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, although not necessarily conforming to or being certified to any version of the Single UNIX Specification
. A Unix-like application
is one that behaves like the corresponding Unix command
. Although there are general philosophies
for Unix design, there is no technical 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 include Linux and BSD. These systems are often used on servers, as well as on personal computers and other devices. Many popular applications, such as the Apache web server and the Bash shell, are also designed to be used on Unix-like systems.
One of the key features of Unix-like systems is their ability to support multiple users and processes simultaneously. This allows users to run multiple programs at the same time, and to share resources such as memory and disk space. This is in contrast to many older operating systems, which were designed to only support a single user or process at a time. Another important feature of Unix-like systems is their modular design. This means that the operating system is made up of many small, interchangeable components that can be added or removed as needed. This makes it easy to customize the operating system to suit the needs of different users or environments.
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
, and XNU
. 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 as Darwin
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.
"Unix-like" systems started to appear in the late 1970s and early 1980s. Many proprietary
versions, such as Idris
(1983), and UniFlex
(1985), aimed to provide businesses with the functionality available to academic users of UNIX.
allowed relatively inexpensive commercial binary sub-licensing of UNIX in 1979, a variety of proprietary systems were developed based on it, including AIX
, 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
Various free, low-cost, and unrestricted substitutes for UNIX emerged in the 1980s and 1990s, including 4.4BSD
, and Minix
. Some of these have in turn been the basis for commercial "Unix-like" systems, such as BSD/OS
. Several versions of (Mac) OS X/macOS running on Intel-based Mac computers have been certified under the Single UNIX Specification
The BSD variants are 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" rather than "UNIX".
, one of the original creators of Unix, expressed his opinion that Unix-like systems such as Linux
''De facto'' ( ; , "in fact") describes practices that exist in reality, whether or not they are officially recognized by laws or other formal norms. It is commonly used to refer to what happens in practice, in contrast with '' de jure'' ("by l ...
'' Unix systems. Eric S. Raymond
and Rob Landley have suggested that there are three kinds of Unix-like systems:
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 systemslargely commercial in naturehave 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 macOS 10.5
and later is a BSD
variant that has been certified, EulerOS
and Inspur K-UX
distributions that have 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.
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
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.
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.
Some non-Unix-like operating systems provide a Unix-like compatibility layer
, with varying degrees of Unix-like functionality.
* IBM z/OS
's UNIX System Services
is sufficiently complete as to be certified as trademark UNIX.
, and MSYS2
each provide a GNU
environment on top of the Microsoft Windows user API
, sufficient for most common open source
software to be compiled and run.
* The MKS Toolkit
are comprehensive interoperability tools which allow the porting of Unix programs to Windows.
* Windows NT-type systems have a POSIX environmental subsystem
* Subsystem for Unix-based Applications
) provides Unix-like functionality as a Windows NT subsystem
* Windows Subsystem for Linux
provides a Linux
-compatible kernel interface developed by Microsoft and containing no Linux code, with Ubuntu
running on top of it.
** Windows Subsystem for Linux version 2
(WSL2) provides a fully functional Linux environment running in a virtual machine.
Other means of Windows-Unix interoperability include:
* The above Windows packages can be used with various X servers for Windows
* Hummingbird Connectivity
provides several ways for Windows machines to connect to Unix and Linux machines, from terminal emulators to X clients and servers, and others
* The Windows Resource Kit
s for versions of Windows NT include a Bourne Shell
, some command-line tools, and a version of Perl
* Hamilton C shell
is a version of csh
written specifically for Windows.
* List of Unix-like systems
* Berkeley Software Distribution
* Linux kernel
and Linux distribution
* List of Linux distributions
* List of Unix commands
* List of operating systems
* Free Software Foundation
and GNU Project
by The Linux Information Project (LINFO)
a history time line graph of most UNIX and Unix-like systems by Éric Lévénez
Operating system families