Unix

Unix(/ˈjuːnɪks/,YOO-niks;trademarked asUNIX) is a family ofmultitasking,multi-usercomputeroperating systemsthat derive from the originalAT&TUnix, whose development started in 1969at theBell Labsresearch center byKen Thompson,Dennis Ritchie,and others.

Unix

Unix System IIIrunning on aPDP-11simulator
Developer	Ken Thompson,Dennis Ritchie,Brian Kernighan,Douglas McIlroy,andJoe OssannaatBell Labs
Written in	Candassembly language
OS family	Unix
Source model	Historicallyproprietary software,while some Unix projects (includingBSDfamily andillumos) areopen-source
Initial release	Development started in 1969 First manual publishedinternallyin November 1971(1971-11) Announced outside Bell Labs in October 1973(1973-10)
Available in	English
Kerneltype	Varies;monolithic,microkernel,hybrid
Influenced by	CTSS,Multics
Default user interface	Command-line interfaceandGraphical(WaylandandX Window System;AndroidSurfaceFlinger;macOSQuartz)
License	Varies; some versions areproprietary,others arefree/open-source software
Official website	www.opengroup.org/unix

Initially intended for use inside theBell System,AT&TlicensedUnix to outside parties in the late 1970s, leading to a variety of both academic and commercial Unix variants from vendors includingUniversity of California, Berkeley(BSD),Microsoft(Xenix),Sun Microsystems(SunOS/Solaris),HP/HPE(HP-UX), andIBM(AIX). In the early 1990s,AT&Tsold its rights in Unix toNovell,which then sold the UNIX trademark toThe Open Group,an industry consortium founded in 1996. The Open Group allows the use of the mark for certified operating systems that comply with theSingle UNIX Specification(SUS).

Early versions of Unix ran onPDP-11computers.

Unix systems are characterized by amodular designthat is sometimes called the "Unix philosophy".According to this philosophy, the operating system should provide a set of simple tools, each of which performs a limited, well-defined function.A unified andinode-basedfilesystemand aninter-process communicationmechanism known as "pipes"serve as the main means of communication,and ashellscripting and command language (theUnix shell) is used to combine the tools to perform complex workflows.

Unix distinguishes itself from its predecessors as the firstportableoperating system: almost the entire operating system is written in theC programming language,which allows Unix to operate on numerous platforms.

Overview

Version 7 Unix,theResearch Unixancestor of all modern Unix systems

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, and as users added their own tools to the system and shared them with colleagues.

At first, Unix was not designed to beportableor formulti-tasking.Later, Unix gradually gained portability, multi-tasking andmulti-usercapabilities in atime-sharingconfiguration. Unix systems are characterized by various concepts: the use ofplain textfor storing data; ahierarchical file system;treating devices and certain types ofinter-process communication(IPC) as files; and the use of a large number ofsoftware tools,small programs that can be strung together through acommand-line interpreterusingpipes,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 KernighanandRob Pikesummarize this inThe Unix Programming Environmentas "the idea that the power of a system comes more from the relationships among programs than from the programs themselves".

By the early 1980s, users began seeing Unix as a potential universal operating system, suitable for computers of all sizes.The Unix environment and theclient–serverprogram model were essential elements in the development of theInternetand the reshaping of computing as centered innetworksrather than in individual computers.

Both Unix and theC programming languagewere developed byAT&Tand distributed to government and academic institutions, which led to both being ported to a wider variety of machine families than any other operating system.

The Unix operating system consists of many libraries and utilities along with the master control program, thekernel.The kernel provides services to start and stop programs, handles thefile systemand 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 distinction ofkernel spacefromuser space,the latter being a lower priority realm where most application programs operate.

History

Main article:History of Unix

The origins of Unix date back to the mid-1960s when theMassachusetts Institute of Technology,Bell Labs,andGeneral Electricwere developingMultics,atime-sharingoperating system for theGE 645mainframe computer. Multics featuredseveral 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 wereKen Thompson,Dennis Ritchie,Douglas McIlroy,andJoe 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 nameUnicsforUniplexed Information and Computing Serviceas apunonMultics,which stood forMultiplexed Information and Computer Services.Brian Kernighantakes credit for the idea, but adds that "no one can remember" the origin of the final spellingUnix.Dennis Ritchie,Doug McIlroy,andPeter G. Neumannalso credit Kernighan.

The operating system was originally written inassembly language,but in 1973, Version 4 Unix was rewritten inC.Version 4 Unix, however, still had muchPDP-11specific code, and was not suitable for porting. The first port to another platform was a port of Version 6, made four years later (1977) at theUniversity of Wollongongfor theInterdata 7/32,followed by a Bell Labs port of Version 7 to theInterdata 8/32during 1977 and 1978.

Bell Labs produced several versions of Unix that are collectively referred to asResearch Unix.In 1975, the first source license forUNIXwas sold toDonald B. Gilliesat theUniversity of Illinois Urbana–ChampaignDepartment of Computer Science (UIUC).

During the late 1970s and early 1980s, the influence of Unix in academic circles led to large-scale adoption of Unix (BSDandSystem V) by commercial startups, which in turn led to Unix fragmenting into multiple, similar — but often slightly and mutually incompatible — systems includingDYNIX,HP-UX,SunOS/Solaris,AIX,andXenix.In the late 1980s,AT&TUnix System LaboratoriesandSun Microsystemsdeveloped System V Release 4 (SVR4), which was subsequently adopted by many commercial Unix vendors.

In the 1990s, Unix andUnix-likesystems grew in popularity and became the operating system of choice forover 90% of the world's top 500 fastest supercomputers,asBSDandLinuxdistributions were developed through collaboration by a worldwide network of programmers. In 2000, Apple releasedDarwin,also a Unix system, which became the core of the Mac OS X operating system, later renamedmacOS.

Unix-like operating systems are widely used in modernservers,workstations,andmobile devices.

Standards

TheCommon Desktop Environment(CDE), part of theCOSE initiative

In the late 1980s, an open operating system standardization effort now known asPOSIXprovided a common baseline for all operating systems;IEEEbased 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, theCommon Open Software Environment(COSE) initiative, which eventually became theSingle UNIX Specification(SUS) administered byThe Open Group.Starting in 1998, the Open Group and IEEE started theAustin Group,to provide a common definition of POSIX and the Single UNIX Specification, which, by 2008, had become the Open Group Base Specification.

In 1999, in an effort towards compatibility, several Unix system vendors agreed on SVR4'sExecutable and Linkable Format(ELF) as the standard for binary and object code files. The common format allows substantial binary compatibility among different Unix systems operating on the same CPU architecture.

TheFilesystem Hierarchy Standardwas created to provide a reference directory layout for Unix-like operating systems; it has mainly been used in Linux.

Components

See also:List of Unix commands

The 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 thekernelof 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 a broad influence.

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-trackmagnetic tape,earning its reputation as a portable system.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 manyto have the canonical early structure:

• Kernel– source code in /usr/sys, composed of several sub-components:
  • conf– configuration and machine-dependent parts, including boot code
  • dev– device drivers for control of hardware (and some pseudo-hardware)
  • 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 source code:
  • ed– text editor, for creating source code files
  • cc–C languagecompiler (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 things such as mathematical functions (libm) or database access. V7 Unix introduced the first version of the modern "Standard I/O" librarystdioas part of the system library. Later implementations increased the number of libraries significantly.
  • make– build manager (introduced inPWB/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 theawkscripting language; later versions and implementations contain many other language compilers and toolsets. Early BSD releases includedPascaltools, and many modern Unix systems also include theGNU Compiler Collectionas well as or instead of a proprietary compiler system.
  • Other tools– including an object-code archive manager (ar), symbol-table lister (nm), compiler-development tools (e.g.lex&yacc), and debugging tools.
• Commands– 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" programmablecommand-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, includingcp,ls,grep,findand many others. Subcategories include:
    • System utilities– administrative tools such asmkfs,fsck,and many others.
    • User utilities– environment management tools such aspasswd,kill,and others.
  • Document formatting– Unix systems were used from the outset for document preparation and typesetting systems, and included many related programs such asnroff,troff,tbl,eqn,refer,andpic.Some modern Unix systems also include packages such asTeXandGhostscript.
  • Graphics– theplotsubsystem 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 includeX11as a standard windowing system andGUI,and many supportOpenGL.
  • Communications– early Unix systems contained no inter-system communication, but did include the inter-user communication programsmailandwrite.V7 introduced the early inter-system communication systemUUCP,and systems beginning with BSD release 4.1c includedTCP/IPutilities.
• Documentation– Unix was one of the first operating systems 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

Impact

See also:Unix-like

Ken ThompsonandDennis Ritchie,principal developers ofResearch Unix

Photo fromUSENIX1984, includingDennis Ritchie(center)

The Unix system had a 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 machines. Unix was originally written inassembly language,but was soon rewritten inC,ahigh-level programming language.Although this followed the lead ofCTSS,MulticsandBurroughs MCP,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 asprinters,terminals,ordisk drives), providing a uniform interface, but at the expense of occasionally requiring additional mechanisms such asioctland mode flags to access features of the hardware that did not fit the simple "stream of bytes" model. ThePlan 9operating 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'sRSX-11M's "group, user" hierarchy evolved intoOpenVMSdirectories,CP/M's volumes evolved intoMS-DOS2.0+ subdirectories, and HP'sMPEgroup.account hierarchy and IBM'sSSPandOS/400library 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. TheUnix shellused the same language for interactive commands as for scripting (shell scripts– there was no separate job control language like IBM'sJCL). Since the shell and OS commands were "just another program", the user could choose (or even write) their 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 the Unix shell.

A fundamental simplifying assumption of Unix was its focus onnewline-delimitedtext 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 everything made Unix pipes especially useful and encouraged the development of simple, general tools that could easily be combined to perform more complicatedad hoctasks. 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 theInternet protocols,e.g.,FTP,SMTP,HTTP,SOAP,andSIP.

Unix popularized a syntax forregular expressionsthat found widespread use. The Unix programming interface became the basis for a widely implemented operating system interface standard (POSIX, see above). TheC programming languagesoon spread beyond Unix, and is now ubiquitous in systems and applications programming.

Early Unix developers were important in bringing the concepts ofmodularityandreusabilityintosoftware engineeringpractice, spawning a "software tools" movement. Over time, the leading developers of 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 theUnix philosophy.

TheTCP/IP networking protocolswere quickly implemented on the Unix versions widely used on relatively inexpensive computers, which contributed to theInternetexplosion of worldwide, real-time connectivity and formed the basis for implementations on many other platforms.

The 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 launch of thefree software movementin 1983.

Free Unix and Unix-like variants

See also:Operating system § Unix and Unix-like operating systems

Console screenshots ofDebian(top, a popularLinux distribution) andFreeBSD(bottom, a popularUnix-likeoperating system)

In 1983,Richard Stallmanannounced theGNU(short for "GNU's Not Unix" ) project, an ambitious effort to create afree softwareUnix-likesystem— "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 yet produced a working kernel, but in 1991Linus Torvaldsreleased theLinux kernelas free software under theGNU General Public License.In addition to their use in theGNUoperating system, many GNU packages – such as theGNU Compiler Collection(and the rest of theGNU toolchain), theGNU C libraryand theGNU 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 includeRed Hat Enterprise Linux,Fedora,SUSE Linux Enterprise,openSUSE,Debian,Ubuntu,Linux Mint,Slackware Linux,Arch LinuxandGentoo.

A free derivative ofBSDUnix,386BSD,was released in 1992 and led to theNetBSDandFreeBSDprojects. With the 1994 settlement of a lawsuit brought against the University of California and Berkeley Software Design Inc. (USL v. BSDi) byUnix System Laboratories,it was clarified that Berkeley had the right to distribute BSD Unix for free if it so desired. Since then, BSD Unix has been developed in several different product branches, includingOpenBSDandDragonFly BSD.

Linux and BSD Unix are increasinglyfilling 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: most or all Unix and Unix-like systems include at least some BSD code, while some include GNU utilities in their distributions.

In a 1999 interview, Dennis Ritchie voiced his opinion that Linux and BSD Unix 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 Unix provided. Linux seems to be among the healthiest of the direct Unix derivatives, though there are also the various 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 Unix and Linux as "the continuation of ideas that were started by Ken and me and many others, many years ago".

OpenSolariswas thefree softwarecounterpart toSolarisdeveloped bySun Microsystems,which included aCDDL-licensed kernel and a primarilyGNUuserland. However,Oraclediscontinued 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 theillumoskernel. As of 2014, illumos remains the only active, open-source System V derivative.

ARPANET

In May 1975, RFC 681 described the development ofNetwork Unixby the Center for Advanced Computation at theUniversity of Illinois Urbana-Champaign.The Unix system was said to "present several interesting capabilities as anARPANETmini-host ". At the time, Unix required a license fromBell Telephone Laboratoriesthat cost US$20,000 for non-university institutions, while universities could obtain a license for a nominal fee of $150. It was noted that Bell was "open to suggestions" for an ARPANET-wide license.

The RFC specifically mentions that Unix "offers powerful local processing facilities in terms of user programs, severalcompilers,aneditorbased onQED,a versatile document preparation system, and an efficientfile systemfeaturing sophisticated access control,mountableand de-mountable volumes, and a unified treatment of peripherals asspecial files."The latter permitted theNetwork Control Program(NCP) to be integrated within the Unix file system, treatingnetwork connectionsas special files that could be accessed through standard UnixI/O calls,which included the added benefit of closing all connections on program exit, should the user neglect to do so. In order "to minimize the amount of code added to the basic Unixkernel",much of the NCP code ran in aswappableuser process, running only when needed.

Branding

See also:List of Unix systems

Promotionallicense platebyDigital Equipment Corporation.Actual license plate is used byJon Hall.

HP 9000workstationrunningHP-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 thetrademarksof Unix to the X/Open Company (nowThe Open Group),and in 1995 sold the related business operations toSanta Cruz Operation(SCO).Whether Novell also sold thecopyrightsto the actual software was the subject of a federal lawsuit in 2006,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 case.Unix vendorSCO Group Inc.accused Novell ofslander of title.

The present owner of the trademarkUNIXis The Open Group, an industry standards consortium. Only systems fully compliant with and certified to theSingle UNIX Specificationqualify as "UNIX" (others are called "Unix-like").

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 theUNIX 98orUNIX 03trademarks today, after the operating system's vendor pays a substantial certification fee and annual trademark royalties to The Open Group.Systems that have been licensed to use the UNIX trademark includeAIX,EulerOS,HP-UX,Inspur K-UX,IRIX,macOS,Solaris,Tru64 UNIX(formerly "Digital UNIX", orOSF/1),andz/OS.Notably, EulerOS and Inspur K-UX are Linux distributions certified as UNIX 03 compliant.

Sometimes a representation likeUn*x,*NIX,or*N?Xis 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 other Unix-like systems that have not met the requirements for UNIX branding from the Open Group.

The Open Group requests thatUNIXalways be used as an adjective followed by a generic term such assystemto help avoid the creation of agenericized trademark.

Unixwas the original formatting,but the usage ofUNIXremains widespread because it was once typeset insmall caps(Unix). According toDennis Ritchie,when presenting the original Unix paper to the third Operating Systems Symposium of the AmericanAssociation for Computing Machinery(ACM), "we had a new typesetter andtroffhad 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 bookshelves, ink pens, bottled glue, diapers, hair driers and food containers.

Several plural forms of Unix are used casually to refer to multiple brands of Unix and Unix-like systems. Most common is the conventionalUnixes,butUnices,treating Unix as aLatinnoun of thethird declension,is also popular. The pseudo-Anglo-Saxonplural formUnixenis not common, although occasionally seen.Sun Microsystems,developer of the Solaris variant, has asserted that the termUnixis itself plural, referencing its many implementations.

See also

• Comparison of operating systemsandfree and proprietary software
• List of operating systems,Unix systems,andUnix commands
• Plan 9 from Bell Labs
• Timeline of operating systems
• Unix time
• Market share of operating systems
• Year 2038 problem

Further reading

General

• Ritchie, D.M.;Thompson, K.(July–August 1978)."The UNIX Time-Sharing System".Bell System Technical Journal.57(6). Archived fromthe originalon November 3, 2010.
• "UNIX History".levenez.RetrievedMarch 17,2005.
• "AIX, FreeBSD, HP-UX, Linux, Solaris, Tru64".UNIXguide.net.RetrievedMarch 17,2005.
• "Linux Weekly News, February 21, 2002".lwn.net.RetrievedApril 7,2006.
• Lions, John:Lions'"Commentary on the Sixth Edition UNIX Operating System".with Source Code,Peer-to-Peer Communications, 1996;ISBN1-57398-013-7

Books

• Salus, Peter H.:A Quarter Century of UNIX,Addison Wesley, June 1, 1994;ISBN0-201-54777-5

Television

• Computer Chronicles(1985). "UNIX".
• Computer Chronicles(1989). "Unix".

Talks

• Ken Thompson (2019)."VCF East 2019 -- Brian Kernighan interviews Ken Thompson"(Interview).
• Marshall Kirk McKusick (2006).History of the Berkeley Software Distributions (three one-hour lectures).

External links

• The UNIX Standard,atThe Open Group.
• The Evolution of the Unix Time-sharing Systemat theWayback Machine(archived April 8, 2015)
• The Creation of the UNIX Operating Systemat theWayback Machine(archived April 2, 2014)
• The Unix Tree: files from historic releases
• Unix History Repository — a git repository representing a reconstructed version of the Unix historyonGitHub
• UnixatCurlie
• The Unix 1st Edition Manual
  • 1st Edition manual rendered to HTML
• AT&T Tech Channel Archive: The UNIX Operating System: Making Computers More Productive (1982)onYouTube(film about Unix featuring Dennis Ritchie, Ken Thompson, Brian Kernighan, Alfred Aho, and more)
• AT&T Tech Channel Archive: The UNIX System: Making Computers Easier to Use (1982)onYouTube(complementary film to the preceding "Making Computers More Productive" )
• audio bsdtalk170 - Marshall Kirk McKusick at DCBSDCon -- on history of tcp/ip (in BSD) -- abridgement of the three lectures on the history of BSD.
• 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 compatibility