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Unix

Evolution of Unix and Unix-like systems

Company /

developer

Ken Thompson, DennisRitchie, Brian Kernighan, Douglas McIlroy, and JoeOssanna at Bell Labs

Programmed inC

OS familyUnix

Working stateActive

Source model

Historically closedsource, now some Unixprojects (BSD family andIllumos) are open

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sourced. 

Initial release1969

 Available

language(s)

English

 Available

programming

languages(s)

C, C++

Kernel typeMonolithic

Default user

interface

Command-line interface& Graphical (X WindowSystem) 

LicenseProprietary

Official websiteunix.org

Unix (officially trademarked as UNIX, sometimes also written as UNIX) is amultitasking, multi-user computer operating system originally developed in 1969by a group of AT&T employees at Bell Labs, including Ken Thompson, DennisRitchie, Brian Kernighan, Douglas McIlroy, Steven Gladstone and Joe Ossanna. TheUnix operating system was first developed in assembly language, but by 1973 hadbeen almost entirely recoded in C, greatly facilitating its further development andporting to other hardware. Today's Unix system evolution is split into variousbranches, developed over time by AT&T as well as various commercial vendors,universities (such as University of California, Berkeley's BSD), and non-profit organizations.The Open Group, an industry standards consortium, owns the UNIX trademark. Only

systems fully compliant with and certified according to the Single UNIXSpecification are qualified to use the trademark; others might be called Unix system-

like or Unix-like, although the Open Group disapproves [1] of this term. However, theterm Unix is often used informally to denote any operating system that closelyresembles the trademarked system.During the late 1970s and early 1980s, the influence of Unix in academic circles ledto large-scale adoption of Unix (particularly of the BSD variant, originating from theUniversity of California, Berkeley) by commercial startups, the most notable of which are Solaris, HP-UX and AIX. Among all variants of Unix, Linux is the most widely used, powering everything from huge data centers to desktop systems tomobile phones to embedded devices such as routers. Mac OS X currently has the

biggest use on personal computers with more than 55 million systems installed.Today, in addition to certified Unix systems such as those already mentioned, Unix-

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like operating systems such as MINIX, Linux and BSD descendants (FreeBSD, NetBSD, OpenBSD, and DragonFly BSD) are commonly encountered. The termtraditional Unix may be used to describe a Unix or an operating system that has thecharacteristics of either Version 7 Unix or UNIX System V. Several graphical operating systems, including MacOS X and Android, can be

referred to as UNIX-like.

Contents1 Overview2 History2.1 1970s2.2 1980s2.3 1990s2.4 2000s3 Standards

4 Components5 Impact 5.1 Free Unix-like operating systems5.2 20385.3 ARPANET6 Branding7 See also8 References9 Further reading10 External links

[edit ] OverviewUnix operating systems are widely used in servers, workstations, and mobiledevices.[2] The Unix environment and the client –server program model wereessential elements in the development of the Internet and the reshaping of computing as centered in networks rather than in individual computers.Originally, Unix was meant to be a programmer's workbench more than to be usedto run application software. The system grew larger as the operating system startedspreading in the academic circle, as users added their own tools to the system andshared them with colleagues.[3] 

Both Unix and the C programming language were developed by AT&T anddistributed to government and academic institutions, which led to both beingported to a wider variety of machine families than any other operating system. As aresult, Unix became synonymous with open systems. Unix was designed to be portable, multi-tasking and multi-user in a time-sharingconfiguration. Unix systems are characterized by various concepts: the use of plaintext 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 lineinterpreter 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. Kernighan and Pike summarize this in The Unix Programming

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Environment as "the idea that the power of a system comes more from therelationships among programs than from the programs themselves."Under Unix, the operating system consists of many utilities along with the mastercontrol program, the kernel. The kernel provides services to start and stopprograms, handles the file system and other common "low level" tasks that most 

programs share, and, perhaps most important, schedules access to hardware toavoid conflicts if two programs try to access the same resource or devicesimultaneously. To mediate such access, the kernel was given special rights on thesystem, leading to the division between user-space and kernel-space.The microkernel concept was introduced in an effort to reverse the trend towardslarger kernels and return to a system in which most tasks were completed bysmaller utilities. In an era when a standard computer consisted of a hard disk forstorage and a data terminal for input and output (I/O), the Unix file model workedquite well, as most I/O was linear. However, modern systems include networkingand other new devices. As graphical user interfaces developed, the file modelproved inadequate to the task of handling asynchronous events such as thosegenerated by a mouse, and in the 1980s non-blocking I/O and the set of inter-process communication mechanisms was augmented (Unix domain sockets, sharedmemory, message queues, semaphores), and functionalities, such as networkprotocols, were moved out of the kernel.

[edit ] HistoryIn the 1960s, Massachusetts Institute of Technology, AT&T Bell Labs, and GeneralElectric developed an experimental operating system called Multics for the GE-645mainframe.[4] Multics introduced many innovations, but had many problems.Bell Labs, frustrated by the size and complexity of Multics but not the aims, slowlypulled out of the project. Their last researchers to leave Multics, Ken Thompson,Dennis Ritchie, M. D. McIlroy, and J. F. Ossanna,[5] decided to redo the work on amuch smaller scale. At the time, Ritchie says "What we wanted to preserve was not just a good environment in which to do programming, but a system around which afellowship could form. We knew from experience that the essence of communalcomputing, as supplied by remote-access, time-shared machines, is not just to typeprograms into a terminal instead of a keypunch, but to encourage closecommunication."[5] While Ken Thompson still had access to the Multics environment, he wrotesimulations for the new file and paging system on it. He also programmed a gamecalled Space Travel, but the game needed a more efficient and less expensive

machine to run on, and eventually a little-used PDP-7 at Bell Labs fit the bill.[6] Onthis PDP-7, a team of Bell Labs researchers led by Thompson and Ritchie, includingRudd Canaday, developed a hierarchical file system, the concepts of computerprocesses and device files, a command-line interpreter, and some small utilityprograms.[5] [edit ] 1970sIn 1970 Peter Neumann coined the project name Unics (UNiplexed Information andComputing Service) as a pun on Multics, (Multiplexing Information and ComputerServices)[7]. Unics could eventually support multiple simultaneous users, and it wasrenamed Unix .Up until this point there had been no financial support from Bell Labs. When theComputer Science Research Group wanted to use Unix on a much larger machine

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than the PDP-7, Thompson and Ritchie managed to trade the promise of adding text processing capabilities to Unix for a PDP-11/20 machine. This led to some financialsupport from Bell. For the first time in 1970, the Unix operating system wasofficially named and ran on the PDP-11/20. It added a text formatting programcalled roff and a text editor. All three were written in PDP-11/20 assembly

language. Bell Labs used this initial text processing system, consisting of Unix, roff,and the editor, for text processing of patent applications. Roff soon evolved intotroff, the first electronic publishing program with a full typesetting capability. TheUNIX Programmer's Manual was published on November 3, 1971.In 1972, Unix was rewritten in the C programming language, contrary to the generalnotion at the time "that something as complex as an operating system, which must deal with time-critical events, had to be written exclusively in assemblylanguage".[8] The migration from assembly language to the higher-level language Cresulted in much more portable software, requiring only a relatively small amount of machine-dependent code to be replaced when porting Unix to other computingplatforms. Under a 1958 consent decree in settlement of an antitrust case, AT&T (the parent organization of Bell Labs) had been forbidden from entering the computer business.Unix could not, therefore, be turned into a product; indeed, under the terms of theconsent decree, Bell Labs was required to license its non-telephone technology toanyone who asked. Ken Thompson quietly began answering requests by shippingout tapes and disk packs – each, according to legend, with a note signed "love,ken”.[9] AT&T made Unix available to universities and commercial firms, as well as theUnited States government, under licenses. The licenses included all source codeincluding the machine-dependent parts of the kernel, which were written in PDP-11

assembly code. Copies of the annotated Unix kernel sources circulated widely in thelate 1970s in the form of a much-copied book by John Lions of the University of NewSouth Wales, the Lions' Commentary on UNIX 6th Edition, with Source Code , whichled to considerable use of Unix as an educational example.Versions of the Unix system were determined by editions of its user manuals. Forexample, "Fifth Edition UNIX" and "UNIX Version 5" have both been used todesignate the same version. Development expanded, with Versions 4, 5, and 6 beingreleased by 1975. These versions added the concept of pipes, which led to thedevelopment of a more modular code-base and quicker development cycles. Version5 and especially Version 6 led to a plethora of different Unix versions both insideand outside Bell Labs, including PWB/UNIX and the first commercial Unix, IS/1. Asmore of Unix was rewritten in C, portability also increased. A group at theUniversity of Wollongong ported Unix to the Interdata 7/32. Bell Labs developedseveral ports for research purposes and internal use at AT&T. Target machinesincluded an Intel 8086-based computer (with custom-built MMU) and the UNIVAC1100.[10] In May 1975 ARPA documented the benefits of the Unix time-sharing system which"presents several interesting capabilities" as an ARPA network mini-host in RFC681. In 1978, UNIX/32V was released for DEC's then new VAX system. By this time, over600 machines were running Unix in some form. Version 7 Unix, the last version of 

Research Unix to be released widely, was released in 1979. Versions 8, 9 and 10were developed through the 1980s but were only released to a few universities,

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though they did generate papers describing the new work. This research led to thedevelopment of Plan 9 from Bell Labs, a new portable distributed system.[edit ] 1980s

A Unix[citation needed ]

desktop running the  X Window System graphical user interface.Shown are a number of client applications common to the MIT X Consortium'sdistribution, including Tom's Window Manager, an X Terminal, Xbiff, xload, and agraphical manual page browser.AT&T licensed UNIX System III, based largely on Version 7, for commercial use, thefirst version launching in 1982. This also included support for the VAX. AT&Tcontinued to issue licenses for older Unix versions. To end the confusion betweenall its differing internal versions, AT&T combined them into UNIX System V Release1. This introduced a few features such as the vi editor and curses from the BerkeleySoftware Distribution of Unix developed at the University of California, Berkeley. This also included support for the Western Electric 3B series of machines. AT&T

provided support for System III and System V through the Unix Support Group(USG), and these systems were sometimes referred to as USG Unix. [citation needed ] In 1983, the U.S. Department of Justice settled its second antitrust case against AT&T and broke up the Bell System. This relieved AT&T from the 1958 consent decree that had prevented them from turning Unix into a product. AT&T promptlyrushed to commercialize Unix System V, a move that nearly killed Unix. [9] The GNUProject was founded the same year by Richard Stallman. Since the newer commercial UNIX licensing terms were not as favorable foracademic use as the older versions of Unix, the Berkeley researchers continued todevelop BSD Unix as an alternative to UNIX System III and V, originally on the PDP-

11 architecture (the 2.xBSD releases, ending with 2.11BSD) and later for the VAX-11(the 4.x BSD releases). Many contributions to Unix first appeared on BSD releases,notably the C shell with job control (modelled on ITS). Perhaps the most important aspect of the BSD development effort was the addition of  TCP/IP network code tothe mainstream Unix kernel. The BSD effort produced several significant releasesthat contained network code: 4.1cBSD, 4.2BSD, 4.3BSD, 4.3BSD-Tahoe ("Tahoe"being the nickname of the Computer Consoles Inc. Power 6/32 architecture that was the first non-DEC release of the BSD kernel), Net/1, 4.3BSD-Reno (to match the"Tahoe" naming, and that the release was something of a gamble), Net/2, 4.4BSD,and 4.4BSD-lite. The network code found in these releases is the ancestor of muchTCP/IP network code in use today, including code that was later released in AT&T

System V UNIX and early versions of Microsoft Windows. The accompanying

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Berkeley sockets API is a de facto standard for networking APIs and has beencopied on many platforms.Other companies began to offer commercial versions of the UNIX System for theirown mini-computers and workstations. Most of these new Unix flavors weredeveloped from the System V base under a license from AT&T; however, others

were based on BSD instead. One of the leading developers of BSD, Bill Joy, went onto co-found Sun Microsystems in 1982 and created SunOS for their workstationcomputers. In 1980, Microsoft announced its first Unix for 16-bit microcomputerscalled Xenix, which the Santa Cruz Operation (SCO) ported to the Intel 8086processor in 1983, and eventually branched Xenix into SCO UNIX in 1989.During this period (before PC compatible computers with MS-DOS becamedominant), industry observers expected that UNIX, with its portability and richcapabilities, was likely to become the industry standard operating system formicrocomputers.[11] In 1984 several companies established the X/Open consortiumwith the goal of creating an open system specification based on UNIX. Despite earlyprogress, the standardization effort collapsed into the " Unix wars", with variouscompanies forming rival standardization groups. The most successful Unix-relatedstandard turned out to be the IEEE's POSIX specification, designed as a compromiseAPI readily implemented on both BSD and System V platforms, published in 1988and soon mandated by the United States government for many of its own systems.AT&T added various features into UNIX System V, such as file locking, systemadministration, STREAMS, new forms of IPC, the Remote File System and TLI. AT&Tcooperated with Sun Microsystems and between 1987 and 1989 merged featuresfrom Xenix, BSD, SunOS, and System V into System V Release 4 (SVR4),independently of X/Open. This new release consolidated all the previous featuresinto one package, and heralded the end of competing versions. It also increased

licensing fees.During this time a number of vendors including Digital Equipment, Sun, Addamaxand others began building trusted versions of UNIX for high security applications,mostly designed for military and law enforcement applications.[edit ] 1990sIn 1990, the Open Software Foundation released OSF/1, their standard Uniximplementation, based on Mach and BSD. The Foundation was started in 1988 andwas funded by several Unix-related companies that wished to counteract thecollaboration of AT&T and Sun on SVR4. Subsequently, AT&T and another group of licensees formed the group UNIX International in order to counteract OSF. Thisescalation of conflict between competing vendors again gave rise to the phrase Unix 

wars.In 1991, a group of BSD developers (Donn Seeley, Mike Karels, Bill Jolitz, and Trent Hein) left the University of California to found Berkeley Software Design, Inc ( BSDI).BSDI produced a fully functional commercial version of BSD Unix for theinexpensive and ubiquitous Intel platform, which started a wave of interest in theuse of inexpensive hardware for production computing. Shortly after it wasfounded, Bill Jolitz left BSDI to pursue distribution of 386BSD, the free softwareancestor of FreeBSD, OpenBSD, and NetBSD. In 1991, Linus Torvalds began work on Linux, a Unix clone that initially ran on IBMPC compatible computers.

By 1993 most commercial vendors had changed their variants of Unix to be basedon System V with many BSD features added. The creation of the Common Open

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Software Environment (COSE) initiative that year by the major players in Unixmarked the end of the most notorious phase of the Unix wars, and was followed bythe merger of UI and OSF in 1994. The new combined entity, which retained the OSFname, stopped work on OSF/1. By that time the only vendor using it was DigitalEquipment Corporation, which continued its own development, rebranding their

product Digital UNIX in early 1995.Shortly after UNIX System V Release 4 was produced, AT&T sold all its rights toUNIX to Novell. Dennis Ritchie likened this sale to the Biblical story of Esau sellinghis birthright for the proverbial mess of pottage.[12] Novell developed its ownversion, UnixWare, merging its NetWare with UNIX System V Release 4. Novell triedto use this as a marketing tool against Windows NT, but their core markets sufferedconsiderably.In 1993, Novell decided to transfer the UNIX trademark and certification rights tothe X/Open Consortium.[13] In 1996, X/Open merged with OSF, creating the OpenGroup. Various standards by the Open Group now define what is and what is not aUNIX operating system, notably the post-1998 Single UNIX Specification. In 1995, the business of administering and supporting the existing UNIX licenses,plus rights to further develop the System V code base, were sold by Novell to theSanta Cruz Operation.[14] Whether Novell also sold the copyrights is currently thesubject of litigation (see below).In 1997, Apple Computer sought out a new foundation for its Macintosh operatingsystem and chose NEXTSTEP, an operating system developed by NeXT. The coreoperating system, which was based on BSD and the Mach kernel, was renamedDarwin after Apple acquired it. The deployment of Darwin in Mac OS X makes it,according to a statement made by an Apple employee at a USENIX conference, themost widely used Unix-based system in the desktop computer market.

[edit ] 2000sIn 2000, SCO sold its entire UNIX business and assets to Caldera Systems, whichlater on changed its name to The SCO Group. The bursting of the dot-com bubble (2001–2003) led to significant consolidation of versions of Unix. Of the many commercial variants of Unix that were born in the1980s, only Solaris, HP-UX, and AIX were still doing relatively well in the market,though SGI's IRIX persisted for quite some time. Of these, Solaris had the largest market share in 2005.[15] In 2003, the SCO Group started legal action against various users and vendors of Linux. SCO had alleged that Linux contained copyrighted Unix code now owned byThe SCO Group. Other allegations included trade-secret violations by IBM, orcontract violations by former Santa Cruz customers who had since converted toLinux. However, Novell disputed the SCO Group's claim to hold copyright on theUNIX source base. According to Novell, SCO (and hence the SCO Group) areeffectively franchise operators for Novell, which also retained the core copyrights,veto rights over future licensing activities of SCO, and 95% of the licensing revenue.The SCO Group disagreed with this, and the dispute resulted in the SCO v. Novell  lawsuit. On August 10, 2007, a major portion of the case was decided in Novell'sfavor (that Novell had the copyright to UNIX, and that the SCO Group hadimproperly kept money that was due to Novell). The court also ruled that "SCO isobligated to recognize Novell's waiver of SCO's claims against IBM and Sequent".

After the ruling, Novell announced they have no interest in suing people over Unixand stated, "We don't believe there is Unix in Linux". [16][17][18] SCO successfully got 

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the 10th Circuit Court of Appeals to partially overturn this decision on August 24,2009 which sent the lawsuit back to the courts for a jury trial . [19][20][21] On March 30, 2010, following a jury trial, Novell, and not The SCO Group, was"unanimously [found]" to be the owner of the UNIX and UnixWare copyrights. [22] The SCO Group, through bankruptcy trustee Edward Cahn, decided to continue the

lawsuit against IBM for causing a decline in SCO revenues. [23] See also: SCO-Linux controversiesIn 2005, Sun Microsystems released the bulk of its Solaris system code (based onUNIX System V Release 4) into an open source project called OpenSolaris. New SunOS technologies, notably the ZFS file system, were first released as open source codevia the OpenSolaris project. Soon afterwards, OpenSolaris spawned several non-Sundistributions. In 2010, after Oracle acquired Sun, OpenSolaris was officiallydiscontinued, but the development of derivatives continued.

[edit ] StandardsBeginning in the late 1980s, an open operating system standardization effort now

known as POSIX provided a common baseline for all operating systems; IEEE basedPOSIX around the common structure of the major competing variants of the Unixsystem, publishing the first POSIX standard in 1988. In the early 1990s a separatebut very similar effort was started by an industry consortium, the Common OpenSoftware Environment (COSE) initiative, which eventually became the Single UNIXSpecification administered by The Open Group. Starting in 1998 the Open Groupand IEEE started the Austin Group, to provide a common definition of POSIX and theSingle UNIX Specification.In an effort towards compatibility, in 1999 several Unix system vendors agreed onSVR4's Executable and Linkable Format (ELF) as the standard for binary and object code files. The common format allows substantial binary compatibility among Unixsystems operating on the same CPU architecture.The Filesystem Hierarchy Standard was created to provide a reference directorylayout for Unix-like operating systems, particularly Linux.

[edit ] ComponentsSee also: List of Unix programsThe Unix system is composed of several components that are normally packagedtogether. By including – in addition to the kernel of an operating system – thedevelopment environment, libraries, documents, and the portable, modifiablesource-code for all of these components, Unix was a self-contained software system.This was one of the key reasons it emerged as an important teaching and learningtool and has had such a broad influence.The inclusion of these components did not make the system large – the original V7UNIX distribution, consisting of copies of all of the compiled binaries plus all of thesource code and documentation occupied less than 10MB, and arrived on a single 9 -track magnetic tape. The printed documentation, typeset from the on-line sources,was contained in two volumes.The names and filesystem locations of the Unix components have changedsubstantially across the history of the system. Nonetheless, the V7 implementationis considered by many to have the canonical early structure:Kernel – source code in /usr/sys, composed of several sub-components:

conf  – configuration and machine-dependent parts, including boot codedev  – device drivers for control of hardware (and some pseudo-hardware)

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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 invariablesDevelopment Environment  – Early versions of Unix contained a development 

environment sufficient to recreate the entire system from source code:cc – C language compiler (first appeared in V3 Unix)as – machine-language assembler for the machineld  – linker, for combining object fileslib – object-code libraries (installed in /lib or /usr/lib). libc, the system library withC run-time support, was the primary library, but there have always been additionallibraries for such things as mathematical functions (libm) or database access. V7Unix introduced the first version of the modern "Standard I/O" library stdio as part of the system library. Later implementations increased the number of librariessignificantly.make – build manager (introduced in PWB/UNIX), for effectively automating thebuild processinclude – header files for software development, defining standard interfaces andsystem invariantsOther languages – V7 Unix contained a Fortran-77 compiler, a programmablearbitrary-precision calculator (bc, dc), and the awk scripting language, and laterversions and implementations contain many other language compilers and toolsets.Early BSD releases included Pascal tools, and many modern Unix systems alsoinclude the GNU Compiler Collection as well as or instead of a proprietary compilersystem.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-levelprograms) for system operation and maintenance (e.g. cron), commands of generalutility (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 userinterface on Unix before window systems appeared, and even afterward (within a"command window").Utilities – the core tool kit of the Unix command set, including cp, ls, grep , find andmany 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 asnroff , troff , tbl , eqn, refer , and  pic. Some modern Unix systems also include packagessuch as TeX and Ghostscript. Graphics – The  plot subsystem provided facilities for producing simple vector plotsin a device-independent format, with device-specific interpreters to display suchfiles. Modern Unix systems also generally include X11 as a standard windowingsystem 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

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introduced the early inter-system communication system UUCP, and systemsbeginning with BSD release 4.1c included TCP/IP utilities.Documentation – Unix was the first operating system to include all of itsdocumentation 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 andtroff 

[edit ] Impact See also: Unix-likeThe Unix system had significant impact on other operating systems. It won itssuccess by:Direct interaction.Moving away from the total control of businesses like IBM and DEC.AT&T being willing to give the software away for free.

Running on cheap hardware.Being easy to adopt and move to different machines.It was written in high level language rather than assembly language (which hadbeen thought necessary for systems implementation on early computers). Althoughthis followed the lead of Multics and Burroughs, it was Unix that popularized theidea.Unix had a drastically simplified file model compared to many contemporaryoperating systems, treating all kinds of files as simple byte arrays. The file systemhierarchy contained machine services and devices (such as printers, terminals, ordisk drives), providing a uniform interface, but at the expense of occasionallyrequiring 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 9operating system pushed this model even further and eliminated the need foradditional mechanisms.Unix also popularized the hierarchical file system with arbitrarily nestedsubdirectories, originally introduced by Multics. Other common operating systemsof 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 majorproprietary operating systems eventually added recursive subdirectory capabilitiesalso patterned after Multics. DEC's RSX-11M's "group, user" hierarchy evolved intoVMS directories, CP/M's volumes evolved into MS-DOS 2.0+ subdirectories, and

HP's MPE group.account hierarchy and IBM's SSP and OS/400 library systems werefolded into broader POSIX file systems.Making the command interpreter an ordinary user-level program, with additionalcommands provided as separate programs, was another Multics innovationpopularized by Unix. The Unix shell used the same language for interactivecommands as for scripting (shell scripts – there was no separate job controllanguage like IBM's JCL). Since the shell and OS commands were "just anotherprogram", the user could choose (or even write) his own shell. New commandscould be added without changing the shell itself. Unix's innovative command-linesyntax for creating modular chains of producer-consumer processes (pipelines)made a powerful programming paradigm (coroutines) widely available. Many latercommand-line interpreters have been inspired by the Unix shell.

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A fundamental simplifying assumption of Unix was its focus on ASCII text for nearlyall file formats. There were no "binary" editors in the original version of Unix – theentire system was configured using textual shell command scripts. The commondenominator 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 be easilycombined to perform more complicated ad hoc tasks. The focus on text and bytesmade the system far more scalable and portable than other systems. Over time, text-based applications have also proven popular in application areas, such as printinglanguages (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. TheUnix programming interface became the basis for a widely implemented operatingsystem interface standard (POSIX, see above).The C programming language soon spread beyond Unix, and is now ubiquitous insystems and applications programming.Early Unix developers were important in bringing the concepts of  modularity andreusability into software engineering practice, spawning a "software tools"movement.Unix provided the TCP/IP networking protocol on relatively inexpensive computers,which contributed to the Internet explosion of worldwide real-time connectivity,and which formed the basis for implementations on many other platforms. This alsoexposed numerous security holes in the networking implementations.The Unix policy of extensive on-line documentation and (for many years) readyaccess to all system source code raised programmer expectations, and contributedto the 1983 launch of the free software movement. 

Over time, the leading developers of Unix (and programs that ran on it) establisheda 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 Unixphilosophy. The Unix philosophy encourages Software prototyping, Rapidapplication development, Iterative and incremental development over the Waterfallmodel; prototype before polishing; get it working before you optimize it.[edit ] Free Unix-like operating systemsIn 1983, Richard Stallman announced the GNU project, an ambitious effort to createa free software Unix-like system; "free" in that everyone who received a copy wouldbe free to use, study, modify, and redistribute it. The GNU project's own kerneldevelopment project, GNU Hurd, had not produced a working kernel, but in 1991Linus Torvalds released the Linux kernel as free software under the GNU GeneralPublic License. In addition to their use in the GNU/Linux operating system, manyGNU packages – such as the GNU Compiler Collection (and the rest of the GNUtoolchain), the GNU C library and the GNU core utilities – have gone on to playcentral roles in other free Unix systems as well.Linux distributions, comprising Linux and large collections of compatible softwarehave become popular both with individual users and in business. Populardistributions include Red Hat Enterprise Linux, Fedora, SUSE Linux Enterprise, openSUSE, Debian GNU/Linux, Ubuntu, Mandriva Linux, Slackware Linux andGentoo. 

A free derivative of BSD Unix, 386BSD, was also released in 1992 and led to theNetBSD and FreeBSD projects. With the 1994 settlement of a lawsuit that UNIX

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Systems Laboratories brought against the University of California and BerkeleySoftware Design Inc. (USL v. BSDi), it was clarified that Berkeley had the right todistribute BSD Unix – for free, if it so desired. Since then, BSD Unix has beendeveloped in several different directions, including OpenBSD and DragonFly BSD. Linux and BSD are now rapidly occupying much of the market traditionally

occupied by proprietary Unix operating systems, as well as expanding into newmarkets such as the consumer desktop and mobile and embedded devices. Due tothe modularity of the Unix design, sharing bits and pieces is relatively common;consequently, most or all Unix and Unix-like systems include at least some BSDcode, and modern systems also usually include some GNU utilities in theirdistributions.OpenSolaris is a relatively recent addition to the list of operating systems based onfree software licenses marked as such by FSF and OSI. It includes a number of derivatives that combines CDDL-licensed kernel and system tools and also GNUuserland and is currently the only open source System V derivative available.

[edit ] 2038Main article: Year 2038 problemUnix stores system time values as the number of seconds from midnight January 1,1970 (the "Unix Epoch") in variables of type time_t, historically defined as "signedlong". On January 19, 2038 on 32 bit Unix systems, the current time will roll overfrom a zero followed by 31 ones (0x7FFFFFFF) to a one followed by 31 zeros(0x80000000), which will reset time to the year 1901 or 1970, depending onimplementation, because that toggles the sign bit. Since times before 1970 are rarely represented in Unix time, one possible solutionthat is compatible with existing binary formats would be to redefine time_t as"unsigned 32-bit integer". However, such a kludge merely postpones the problem to

February 7, 2106, and could introduce bugs in software that computes timedifferences.Some Unix versions have already addressed this. For example, in Sola ris and Linuxin 64-bit mode, time_t is 64 bits long, meaning that the OS itself and 64-bit applications will correctly handle dates for some 292 billion years. Existing 32 -bit applications using a 32-bit time_t continue to work on 64-bit Solaris systems but are still prone to the 2038 problem. Some vendors have introduced an alternative64-bit type and corresponding API, without addressing uses of the standard time_t.[edit ] ARPANETIn May 1975 ARPA documented in RFC 681 detailed very specifically why Unix was

the operating system of choice for use as an ARPANET mini-host. The evaluationprocess was also documented. Unix required a license that was very expensive with$20,000(US) for non-university users and $150 for an educational license. It wasnoted that for an ARPA network-wide license Bell "were open to suggestions in that area".Specific features found beneficial were:Local processing facilities.Compilers. Editor. Document preparation system. Efficient file system and access control.

Mountable and de-mountable volumes.Unified treatment of peripherals as special files. 

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The network control program (NCP) was integrated within the Unix file system.Network connections treated as special files which can be accessed throughstandard Unix I/O calls. The system closes all files on program exit."desirable to minimize the amount of code added to the basic Unix kernel".

[edit ] BrandingSee also: List of Unix systemsIn October 1993, Novell, the company that owned the rights to the Unix System Vsource at the time, transferred the trademarks of Unix to the X/Open Company(now The Open Group),[13] and in 1995 sold the related business operations to SantaCruz Operation.[14] Whether Novell also sold the copyrights to the actual softwarewas the subject of a 2006 federal lawsuit, SCO v. Novell, which Novell won. The casewas appealed, but on Aug 30, 2011, the United States Court of Appeals for the TenthCircuit affirmed the trial decisions, closing the case. [24] Unix vendor SCO Group Inc.accused Novell of slander of title. 

The present owner of the trademark UNIX is The Open Group, an industry standardsconsortium. Only systems fully compliant with and certified to the Single UNIXSpecification qualify as "UNIX" (others are called "Unix system-like" or "Unix-like").By decree of The Open Group, the term "UNIX" refers more to a class of operatingsystems than to a specific implementation of an operating system; those operatingsystems which meet The Open Group's Single UNIX Specification should be able tobear the UNIX 98 or UNIX 03 trademarks today, after the operating system's vendorpays a fee to The Open Group. Systems licensed to use the UNIX trademark includeAIX, HP-UX, IRIX, Solaris, Tru64 (formerly "Digital UNIX"), A/UX, Mac OS X,[25][26] and a part of z/OS. Sometimes a representation like Un*x , *NIX , or *N?X is used to indicate all operatingsystems similar to Unix. This comes from the use of the asterisk (* ) and the questionmark characters as wildcard indicators in many utilities. This notation is also usedto describe other Unix-like systems, e.g. Linux, BSD, etc., that have not met therequirements for UNIX branding from the Open Group.The Open Group requests that UNIX is always used as an adjective followed by ageneric term such as system to help avoid the creation of a genericized trademark. "Unix" was the original formatting, but the usage of "UNIX" remains widespreadbecause, according to Dennis Ritchie, when presenting the original Unix paper tothe third Operating Systems Symposium of the American Association for ComputingMachinery, “we had a new typesetter and troff had just been invented and we were

intoxicated by being able to produce small caps.” [27] Many of the operating system'spredecessors and contemporaries used all-uppercase lettering, so many peoplewrote the name in upper case due to force of habit.Several plural forms of Unix are used casually to refer to multiple brands of Unixand Unix-like systems. Most common is the conventional Unixes, but Unices, treatingUnix as a Latin noun of the third declension, is also popular. The pseudo-Anglo-Saxon plural form Unixen is not common, although occasionally seen. Trademarknames can be registered by different entities in different countries and trademarklaws in some countries allow the same trademark name to be controlled by twodifferent entities if each entity uses the trademark in easily distinguishablecategories. The result is that Unix has been used as a brand name for variousproducts including book shelves, ink pens, bottled g lue, diapers, hair driers andfood containers.[28] 

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[edit ] See also

Book: Unix  

Wikipedia books are collections of articles that can be downloaded or ordered in print.  

Comparison of operating systemsComparison of open source and closed sourceHungarian Unix PortalList of operating systemsList of Unix utilitiesList of Unix systemsMarket share of operating systemsOperating systems timeline

Plan 9 from Bell Labs[edit ] References^ What is a "Unix-like" operating system? Unix.org FAQ^ 8:30 AM (2010-09-29). "Unix’s Revenge". asymco.http://www.asymco.com/2010/09/29/unixs-revenge/. Retrieved 2010-11-09.^ Powers, Shelley; Peek, Jerry; O'Reilly, Tim; Loukides, Mike (2002). Unix Power 

Tools. ISBN 0-596-00330-7.  ^ Stuart, Brian L. (2009). Principles of operating systems: design & applications .Boston, Massachusetts: Thompson Learning. p. 23. ISBN 1-4188-3769-5. ^ a b c  Ritchie, Dennis M.. "The Evolution of the Unix Time-sharing System". 

http://cm.bell-labs.com/cm/cs/who/dmr/hist.html. Retrieved 2009-11-29.^ "The Creation of the UNIX* Operating System: The famous PDP-7 comes to therescue". Bell-labs.com. http://www.bell-labs.com/history/unix/pdp7.html. Retrieved 2010-11-09.^ Salus, Peter H. (1994).  A Quarter Century of UNIX . Addison Wesley. p. 9. ISBN 0-201-54777-5. ^ Stallings, William. "Operating Systems: Internals and Design Principles" 5th ed,page 91. Pearson Education, Inc. 2005.^ a b "Origins and History of Unix, 1969–1995". Faqs.org.http://www.faqs.org/docs/artu/ch02s01.html. Retrieved 2010-11-09.^ D. E. Bodenstab, T. F. Houghton, K. A. Kelleman, G. Ronkin, and E. P. Schan

(October 1984). "UNIX Operating System Porting Experiences" (PDF).  AT&T Bell Laboratories Technical Journal, Vol. 63, No. 8 Part 2 . p. 9. http://www.cs.bell-labs.com/who/dmr/otherports/newp.pdf. Retrieved 2009-04-09.^ "UNIX". The Computer Chronicles. 1985.^ "comp.unix.questions | Google Groups". Groups.google.com.http://groups.google.com/group/comp.unix.questions/browse_frm/thread/2f0b5e719fa3a3ec/3fa5e5fe4d58f96b. Retrieved 2010-11-09.^ a b Chuck Karish View profile More options. "The name UNIX is now theproperty of X/Open – comp.std.unix | Google Groups". Groups.google.com.http://groups.google.com/group/comp.std.unix/msg/c9974cf0022884f8. Retrieved

2010-11-09.

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^ a b "HP, Novell and SCO To Deliver High-Volume UNIX OS With Advanced NetworkAnd Enterprise Services". Novell.com. 1995-09-20.http://www.novell.com/news/press/archive/1995/09/pr95220.html. Retrieved2010-11-09.^ Stephen Shankland (December 7, 2005). "Itanium: A cautionary tale". Tech News.

ZDNet. Archived from the original on 2006-09-23.http://web.archive.org/web/20060923185730/http://news.zdnet.com/2100-9584_22-5984747.html. Retrieved 2006-10-04. "In the third quarter of this year,7,845 Itanium servers were sold, according to research by Gartner. That compareswith 62,776 machines with Sun Microsystems' UltraSparc, 31,648 with IBM'sPower, and 9,147 with HP's PA-RISC."^ "Memorandum and Decision Order in SCO v. Novell". Groklaw.net.http://www.groklaw.net/staticpages/index.php?page=20070810205256644.  Retrieved 2010-11-09.^ Tuxrocks.com^ Novell Won't Pursue Unix Copyrights August 15, 2007^ Groklaw.net August 24, 2009^ Novell.com August 24, 2009^ Wired.com August 24, 2009^ "03/30/2010 – 846 – JURY VERDICT for Defendant Novell. (slm) (Entered:03/30/2010)" (PDF). http://www.groklaw.net/pdf2/Novell-846.pdf. Retrieved2010-11-09.^ Jury says Novell owns Unix copyrights^ Jones, Pamela. "SCO Files Docketing Statement and We Find Out What Its AppealWill Be About". Groklaw . Groklaw.net.http://groklaw.net/article.php?story=20100723230825165. Retrieved 12 April

2011.^ The Open Group. "Mac OS X v10.5 Leopard on Intel-based Macintosh computerscertification". http://www.opengroup.org/openbrand/register/brand3555.htm. Retrieved 2007-06-12.^ The Open Group. "Mac OS X v10.6 Snow Leopard certification". http://www.opengroup.org/openbrand/register/brand3581.htm.  ^ "Unix". Catb.org. http://catb.org/jargon/html/U/Unix.html.Retrieved 2010-11-09.^ "Autres Unix, autres moeurs (OtherUnix)". Cm.bell-labs.com. 2000-04-01.http://cm.bell-labs.com/cm/cs/who/dmr/otherunix.html. Retrieved 2010-11-09.Ritchie, D.M.; Thompson, K., The UNIX Time-Sharing System (The Bell SystemTechnical Journal, July–August 1978, Vol. 57, No. 6, Part 2) "UNIX History". www.levenez.com. http://www.levenez.com/unix/. Retrieved 17March 2005."AIX, FreeBSD, HP-UX, Linux, Solaris, Tru64". UNIXguide.net .http://www.unixguide.net/. Retrieved 17 March 2005."Linux Weekly News, February 21, 2002". lwn.net .http://lwn.net/2002/0221/bigpage.php3. Retrieved 7 April 2006.Lions, John: Lions' "Commentary on the Sixth Edition UNIX Operating System". 

http://www.lemis.com/grog/Documentation/Lions/.with Source Code, Peer-to-PeerCommunications, 1996; ISBN 1-57398-013-7

UNIX Shell Programming, Yashawant Kanetkar[edit ] Further reading

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BooksSalus, Peter H.:  A Quarter Century of UNIX , Addison Wesley, June 1, 1994; ISBN 0-201-54777-5TelevisionComputer Chronicles (1985). "UNIX".

Computer Chronicles (1989). "Unix".[edit ] External links

The Wikibook Guide to Unix  has a page on the topic of Commands 

The UNIX System, at The Open Group. The Evolution of the Unix Time-sharing SystemThe Creation of the UNIX Operating SystemThe Unix Tree: files from historic releasesThe Unix and Linux ForumsUNIXhelp for usersUnix at the Open Directory Project The Unix 1st Edition Manuals. 

[show]v · d · eUnix command-line interface 

programs and shell builtins (more) 

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[show]v · d · eUnix and Unix-like operating

systems 

[show]v · d · eOperating system 

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Architectures

General Monolithic kernel · Microkernel

Subtypes Exokernel · Nanokernel · Hybrid

ComponentsUser/Kernel space · Server · Loadable kernel module · Device driver

Concepts

Process · Process control block · Interrupt · Thread · Context switch · 

Scheduling

CPUmodes

Protected mode · Supervisor mode

Scheduling algorithm

Cooperative multitasking · Preemptive multitasking · Round-robin

scheduling · Fixed priority pre-emptive scheduling · Multilevel feedbackqueue · Shortest job next 

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