Unix (/ˈjuːnɪks/; trademarked as UNIX) is a family of multitasking,
multiuser computer operating systems that derive from the original
AT&T Unix, development starting in the 1970s at the Bell Labs
research center by Ken Thompson, Dennis Ritchie, and others.
Initially intended for use inside the Bell System, AT&T licensed
Unix to outside parties in the late 1970s, leading to a variety of
both academic and commercial
Unix variants from vendors like the
University of California, Berkeley
University of California, Berkeley (BSD),
Microsoft (Xenix), IBM
Sun Microsystems (Solaris). In the early 1990s, AT&T
sold its rights in
Unix to Novell, which then sold its
Santa Cruz Operation (SCO) in 1995. The UNIX trademark
passed to The Open Group, a neutral industry consortium, which allows
the use of the mark for certified operating systems that comply with
Single UNIX Specification (SUS). As of 2014, the
Unix version with
the largest installed base is Apple's macOS.
Unix systems are characterized by a modular design that is sometimes
called the "
Unix philosophy". This concept entails that the operating
system provides a set of simple tools that each perform a limited,
well-defined function, with a unified filesystem as the main means
of communication, and a shell scripting and command language to
combine the tools to perform complex workflows.
itself from its predecessors as the first portable operating system:
almost the entire operating system is written in the C programming
language, thus allowing
Unix to reach numerous platforms.
7 See also
9 Further reading
10 External links
Version 7 Unix, the
Research Unix ancestor of all modern
Unix was originally meant to be a convenient platform for programmers
developing software to be run on it and on other systems, rather than
for non-programmers. The system grew larger as the operating
system started spreading in academic circles, as users added their own
tools to the system and shared them with colleagues.
Unix was designed to be portable, multi-tasking and multi-user in a
Unix systems are characterized by various
concepts: the use of plain text for storing data; a hierarchical file
system; treating devices and certain types of inter-process
communication (IPC) as files; and the use of a large number of
software tools, small programs that can be strung together through a
command-line interpreter using pipes, as opposed to using a single
monolithic program that includes all of the same functionality. These
concepts are collectively known as the "
Unix philosophy". Brian
Rob Pike summarize this in The
Environment as "the idea that the power of a system comes more from
the relationships among programs than from the programs
In an era when a standard computer consisted of a hard disk for
storage and a data terminal for input and output (I/O), the
model worked quite well, as I/O was generally linear. In the 1980s,
non-blocking I/O and the set of inter-process communication mechanisms
were augmented with
Unix domain sockets, shared memory, message
queues, and semaphores, and network sockets were added to support
communication with other hosts. As graphical user interfaces
developed, the file model proved inadequate to the task of handling
asynchronous events such as those generated by a mouse.
By the early 1980s, users began seeing
Unix as a potential universal
operating system, suitable for computers of all sizes. The
Unix environment and the client–server program model were essential
elements in the development of the
Internet and the reshaping of
computing as centered in networks rather than in individual computers.
Unix and the C programming language were developed by AT&T
and distributed to government and academic institutions, which led to
both being ported to a wider variety of machine families than any
other operating system.
Under Unix, the operating system consists of many libraries and
utilities along with the master control program, the kernel. The
kernel provides services to start and stop programs, handles the file
system and other common "low-level" tasks that most programs share,
and schedules access to avoid conflicts when programs try to access
the same resource or device simultaneously. To mediate such access,
the kernel has special rights, reflected in the division between user
space and kernel space.
The microkernel concept was introduced in an effort to reverse the
trend towards larger kernels and return to a system in which most
tasks were completed by smaller utilities. In microkernel
implementations, functions such as network protocols could be moved
out of the kernel, while conventional (monolithic) Unix
implementations have network protocol stacks as part of the kernel.
Main article: History of Unix
Ken Thompson (sitting) and
Dennis Ritchie working together at a PDP-11
The origins of
Unix date back to the mid-1960s when the Massachusetts
Institute of Technology, Bell Labs, and
General Electric were
developing Multics, a time-sharing operating system for the GE-645
Multics featured several innovations, but also
presented severe problems. Frustrated by the size and complexity of
Multics, but not by its goals, individual researchers at Bell Labs
started withdrawing from the project. The last to leave were Ken
Thompson, Dennis Ritchie, Douglas McIlroy, and Joe Ossanna, who
decided to reimplement their experiences in a new project of smaller
scale. This new operating system was initially without organizational
backing, and also without a name.
The new operating system was a single-tasking system. In 1970, the
group coined the name Unics for Uniplexed Information and Computing
Service (pronounced "eunuchs"), as a pun on Multics, which stood for
Multiplexed Information and Computer Services.
Brian Kernighan takes
credit for the idea, but adds that "no one can remember" the origin of
the final spelling Unix. Dennis Ritchie, Doug McIlroy, and
Peter G. Neumann also credit Kernighan.
The operating system was originally written in assembly language, but
in 1972, the code was migrated to the C programming language. This
new language which was still in development to address shortcomings,
such as byte addressability, of its predecessor, the B programming
language, would become historically tied to the progress in
development of Unix. As a high-level programming language it greatly
improved portability of the
Unix software, requiring only a relatively
small amount of machine-dependent code to be replaced when porting to
other computing platforms.
Bell Labs produced several versions of
Unix that are collectively
referred to as "Research Unix". In 1975, the first source license for
UNIX was sold to
Donald B. Gillies
Donald B. Gillies at the
University of Illinois
Department of Computer Science. UIUC graduate student Greg
Chesson, who had worked on the UNIX kernel at Bell Labs, was
instrumental in negotiating the terms of the license.
During the late 1970s and early 1980s, the influence of
academic circles led to large-scale adoption of
System V) by commercial startups, including Sequent, HP-UX,
Solaris, AIX, and Xenix. In the late 1980s, AT&T
Sun Microsystems developed System V
Release 4 (SVR4), which was subsequently adopted by many
In the 1990s,
Unix-like systems grew in popularity as
Linux distributions were developed through collaboration by a
worldwide network of programmers. In 2000, Apple released Darwin, also
Unix system, which became the core of the Mac OS X operating system,
which was later renamed macOS.
Unix operating systems are widely used in modern servers,
workstations, and mobile devices.
Common Desktop Environment
Common Desktop Environment (CDE), part of the COSE initiative
In the late 1980s, an open operating system standardization effort now
POSIX provided a common baseline for all operating systems;
POSIX around the common structure of the major competing
variants of the
Unix system, publishing the first
POSIX standard in
1988. In the early 1990s, a separate but very similar effort was
started by an industry consortium, the Common Open Software
Environment (COSE) initiative, which eventually became the Single UNIX
Specification (SUS) administered by The Open Group. Starting in 1998,
the Open Group and
IEEE started the Austin Group, to provide a common
POSIX and the Single UNIX Specification, which, by 2008,
had become the Open Group Base Specification.
In 1999, in an effort towards compatibility, several
vendors agreed on SVR4's
Executable and Linkable Format
Executable and Linkable Format (ELF) as the
standard for binary and object code files. The common format allows
substantial binary compatibility among
Unix systems operating on the
same CPU architecture.
Filesystem Hierarchy Standard was created to provide a reference
directory layout for
Unix-like operating systems, and has mainly been
used in Linux.
See also: List of
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Unix system is composed of several components that were originally
packaged together. By including the development environment,
libraries, documents and the portable, modifiable source code for all
of these components, in addition to the kernel of an operating system,
Unix was a self-contained software system. This was one of the key
reasons it emerged as an important teaching and learning tool and has
had such a broad influence.[according to whom?]
The inclusion of these components did not make the system
large – the original V7 UNIX distribution, consisting of
copies of all of the compiled binaries plus all of the source code and
documentation occupied less than 10 MB and arrived on a single
nine-track magnetic tape. The printed documentation, typeset from the
online sources, was contained in two volumes.
The names and filesystem locations of the
Unix components have changed
substantially across the history of the system. Nonetheless, the V7
implementation is considered by many[who?] to have the canonical early
Kernel – source code in /usr/sys, composed of several
conf – configuration and machine-dependent parts, including
dev – device drivers for control of hardware (and some
sys – operating system "kernel", handling memory management,
process scheduling, system calls, etc.
h – header files, defining key structures within the system
and important system-specific invariables
Development environment – early versions of
Unix contained a
development environment sufficient to recreate the entire system from
cc – C language compiler (first appeared in V3 Unix)
as – machine-language assembler for the machine
ld – linker, for combining object files
lib – object-code libraries (installed in /lib or /usr/lib).
libc, the system library with C run-time support, was the primary
library, but there have always been additional libraries for such
things as mathematical functions (libm) or database access. V7 Unix
introduced the first version of the modern "Standard I/O" library
stdio as part of the system library. Later implementations increased
the number of libraries significantly.
make – build manager (introduced in PWB/UNIX), for effectively
automating the build process
include – header files for software development, defining
standard interfaces and system invariants
Other languages – V7
Unix contained a Fortran-77 compiler, a
programmable arbitrary-precision calculator (bc, dc), and the awk
scripting language; later versions and implementations contain many
other language compilers and toolsets. Early
BSD releases included
Pascal tools, and many modern
Unix systems also include the GNU
Compiler Collection as well as or instead of a proprietary compiler
Other tools – including an object-code archive manager (ar),
symbol-table lister (nm), compiler-development tools (e.g. lex &
yacc), and debugging tools.
Unix makes little distinction between commands
(user-level programs) for system operation and maintenance (e.g.
cron), commands of general utility (e.g. grep), and more
general-purpose applications such as the text formatting and
typesetting package. Nonetheless, some major categories are:
sh – the "shell" programmable command-line interpreter, the
primary user interface on
Unix before window systems appeared, and
even afterward (within a "command window").
Utilities – the core toolkit of the
Unix command set,
including cp, ls, grep, find and many others. Subcategories include:
System utilities – administrative tools such as mkfs, fsck,
and many others.
User utilities – environment management tools such as passwd,
kill, and others.
Document formatting –
Unix systems were used from the outset
for document preparation and typesetting systems, and included many
related programs such as nroff, troff, tbl, eqn, refer, and pic. Some
Unix systems also include packages such as
TeX and Ghostscript.
Graphics – the plot subsystem provided facilities for
producing simple vector plots in a device-independent format, with
device-specific interpreters to display such files. Modern Unix
systems also generally include
X11 as a standard windowing system and
GUI, and many support OpenGL.
Communications – early
Unix systems contained no inter-system
communication, but did include the inter-user communication programs
mail and write. V7 introduced the early inter-system communication
system UUCP, and systems beginning with
BSD release 4.1c included
Unix was the first operating
system to include all of its documentation online in machine-readable
form. The documentation included:
man – manual pages for each command, library component, system
call, header file, etc.
doc – longer documents detailing major subsystems, such as the
C language and troff
See also: Unix-like
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Ken Thompson and Dennis Ritchie, principal developers of Research Unix
USENIX 1984, including
Dennis Ritchie (center)
Plan 9 from
Bell Labs extends
Unix design principles, and was
developed as a successor to Unix
Unix system had significant impact on other operating systems. It
achieved its reputation by its interactivity, by providing the
software at a nominal fee for educational use, by running on
inexpensive hardware, and by being easy to adapt and move to different
Unix was originally written in assembly language (which had
been thought necessary for system implementations on early computers),
but was soon rewritten in C, a high-level programming language.
Although this followed the lead of
Multics and Burroughs, it was Unix
that popularized the idea.
Unix had a drastically simplified file model compared to many
contemporary operating systems: treating all kinds of files as simple
byte arrays. The file system hierarchy contained machine services and
devices (such as printers, terminals, or disk drives), providing a
uniform interface, but at the expense of occasionally requiring
additional mechanisms such as ioctl and mode flags to access features
of the hardware that did not fit the simple "stream of bytes" model.
The Plan 9 operating system pushed this model even further and
eliminated the need for additional mechanisms.
Unix also popularized the hierarchical file system with arbitrarily
nested subdirectories, originally introduced by Multics. Other common
operating systems of the era had ways to divide a storage device into
multiple directories or sections, but they had a fixed number of
levels, often only one level. Several major proprietary operating
systems eventually added recursive subdirectory capabilities also
patterned after Multics. DEC's RSX-11M's "group, user" hierarchy
evolved into VMS directories, CP/M's volumes evolved into
subdirectories, and HP's MPE group.account hierarchy and IBM's SSP and
OS/400 library systems were folded into broader
POSIX file systems.
Making the command interpreter an ordinary user-level program, with
additional commands provided as separate programs, was another Multics
innovation popularized by Unix. The
Unix shell used the same language
for interactive commands as for scripting (shell scripts –
there was no separate job control language like IBM's JCL). Since the
shell and OS commands were "just another program", the user could
choose (or even write) his own shell. New commands could be added
without changing the shell itself. Unix's innovative command-line
syntax for creating modular chains of producer-consumer processes
(pipelines) made a powerful programming paradigm (coroutines) widely
available. Many later command-line interpreters have been inspired by
A fundamental simplifying assumption of
Unix was its focus on
newline-delimited text for nearly all file formats. There were no
"binary" editors in the original version of Unix – the entire
system was configured using textual shell command scripts. The common
denominator in the I/O system was the byte – unlike
"record-based" file systems. The focus on text for representing nearly
Unix pipes especially useful, and encouraged the
development of simple, general tools that could be easily combined to
perform more complicated ad hoc tasks. The focus on text and bytes
made the system far more scalable and portable than other systems.
Over time, text-based applications have also proven popular in
application areas, such as printing languages (PostScript, ODF), and
at the application layer of the
Internet protocols, e.g., FTP, SMTP,
HTTP, SOAP, and SIP.
Unix popularized a syntax for regular expressions that found
widespread use. The
Unix programming interface became the basis for a
widely implemented operating system interface standard (POSIX, see
above). The C programming language soon spread beyond Unix, and is now
ubiquitous in systems and applications programming.
Unix developers were important in bringing the concepts of
modularity and reusability into software engineering practice,
spawning a "software tools" movement. Over time, the leading
Unix (and programs that ran on it) established a set of
cultural norms for developing software, norms which became as
important and influential as the technology of
Unix itself; this has
been termed the
TCP/IP networking protocols were quickly implemented on the Unix
versions widely used on relatively inexpensive computers, which
contributed to the
Internet explosion of worldwide real-time
connectivity, and which formed the basis for implementations on many
Unix policy of extensive on-line documentation and (for many
years) ready access to all system source code raised programmer
expectations, and contributed to the 1983 launch of the free software
Operating system §
Console screenshots of
Debian (top, a popular
Linux distribution) and
BSD (bottom, a popular
Unix-like operating system)
Richard Stallman announced the
GNU (short for "GNU's Not
Unix") project, an ambitious effort to create a free software
Unix-like system; "free" in the sense that everyone who received a
copy would be free to use, study, modify, and redistribute it. The GNU
project's own kernel development project,
GNU Hurd, had not produced a
working kernel, but in 1991
Linus Torvalds released the
as free software under the
GNU General Public License. In addition to
their use in the
Linux operating system, many
GNU packages –
such as the
GNU Compiler Collection
GNU Compiler Collection (and the rest of the GNU
GNU C library and the
GNU core utilities – have
gone on to play central roles in other free
Unix systems as well.
Linux distributions, consisting of the
Linux kernel and large
collections of compatible software have become popular both with
individual users and in business. Popular distributions include Red
Hat Enterprise Linux, Fedora, SUSE
Linux Enterprise, openSUSE, Debian
Linux Mint, Mandriva Linux, Slackware Linux, and
A free derivative of
BSD Unix, 386BSD, was released in 1992 and led to
BSD and Free
BSD projects. With the 1994 settlement of a lawsuit
brought against the
University of California and Berkeley Software
Design Inc. (USL v. BSDi) by UNIX Systems Laboratories, it was
clarified that Berkeley had the right to distribute
Unix for free,
if it so desired. Since then,
Unix has been developed in several
different product branches, including Open
BSD and DragonFly BSD.
BSD are increasingly filling the market needs traditionally
served by proprietary
Unix operating systems, as well as expanding
into new markets such as the consumer desktop and mobile and embedded
devices. Because of the modular design of the
Unix model, sharing
components is relatively common; consequently, most or all
Unix-like systems include at least some
BSD code, and some systems
GNU utilities in their distributions.
In a 1999 interview,
Dennis Ritchie voiced his opinion that
BSD operating systems are a continuation of the basis of the Unix
design, and are derivatives of Unix:
"I think the
Linux phenomenon is quite delightful, because it draws so
strongly on the basis that
Linux seems to be the among
the healthiest of the direct
Unix derivatives, though there are also
BSD systems as well as the more official offerings from
the workstation and mainframe manufacturers."
In the same interview, he states that he views both
"the continuation of ideas that were started by Ken and me and many
others, many years ago."
OpenSolaris was the open-source counterpart to Solaris developed by
Sun Microsystems, which included a CDDL-licensed kernel and a
GNU userland. However, Oracle discontinued the project upon
their acquisition of Sun, which prompted a group of former Sun
employees and members of the
OpenSolaris community to fork OpenSolaris
into the illumos kernel. As of 2014, illumos remains the only active
System V derivative.
In May 1975, RFC 681 described the development of Network
Unix by the
Center for Advanced Computation at the
University of Illinois. The
system was said to "present several interesting capabilities as an
ARPANET mini-host". At the time
Unix required a license from Bell
Laboratories that at $20,000(US) was very expensive for non-university
users, while an educational license cost just $150. It was noted that
Bell was "open to suggestions" for an ARPANET-wide license.
Specific features found beneficial were the local processing
facilities, compilers, editors, a document preparation system,
efficient file system and access control, mountable and unmountable
volumes, unified treatment of peripherals as special files,
integration of the network control program (NCP) within the
system, treatment of network connections as special files that can be
accessed through standard
Unix I/O calls, closing of all files on
program exit, and the decision to be "desirable to minimize the amount
of code added to the basic
See also: List of
Promotional license plate by Digital Equipment Corporation
HP9000 workstation running HP-UX, a certified
Unix operating system
In October 1993, Novell, the company that owned the rights to the Unix
System V source at the time, transferred the trademarks of
Unix to the
X/Open Company (now The Open Group), and in 1995 sold the related
business operations to
Santa Cruz Operation (SCO). Whether Novell
also sold the copyrights to the actual software was the subject of a
2006 federal lawsuit, SCO v. Novell, which
Novell won. The case was
appealed, but on August 30, 2011, the United States Court of Appeals
for the Tenth Circuit affirmed the trial decisions, closing the
SCO Group Inc. accused
Novell of slander of
The present owner of the trademark UNIX is The Open Group, an industry
standards consortium. Only systems fully compliant with and certified
Single UNIX Specification qualify as "UNIX" (others are called
By decree of The Open Group, the term "UNIX" refers more to a class of
operating systems than to a specific implementation of an operating
system; those operating systems which meet The Open Group's Single
UNIX Specification should be able to bear the
UNIX 98 or UNIX 03
trademarks today, after the operating system's vendor pays a
substantial certification fee and annual trademark royalties to The
Open Group. Systems licensed to use the UNIX trademark include
AIX, HP-UX, Inspur K-UX, IRIX, Solaris, Tru64 UNIX
(formerly "Digital UNIX", or OSF/1), macOS, and a part of
Inspur K-UX is a
Linux distribution certified as
UNIX 03 compliant.
Sometimes a representation like Un*x, *NIX, or *N?X is used to
indicate all operating systems similar to Unix. This comes from the
use of the asterisk (*) and the question mark characters as wildcard
indicators in many utilities. This notation is also used to describe
Unix-like systems that have not met the requirements for UNIX
branding from the Open Group.
The Open Group requests that UNIX is always used as an adjective
followed by a generic term such as system to help avoid the creation
of a genericized trademark.
Unix was the original formatting, but the usage of UNIX remains
widespread because it was once typeset in small caps (Unix). According
to Dennis Ritchie, when presenting the original
Unix paper to the
third Operating Systems Symposium of the American Association for
Computing Machinery (ACM), "we had a new typesetter and troff had just
been invented and we were intoxicated by being able to produce small
caps." Many of the operating system's predecessors and
contemporaries used all-uppercase lettering, so many people wrote the
name in upper case due to force of habit. It is not an acronym.
Trademark names can be registered by different entities in different
countries and trademark laws in some countries allow the same
trademark name to be controlled by two different entities if each
entity uses the trademark in easily distinguishable categories. The
result is that
Unix has been used as a brand name for various products
including book shelves, ink pens, bottled glue, diapers, hair driers
and food containers.
Several plural forms of
Unix are used casually to refer to multiple
Unix-like systems. Most common is the conventional
Unixes, but Unices, treating
Unix as a
Latin noun of the third
declension, is also popular. The pseudo-Anglo-Saxon plural form Unixen
is not common, although occasionally seen. Sun Microsystems, developer
of the Solaris variant, has asserted that the term
Unix is itself
plural, referencing its many implementations.
Comparison of operating systems and open-source and closed-source
List of operating systems,
Unix systems, and
Market share of operating systems
Operating systems timeline
Plan 9 from Bell Labs
Year 2038 problem
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Wikimedia Commons has media related to Unix.
The Wikibook Guide to
Unix has a page on the topic of: Commands
The UNIX System, at The Open Group.
The Evolution of the
Time-sharing System at the Wayback Machine
(archived 8 April 2015)
The Creation of the UNIX Operating System at the Wayback Machine
(archived 2 April 2014)
Unix Tree: files from historic releases
Unix at Curlie (based on DMOZ)
Unix 1st Edition Manuals.
1982 film about
Unix featuring Dennis Ritchie, Ken Thompson, Brian
Kernighan, Alfred Aho, and more
A History of UNIX before Berkeley: UNIX Evolution: 1975-1984
BYTE Magazine, September 1986: UNIX and the MC68000 – a
software perspective on the MC68000 CPU architecture and UNIX
Unix command-line interface programs and shell builtins
true and false
Unix SUS2008 utilities
Unix-like operating systems
Italics indicate discontinued branches. Category
Loadable kernel module
Process control block
Multilevel feedback queue
Shortest job next
Memory management and
General protection fault
Storage access and
Virtual file system
Virtual tape library
Classic Mac OS