A Unix-like (sometimes referred to as UN*X or *nix)

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...

is one that behaves in a manner similar to a

Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...

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 This is a list of Unix commands as specified by IEEE Std 1003.1-2008, which is part of the Single UNIX Specification (SUS). These commands can be found on Unix operating systems and most Unix-like operating systems. List See also * List of G ...

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses ** Thin-shell structure Science Biology * Seashell, a hard o ...

. Although there are general

philosophies Philosophical schools of thought and philosophical movements. A Absurdism - Action, philosophy of - Actual idealism - Actualism - Advaita Vedanta - Aesthetic Realism - Aesthetics - African philosophy - Afrocentrism - Agential realism - ...

for Unix design, there is no

technical standard A technical standard is an established norm or requirement for a repeatable technical task which is applied to a common and repeated use of rules, conditions, guidelines or characteristics for products or related processes and production methods, ...

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.

Definition

The Open Group The Open Group is a global consortium that seeks to "enable the achievement of business objectives" by developing "open, vendor-neutral technology standards and certifications." It has over 840 member organizations and provides a number of servi ...

owns the

UNIX Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...

trademark A trademark (also written trade mark or trade-mark) is a type of intellectual property consisting of a recognizable sign, design, or expression that identifies products or services from a particular source and distinguishes them from othe ...

and administers the Single UNIX Specification, with the "UNIX" name being used as a

certification mark A certification mark (or conformity mark) on a commercial product indicates the existence of an accepted product standard or regulation and a claim that the manufacturer has verified compliance with those standards or regulations. The specifi ...

. 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 A generic trademark, also known as a genericized trademark or proprietary eponym, is a trademark or brand name that, because of its popularity or significance, has become the generic term for, or synonymous with, a general class of products ...

. Some add a

wildcard character In software, a wildcard character is a kind of placeholder represented by a single character, such as an asterisk (), which can be interpreted as a number of literal characters or an empty string. It is often used in file searches so the full na ...

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 Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, w ...

, Minix, Ultrix, Xenix, 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/

macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...

illumos Illumos (stylized as illumos) is a partly free and open-source Unix operating system. It is based on OpenSolaris, which was based on System V Release 4 (SVR4) and the Berkeley Software Distribution (BSD). Illumos comprises a kernel, device d ...

/ 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 (1978), UNOS (1982), Coherent (1983), and UniFlex (1985), aimed to provide businesses with the functionality available to academic users of UNIX. When

AT&T AT&T Inc. is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's largest telecommunications company by revenue and the third largest provider of mobile te ...

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 Tru64 UNIX is a discontinued 64-bit UNIX operating system for the Alpha instruction set architecture (ISA), currently owned by Hewlett-Packard (HP). Previously, Tru64 UNIX was a product of Compaq, and before that, Digital Equipment Corporation (DE ...

, Ultrix, and Xenix. These largely displaced the proprietary clones. Growing incompatibility among these systems led to the creation of interoperability standards, including

POSIX The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inter ...

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 BSD/OS (originally called BSD/386 and sometimes known as BSDi) is a discontinued proprietary version of the BSD operating system developed by Berkeley Software Design, Inc. (BSDi). BSD/OS had a reputation for reliability in server roles; the ren ...

and

. 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 Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial Research and development, research and scientific developm ...

. 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".

Categories

Dennis Ritchie, one of the original creators of Unix, expressed his opinion that Unix-like systems such as

are ''

de facto ''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 la ...

'' 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

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 The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant u ...

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 The Open Group is a global consortium that seeks to "enable the achievement of business objectives" by developing "open, vendor-neutral technology standards and certifications." It has over 840 member organizations and provides a number of servi ...

to meet the Single UNIX Specification and are allowed to carry the UNIX name. Most such systems are commercial derivatives of the

System V Unix System V (pronounced: "System Five") is one of the first commercial versions of the Unix operating system. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, numbered 1, 2, 3, an ...

code base in one form or another, although Apple

10.5 and later is a BSD variant that has been certified, EulerOS and Inspur K-UX are

distributions that have been certified, and a few other systems (such as IBM

z/OS z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest: * ...

) earned the trademark through a

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

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. 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 varying degrees of Unix-like functionality. * IBM

's UNIX System Services is sufficiently complete as to be certified as trademark UNIX. * Cygwin, MSYS, and

MSYS2 Mingw-w64 is a free and open source software development environment to create ( cross-compile) Microsoft Windows PE applications. It was forked in 2005–2010 from MinGW (''Minimalist GNU for Windows''). Mingw-w64 includes a port of the GNU C ...

each provide a

GNU GNU () is an extensive collection of free software (383 packages as of January 2022), which can be used as an operating system or can be used in parts with other operating systems. The use of the completed GNU tools led to the family of operat ...

environment on top of the Microsoft Windows user API, sufficient for most common open source software to be compiled and run. * The MKS Toolkit and

UWIN UWIN is a computer software package created by David Korn which allows programs written for the operating system Unix to be built and run on Microsoft Windows with few, if any, changes. Some of the software development was subcontracted to Wipro ...

are comprehensive interoperability tools which allow the porting of Unix programs to Windows. * Windows NT-type systems have a

environmental subsystem. *

Subsystem for Unix-based Applications Interix was an optional, POSIX-conformant Unix subsystem for Windows NT operating systems. Interix was a component of Windows Services for UNIX, and a superset of the Microsoft POSIX subsystem. Like the POSIX subsystem, Interix was an environment ...

(previously Interix) provides Unix-like functionality as a Windows NT

subsystem A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and expresse ...

(discontinued). *

Windows Subsystem for Linux Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ser ...

provides a

-compatible kernel interface developed by Microsoft and containing no Linux code, with

Ubuntu Ubuntu ( ) is a Linux distribution based on Debian and composed mostly of free and open-source software. Ubuntu is officially released in three editions: '' Desktop'', ''Server'', and ''Core'' for Internet of things devices and robots. All ...

user-mode binaries 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 Kits for versions of Windows NT include a Bourne Shell, some command-line tools, and a version of

Perl Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...

* Hamilton C shell is a version of csh written specifically for Windows.

References

External links

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 * {{DEFAULTSORT:Unix-Like Unix Operating system families