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.
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.
Definition 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.
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.
History
"Unix-like" systems started to appear in the late 1970s and early 1980s.
Many proprietary versions, such as Idris, UNOS, Coherent, and UniFlex, 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.
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 Leopard, Mac OS X Snow Leopard, OS X Mountain Lion, and OS X Mavericks, 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,
they are referred to as "UNIX-like".
Categories 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 Landley 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 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 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, including having a "program
which manages your login and command line sessions"; 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 activities 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 LSB
work group.
Compatibility layers 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 GNU environment on top of the Microsoft Windows user API,
sufficient for most common open source software to be compiled and run.
Subsystem for Unix-based Applications provides Unix-like functionality as a Windows NT subsystem.
See also Berkeley Software Distribution
Linux distribution List of Linux distributions
List of Unix utilities List of operating systems
References
External links Unix-like Definition by The Linux Information
Project 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