982), Coherent (1983), and UniFlex (1985), aimed to provide businesses with the functionality available to academic users of UNIX.
When AT&T allowed relatively inexpensive commercial binary sub-licensing of UNIX in 1979, 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 among 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 OS X. Notably, Mac OS X 10.5, Mac OS X 10.6, and OS X Mountain Lion running on Intel Macs are 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, Mac OS X 10.6 Snow Leopard, and OS X 10.8 Mountain Lion), they are referred to as "UNIX-like".
Dennis Ritchie, one of the original creators of Unix, expressed his opinion that Unix-like systems such as Linux are de facto Unix systems. Eric S. Raymond and Rob Langley have 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, although Apple Mac OS X 10.5 and later is a BSD variant that has been certified, and a few other systems (such as IBM 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 for commercial closed source systems. 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. There have been some acitivities to make Linux POSIX compliant, with Josey having prepared a list of differences between the POSIX standard and the Linux Standard Base specification, but in August 2005, this project was shut down because of missing interest at the Linux/FSF side.
Some non-Unix-like operating systems provide a Unix-like compatibility layer, with variable degrees of Unix-like functionality.
- IBM z/OS's UNIX System Services is sufficiently complete to be certified as trademark UNIX.
- Cygwin and MSYS both provide a reasonably complete GNU environment, sufficient for most common open source software to be compiled and run, with some emulation of Linux, on top of the Microsoft Windows user API.
- Interix provides Unix-like functionality as a Windows NT subsystem.
- Berkeley Software Distribution
- Linux distribution
- List of Linux distributions
- List of Unix utilities
- List of operating systems
- Trademark Guidelines The Open Group.
- Eric S. Raymond; Guy L. Steele Jr.. "UN*X". The Jargon File. Retrieved 2009-01-22.
- Gray v. Novell, X/Open Company, The SCO Group (11th Cir. 7 January 2011). Text
- "More Wayne Gray. No! Again? Still?! Yes. He Wants to Reopen Discovery in the USPTO Dispute". Groklaw. 22 April 2011. Retrieved 5 February 2013.
- Register of Open Branded Products The Open Group
- Interview with Dennis M. Ritchie Manuel Benet, LinuxFocus, July 1999
- The meaning of 'Unix' Eric Raymond and Rob Langley, OSI Position Paper on the SCO-vs.-IBM Complaint
- Andrew Josey (20 August 2005). "Conflicts between ISO/IEC 9945 (POSIX) and the Linux Standard Base". The Open Group. Retrieved 23 July 23 2012.
- Unix-like Definition by The Linux Information Project (LINFO)
- 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 (Dead link)