Unix (officially trademarked as UNIX, sometimes also written as Unix with small caps In typography, small capitals are uppercase (capital) characters set at the same height as surrounding lowercase (small) letters or text figures. They are used in running text to prevent capitalized words from appearing too large on the page, and as a method of emphasis or distinctiveness for text alongside or instead of italics, or when boldface) is a computer operating system An operating system is an interface between hardware and user; an OS is responsible for the management and coordination of activities and the sharing of the resources of the computer. The operating system acts as a host for computing applications that are run on the machine. As a host, one of the purposes of an operating system is to handle the originally developed in 1969 by a group of AT&T AT&T Corporation, originally the American Telephone & Telegraph Company, is an American telecommunications company that provided voice, video, data, and Internet telecommunications and professional services to businesses, consumers, and government agencies. During its long history, AT&T was at times the world's largest telephone employees at Bell Labs Bell Laboratories is the research organization of Alcatel-Lucent and previously of the American Telephone & Telegraph Company (AT&T), including Ken Thompson Kenneth Lane Thompson , commonly referred to as Ken Thompson (or simply ken in hacker circles), is an American pioneer of computer science notable for his work with the B programming language and his shepherding of the Unix and Plan 9 operating systems, Dennis Ritchie Dennis MacAlistair Ritchie is an American computer scientist notable for his influence on C and other programming languages, and on operating systems such as Multics and Unix. He received the Turing Award in 1983 and the National Medal of Technology in 1998. Ritchie was the head of Lucent Technologies System Software Research Department when he, Douglas McIlroy Malcolm Douglas McIlroy is a mathematician, engineer, and programmer. As of 2007 he is an Adjunct Professor of Computer Science at Dartmouth College. Dr. McIlroy is best known for having originally developed the Unix pipeline implementation, software componentry and several Unix tools, such as spell, diff, sort, join, graph, speak, and tr, and Joe Ossanna Joseph F. Ossanna , received his BSEE from Wayne State University, Detroit, Michigan, in 1952. He was since a member of the Technical Staff of the Bell Telephone Laboratories, Murray Hill, New Jersey. He was actively engaged in the software design of Multics (Multiplexed Information and Computing Service), a general purpose programming system used. Today's Unix systems are split into various branches, developed over time by AT&T as well as various commercial vendors and non-profit A nonprofit organization is an organization that does not distribute its surplus funds to owners or shareholders, but instead uses them to help pursue its goals. Examples of NPOs include charities (i.e. charitable organizations) , trade unions, and public arts organizations. Most governments and government agencies meet this definition, but in organizations.

As of 2007, the owner of the trademark A trademark or trade mark is a distinctive sign or indicator used by an individual, business organization, or other legal entity to identify that the products or services to consumers with which the trademark appears originate from a unique source, and to distinguish its products or services from those of other entities is The Open Group The Open Group is an industry consortium to set vendor- and technology-neutral open standards for computing infrastructure. It was formed when X/Open merged with the Open Software Foundation in 1996. The Open Group is most famous as the certifying body for the UNIX trademark, in the past the group was best known for its publication of the Single, an industry standards consortium. Only systems fully compliant with and certified according to the Single UNIX Specification The Single UNIX Specification is the collective name of a family of standards for computer operating systems to qualify for the name "Unix". The SUS is developed and maintained by the Austin Group, based on earlier work by the IEEE and The Open Group are qualified to use the trademark; others are called "Unix system-like" or "Unix-like A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification".

During the late 1970s and early 1980s, the influence of Unix in academic circles led to large-scale adoption of Unix (particularly of the BSD Berkeley Software Distribution is the UNIX operating system derivative developed and distributed by the Computer Systems Research Group of the University of California, Berkeley, from 1977 to 1995 variant, originating from the University of California, Berkeley The University of California, Berkeley is a public research university located in Berkeley, California, United States. The oldest of the ten major campuses affiliated with the University of California, Berkeley offers some 300 undergraduate and graduate degree programs in a wide range of disciplines. The university occupies 6,651 acres (2,692 ha)) by commercial startups, the most notable of which are Solaris Solaris is a UNIX-based operating system introduced by Sun Microsystems in 1992 as the successor to SunOS, HP-UX HP-UX is Hewlett-Packard's proprietary implementation of the Unix operating system, based on System V (initially System III). It runs on the HP 9000 PA-RISC-based range of processors and HP Integrity Intel's Itanium-based systems, and was also available for later Apollo/Domain systems. Earlier versions also ran on the HP 9000 Series 200, 300, and 4 and AIX AIX is the name given to a series of proprietary operating systems sold by IBM for several of its computer system platforms, based on UNIX System V with 4.3BSD-compatible command and programming interface extensions. Today, in addition to certified Unix systems such as those already mentioned, Unix-like A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification operating systems such as Linux Linux is a generic term referring to Unix-like computer operating systems based on the Linux kernel. Their development is one of the most prominent examples of free and open source software collaboration; typically all the underlying source code can be used, freely modified, and redistributed by anyone under the terms of the GNU GPL and other free and BSD Berkeley Software Distribution is the UNIX operating system derivative developed and distributed by the Computer Systems Research Group of the University of California, Berkeley, from 1977 to 1995 are commonly encountered. The term "traditional Unix" may be used to describe a Unix or an operating system that has the characteristics of either Version 7 Unix Seventh Edition Unix, also called Version 7 Unix, Version 7 or just V7, was an important early release of the Unix operating system. V7, released in 1979, was the last Bell Laboratories release to see widespread distribution before the commercialization of Unix by AT&T in the early 1980s. V7 was originally developed for Digital Equipment or UNIX System V Unix System V, commonly abbreviated SysV , is one of the versions of the Unix operating system. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4. System V Release 4, or SVR4, was commercially the most successful version, being the result of an effort,.

Overview

Unix operating systems are widely used in both servers A server is any combination of hardware or software designed to provide services to clients. When used alone, the term typically refers to a computer which may be running a server operating system, but is commonly used to refer to any software or dedicated hardware capable of providing services and workstations A workstation is a high-end microcomputer designed for technical or scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating systems. The term workstation has also been used to refer to a mainframe computer terminal or a PC connected to a. The Unix environment and the client-server Client-server computing or networking is a distributed application architecture that partitions tasks or work loads between service providers and service requesters, called clients. Often clients and servers operate over a computer network on separate hardware. A server is a high-performance host that is a registering unit and shares its resources program model were essential elements in the development of the Internet The Internet is a global system of interconnected computer networks that use the standardized Internet Protocol Suite . It is a network of networks that consists of millions of private and public, academic, business, and government networks of local to global scope that are linked by copper wires, fiber-optic cables, wireless connections, and and the reshaping of computing as centered in networks Computer networking is the engineering discipline concerned with communication between computer systems or devices. Networking, routers, routing protocols, and networking over the public Internet have their specifications defined in documents called RFCs. Computer networking is sometimes considered a sub-discipline of telecommunications, computer rather than in individual computers.

Both Unix and the C programming language C is one of the most popular programming languages. It is widely used on many different software platforms, and there are few computer architectures for which a C compiler does not exist. C has greatly influenced many other popular programming languages, most notably C++, which originally began as an extension to C 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. As a result, Unix became synonymous with "open systems".

Unix was designed to be portable In computer science, porting is the process of adapting software so that an executable program can be created for a computing environment that is different from the one for which it was originally designed . The term is also used in a general way to refer to the changing of software/hardware to make them usable in different environments, multi-tasking In computing, multitasking is a method by which multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is actively executing instructions for that task. Multitasking solves the problem by and multi-user in a time-sharing Time-sharing is sharing a computing resource among many users by multitasking. Its introduction in the 1960s, and emergence as the prominent model of computing in the 1970s, represents a major historical shift in the history of computing. By allowing a large number of users to interact simultaneously on a single computer, time-sharing dramatically configuration. Unix systems are characterized by various concepts: the use of plain text The encoding has traditionally been either ASCII, one of its many derivatives such as ISO/IEC 646 etc., or sometimes EBCDIC. No other encodings are used in plain text files which neither contain any structural tags such as heading marks, nor any typographic markers like bold face, italics, etc for storing data; a hierarchical file system In computing, a file system is a method for storing and organizing computer files and the data they contain to make it easy to find and access them. File systems may use a data storage device such as a hard disk or CD-ROM and involve maintaining the physical location of the files, they might provide access to data on a file server by acting as; treating devices and certain types of inter-process communication Inter-process communication is a set of techniques for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC techniques are divided into methods for message passing, synchronization, shared memory, and remote procedure calls (RPC). The method of IPC used (IPC) as files; and the use of a large number of software tools A programming tool or software development tool is a program or application that software developers use to create, debug, maintain, or otherwise support other programs and applications. The term usually refers to relatively simple programs that can be combined together to accomplish a task, much as one might use multiple hand tools to fix a, small programs that can be strung together through a command line interpreter A command-line interpreter is a computer program that reads lines of text entered by a user and interprets them in the context of a given operating system or programming language using pipes In Unix-like computer operating systems, a pipeline is the original software pipeline: a set of processes chained by their standard streams, so that the output of each process feeds directly as input (stdin) of the next one. Each connection is implemented by an anonymous pipe. Filter programs are often used in this configuration. The concept was, as opposed to using a single monolithic program that includes all of the same functionality. These concepts are known as the Unix philosophy.

Under Unix, the "operating system" consists of many of these utilities along with the master control program, the kernel In computing, the kernel is the central component of most computer operating systems. Its responsibilities include managing the system's resources . Usually as a basic component of an operating system, a kernel can provide the lowest-level abstraction layer for the resources (especially memory, processors and I/O devices) that application software. The kernel provides services to start and stop programs, handles the file system In computing, a file system is a method for storing and organizing computer files and the data they contain to make it easy to find and access them. File systems may use a data storage device such as a hard disk or CD-ROM and involve maintaining the physical location of the files, they might provide access to data on a file server by acting as and other common "low level" tasks that most programs share, and, perhaps most importantly, schedules access to hardware to avoid conflicts if two programs try to access the same resource or device simultaneously. To mediate such access, the kernel was given special rights on the system, leading to the division between user-space and kernel-space.

The microkernel In computer science, a microkernel is a computer kernel that provides the mechanisms needed to implement an operating system, such as low-level address space management, thread management, and inter-process communication. If the hardware provides multiple privilege levels, then the microkernel is the only software executing at the most privileged 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 an era when a "normal" computer consisted of a hard disk for storage and a data terminal for input and output (I/O), the Unix file model worked quite well as most I/O was "linear". However, modern systems include networking and other new devices. As graphical user interfaces developed, the file model proved inadequate to the task of handling asynchronous events such as those generated by a mouse In computing, a mouse is a pointing device that functions by detecting two-dimensional motion relative to its supporting surface. Physically, a mouse consists of an object held under one of the user's hands, with one or more buttons. It sometimes features other elements, such as "wheels", which allow the user to perform various system-, and in the 1980s non-blocking I/O and the set of inter-process communication Inter-process communication is a set of techniques for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC techniques are divided into methods for message passing, synchronization, shared memory, and remote procedure calls (RPC). The method of IPC used mechanisms was augmented (sockets, shared memory In computing, shared memory is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Depending on context, programs may run on a single processor or on multiple separate processors. Using memory for communication inside a single program, for example among its, message queues In computer science, message queues and mailboxes are software-engineering components used for interprocess communication, or for inter-thread communication within the same process. They use a queue for messaging – the passing of control or of content. Group communication systems provide similar kinds of functionality, semaphores In computer science, a semaphore is a protected variable or abstract data type which constitutes the classic method for restricting access to shared resources such as shared memory in a parallel programming environment. A counting semaphore is a counter for a set of available resources, rather than a locked/unlocked flag of a single resource. It), and functionalities such as network protocols were moved out of the kernel.

History

In the 1960s, Massachusetts Institute of Technology The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts, United States. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological research. MIT is one of two private land-grant universities[b] and is also a sea-, AT&T Bell Labs, and General Electric The General Electric Company, or GE , is a multinational American technology and services conglomerate incorporated in the State of New York. In 2009, Forbes ranked General Electric as the world's largest company worked on an experimental operating system called Multics Multics was an extremely influential early time-sharing operating system. The project was started in 1964. The last known running Multics installation was shut down on October 30, 2000 (Multiplexed Information and Computing Service), which was designed to run on the GE-645 mainframe computer Mainframes are computers used mainly by large organizations for critical applications, typically bulk data processing such as census, industry and consumer statistics, ERP, and financial transaction processing. (Eventually Multics became a commercial product, although sales did not meet expectations). Multics was an interactive operating system with many novel capabilities, including enhanced security.

AT&T Bell Labs pulled out of the Multics project and deployed its resources elsewhere. One of the developers on the Bell Labs team, Ken Thompson, continued to develop for the GE-645 mainframe, and wrote a game for that computer called Space Travel.^[1] However, he found that the game was too slow on the GE machine and was expensive, costing $75 per execution in scarce computing time.^[2]

Thompson thus re-wrote the game in assembly language for Digital Equipment Corporation's PDP-7 with help from Dennis Ritchie. This experience, combined with his work on the Multics project, led Thompson to start a new operating system for the PDP-7. Thompson and Ritchie led a team of developers, including Rudd Canaday, at Bell Labs developing a file system as well as the new multi-tasking operating system itself. They included a command line interpreter and some small utility programs.^[3]

1970s

In the 1970s the project was named Unics, and eventually could support two simultaneous users. Brian Kernighan invented this name as a contrast to Multics; the spelling was later changed to Unix.

Up until this point there had been no financial support from Bell Labs. When the Computer Science Research Group wanted to use Unix on a much larger machine 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 financial support from Bell. For the first time in 1970, the Unix operating system was officially named and ran on the PDP-11/20. It added a text formatting program called roff and a text editor. All three were written in PDP-11/20 assembly language. Bell Labs used this initial "text processing system", made up of Unix, roff, and the editor, for text processing of patent applications. Roff soon evolved into troff, the first electronic publishing program with a full typesetting capability. The UNIX Programmer's Manual was published on November 3, 1971.

In 1973, Unix was rewritten in the C programming language, contrary to the general notion at the time "that something as complex as an operating system, which must deal with time-critical events, had to be written exclusively in assembly language".^[4] The migration from assembly language to the higher-level language C resulted in much more portable software, requiring only a relatively small amount of machine-dependent code to be replaced when porting Unix to other computing platforms.

AT&T made Unix available to universities and commercial firms, as well as the United States government under licenses. The licenses included all source code including 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 the late 1970s in the form of a much-copied book by John Lions of the University of New South Wales, the Lions' Commentary on UNIX 6th Edition, with Source Code, which led to considerable use of Unix as an educational example.

Versions of the Unix system were determined by editions of its user manuals. For example, "Fifth Edition UNIX" and "UNIX Version 5" have both been used to designate the same version. Development expanded, with Versions 4, 5, and 6 being released by 1975. These versions added the concept of pipes, which led to the development of a more modular code-base and quicker development cycles. Version 5 and especially Version 6 led to a plethora of different Unix versions both inside and outside Bell Labs, including PWB/UNIX and the first commercial Unix, IS/1. As more of Unix was rewritten in C, portability also increased. A group at the University of Wollongong ported Unix to the Interdata 7/32. Bell Labs developed several ports for research purposes and internal use at AT&T. Target machines included an Intel 8086-based computer (with custom-built MMU) and the UNIVAC 1100.^[5]

In 1978, UNIX/32V was released for DEC's then new VAX system. By this time, over 600 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 10 were developed through the 1980s but were only released to a few universities, though they did generate papers describing the new work. This research led to the development of Plan 9 from Bell Labs, a new portable distributed system.

1980s

AT&T licensed UNIX System III, based largely on Version 7, for commercial use, the first version launching in 1982. This also included support for the VAX. AT&T continued to issue licenses for older Unix versions. To end the confusion between all its differing internal versions, AT&T combined them into UNIX System V Release 1. This introduced a few features such as the vi editor and curses from the Berkeley Software Distribution of Unix developed at the University of California, Berkeley. This also included support for the Western Electric 3B series of machines.

Since the newer commercial UNIX licensing terms were not as favorable for academic use as the older versions of Unix, the Berkeley researchers continued to develop 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 to the mainstream Unix kernel. The BSD effort produced several significant releases that 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 much TCP/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 Berkeley sockets API is a de facto standard for networking APIs and has been copied on many platforms.

Other companies began to offer commercial versions of the UNIX System for their own mini-computers and workstations. Most of these new Unix flavors were developed 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 on to co-found Sun Microsystems in 1982 and created SunOS for their workstation computers. In 1980, Microsoft announced its first Unix for 16-bit microcomputers called Xenix, which the Santa Cruz Operation (SCO) ported to the Intel 8086 processor in 1983, and eventually branched Xenix into SCO UNIX in 1989.

For a few years during this period (before PC compatible computers with MS-DOS became dominant), industry observers expected that UNIX, with its portability and rich capabilities, was likely to become the industry standard operating system for microcomputers.^[6] In 1984 several companies established the X/Open consortium with the goal of creating an open system specification based on UNIX. Despite early progress, the standardization effort collapsed into the "Unix wars," with various companies forming rival standardization groups. The most successful Unix-related standard turned out to be the IEEE's POSIX specification, designed as a compromise API readily implemented on both BSD and System V platforms, published in 1988 and 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, system administration, STREAMS, new forms of IPC, the Remote File System and TLI. AT&T cooperated with Sun Microsystems and between 1987 and 1989 merged features from Xenix, BSD, SunOS, and System V into System V Release 4 (SVR4), independently of X/Open. This new release consolidated all the previous features into 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, Addamax and others began building trusted versions of UNIX for high security applications, mostly designed for military and law enforcement applications.

1990s

In 1990, the Open Software Foundation released OSF/1, their standard Unix implementation, based on Mach and BSD. The Foundation was started in 1988 and was funded by several Unix-related companies that wished to counteract the collaboration 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. This escalation of conflict between competing vendors gave rise again 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 the inexpensive and ubiquitous Intel platform, which started a wave of interest in the use of inexpensive hardware for production computing. Shortly after it was founded, Bill Jolitz left BSDI to pursue distribution of 386BSD, the free software ancestor of FreeBSD, OpenBSD, and NetBSD.

By 1993 most commercial vendors had changed their variants of Unix to be based on System V with many BSD features added on top. The creation of the COSE initiative that year by the major players in Unix marked the end of the most notorious phase of the Unix wars, and was followed by the merger of UI and OSF in 1994. The new combined entity, which retained the OSF name, stopped work on OSF/1 that year. By that time the only vendor using it was Digital, 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 to UNIX to Novell. (Dennis Ritchie likened this to the Biblical story of Esau selling his birthright for the proverbial "mess of pottage".^[7]) Novell developed its own version, UnixWare, merging its NetWare with UNIX System V Release 4. Novell tried to use this to battle against Windows NT, but their core markets suffered considerably.

In 1993, Novell decided to transfer the UNIX trademark and certification rights to the X/Open Consortium.^[8] In 1996, X/Open merged with OSF, creating the Open Group. Various standards by the Open Group now define what is and what is not a "UNIX" 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 the Santa Cruz Operation.^[9] Whether Novell also sold the copyrights is currently the subject of litigation (see below).

In 1997, Apple Computer sought out a new foundation for its Macintosh operating system and chose NEXTSTEP, an operating system developed by NeXT. The core operating system, which was based on BSD and the Mach kernel, was renamed Darwin 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, the most widely used Unix-based system in the desktop computer market.

2000 to present

In 2000, SCO sold its entire UNIX business and assets to Caldera Systems, which later on changed its name to The SCO Group.

The Dot-com crash (2001–2003) has led to significant consolidation of versions of Unix. Of the many commercial flavors of Unix that were born in the 1980s, only Solaris, HP-UX, and AIX are still doing relatively well in the market, though SGI's IRIX persisted for quite some time. Of these, Solaris has the largest market share.^[10]

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 by The SCO Group. Other allegations included trade-secret violations by IBM, or contract violations by former Santa Cruz customers who had since converted to Linux. However, Novell disputed the SCO Group's claim to hold copyright on the UNIX source base. According to Novell, SCO (and hence the SCO Group) are effectively 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 (the fact that Novell had the copyright to UNIX, and that the SCO Group had improperly kept money that was due to Novell) was decided in Novell's favor. The court also ruled that "SCO is obligated 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 Unix and stated, "We don't believe there is Unix in Linux".^[11][12][13]

In 2005, Sun Microsystems released the bulk of its Solaris system code (based on UNIX System V Release 4) into an open source project called OpenSolaris. New Sun OS technologies such as the ZFS file system are now first released as open source code via the OpenSolaris project; as of 2006^[update] OpenSolaris has spawned several non-Sun distributions such as SchilliX, Belenix, Nexenta, Jaris OS and MarTux.

Standards

Beginning in the late 1980s, an open operating system standardization effort now known as POSIX provided a common baseline for all operating systems; IEEE based 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 administered by The Open Group. Starting in 1998 the Open Group and IEEE started the Austin Group, to provide a common definition of POSIX and the Single UNIX Specification.

In an effort towards compatibility, in 1999 several Unix system vendors agreed on SVR4's 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.

The Filesystem Hierarchy Standard was created to provide a reference directory layout for Unix-like operating systems, particularly Linux.

Components

The Unix system is composed of several components that are normally packaged together. By including — in addition to the kernel of an operating system — the development environment, libraries, documents, and the portable, modifiable source-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 learning tool and has had such 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 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 changed substantially across the history of the system. Nonetheless, the V7 implementation is 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 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:
  • 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, and 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 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" programmable command line interpreter, the primary user interface 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 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 modern 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 TCP/IP utilities.

• Documentation — 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

Unix impact

The Unix system had significant impact on other operating systems.

It was written in high level language as opposed to assembly language (which had been thought necessary for systems implementation on early computers). 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 MS-DOS 2.0+ 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 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 on ASCII 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 everything made 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., Telnet, FTP, SSH, 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.

Early Unix developers were important in bringing the concepts of modularity and reusability 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 also exposed numerous security holes in the networking implementations.

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 1983 launch of the free software 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 the Unix philosophy.

Free Unix-like operating systems

In 1983, Richard Stallman announced the GNU project, an ambitious effort to create a free software Unix-like system; "free" in 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 1992 Linus Torvalds released the Linux kernel 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 (and the rest of the GNU toolchain), the GNU C library and the GNU core utilities — have gone on to play central roles in other free Unix systems as well.

Linux distributions, comprising Linux 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 GNU/Linux, Ubuntu, Mandriva Linux, Slackware Linux and Gentoo.

A free derivative of BSD Unix, 386BSD, was also released in 1992 and led to the NetBSD and FreeBSD projects. With the 1994 settlement of a lawsuit that UNIX Systems Laboratories brought against the University of California and Berkeley Software Design Inc. (USL v. BSDi), 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 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 new markets such as the consumer desktop and mobile and embedded devices. Due to the 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 BSD code, and modern systems also usually include some GNU utilities in their distribution.

2038

Unix 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 "signed 32-bit integer". On January 19, 2038, the current time will roll over from 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 on implementation, because that toggles the sign bit. As many applications use OS library routines for date calculations, the impact of this could be felt much earlier than 2038; for instance, 30-year mortgages may have been calculated incorrectly beginning in the year 2008.

Since times before 1970 are rarely represented in Unix time, one possible solution that 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 compares differences between two sets of time.

Some Unix versions have already addressed this. For example, in Solaris and Linux in 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.

Branding

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),^[8] and in 1995 sold the related business operations to Santa Cruz Operation.^[9] 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 is being appealed.^{[citation needed]} Unix vendor SCO Group Inc. accused Novell of slander of title.

The present owner of the trademark UNIX is The Open Group, an industry standards consortium. Only systems fully compliant with and certified to the Single UNIX Specification 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 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 fee to The Open Group. Systems licensed to use the UNIX trademark include AIX, HP-UX, IRIX, Solaris, Tru64 (formerly "Digital UNIX"), A/UX, Mac OS X 10.5 on Intel platforms,^[14] and a part of z/OS.

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 "*" and "?" characters as "wildcard" characters in many utilities. This notation is also used to describe other Unix-like systems, e.g. Linux, BSD, etc., 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, according to Dennis Ritchie, when presenting the original Unix paper to the third Operating Systems Symposium of the American Association for Computing Machinery, “we had a new typesetter and troff had just been invented and we were intoxicated by being able to produce small caps.”^[15] 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.

Several plural forms of Unix are used to refer to multiple brands of Unix and Unix-like systems. Most common is the conventional "Unixes", but "Unices" (treating Unix as Latin noun of the third declension) is also popular. The Anglo-Saxon plural form "Unixen" is not common, although occasionally seen. 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.^[16]

References

1. ^ Ritchie, Dennis M.. "Space Travel: Exploring the solar system and the PDP-7". http://cm.bell-labs.com/cm/cs/who/dmr/spacetravel.html. Retrieved on 2007-03-11.
2. ^ The Creation of the UNIX* Operating System: The famous PDP-7 comes to the rescue
3. ^ The Creation of the UNIX* Operating System: The UNIX system begins to take shape
4. ^ Stallings, William. "Operating Systems: Internals and Design Principles" 5th ed, page 91. Pearson Education, Inc. 2005.
5. ^ 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. 9. http://www.cs.bell-labs.com/who/dmr/otherports/newp.pdf. Retrieved on 2009-04-09.
6. ^ "UNIX". The Computer Chronicles. 1985. http://www.archive.org/details/UNIX1985.
7. ^ http://groups.google.com/group/comp.unix.questions/browse_frm/thread/2f0b5e719fa3a3ec/3fa5e5fe4d58f96b
8. ^ ^{a b} http://groups.google.com/group/comp.std.unix/msg/c9974cf0022884f8
9. ^ ^{a b} HP, Novell and SCO To Deliver High-Volume UNIX OS With Advanced Network And Enterprise Services
10. ^ Stephen Shankland (December 7, 2005). "Itanium: A cautionary tale". Tech News. ZDNet. http://news.zdnet.com/2100-9584_22-5984747.html. Retrieved on 2006-10-04. "In the third quarter of this year, 7,845 Itanium servers were sold, according to research by Gartner. That compares with 62,776 machines with Sun Microsystems' UltraSparc, 31,648 with IBM's Power, and 9,147 with HP's PA-RISC."
11. ^ Memorandum and Decision Order in SCO v. Novell
12. ^ Memorandum and Decision Order Civil Case No. 2:04CV139DAK
13. ^ Novell Won't Pursue Unix Copyrights August 15, 2007
14. ^ The Open Group. "Mac OS X Version 10.5 Leopard on Intel-based Macintosh computers certification". http://www.opengroup.org/openbrand/register/brand3555.htm. Retrieved on 2007-06-12.
15. ^ Unix
16. ^ Autres Unix, autres moeurs (OtherUnix)

• Ritchie, D.M.; Thompson, K., The UNIX Time-Sharing System (The Bell System Technical Journal, July-August 1978, Vol. 57, No. 6, Part 2)
• Salus, Peter H.: A Quarter Century of UNIX, Addison Wesley, June 1, 1994; ISBN 0-201-54777-5
• "UNIX History". www.levenez.com. http://www.levenez.com/unix/. Retrieved on 17 March 2005.
• "AIX, FreeBSD, HP-UX, Linux, Solaris, Tru64". UNIXguide.net. http://www.unixguide.net/. Retrieved on 17 March 2005.
• "Linux Weekly News, February 21, 2002". lwn.net. http://lwn.net/2002/0221/bigpage.php3. Retrieved on 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-Peer Communications, 1996; ISBN 1-57398-013-7
• UNIX Shell Programming, Yashawant Kanetkar

External links

• The Open Group UNIX System Homepage
• The Unix Forums
• Unix History, a large graphical family tree of Unixes
• Unix Power
• A more complete map of the different Unices and derivatives
• SDF Public Access UNIX System
• The Creation of the UNIX Operating System
• The Unix Acronym List
• The Unix Guru Universe
• The Unix Heritage Society
• The Unix Tree: files from historic releases
• Unix as literature
• Unix at the Open Directory Project
• UNIX Evolution by Ian F. Darwin and Geoffrey Collyer
• Unix History
• Unix tutorial by Douglas Wilhelm Harder, from the University of Waterloo
• What is UNIX, Anyway?

Categories: Operating system families | Unix

See Also:

• Unix
• Common Unix Printing System
• Unix Amiga Delitracker Emulator

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