System V Definition

System V was a milestone version of the UNIX operating system that was first released in 1983. Its most successful and influential version, System V Release 4, was made available in 1989.

UNIX was originally developed by Ken Thompson in 1969 at Bell Labs, the research and development arm of AT&T (The American Telephone and Telegraph Corporation), the former U.S. telecommunications monopoly. In 1973 it was rewritten in the C programming language, which had recently been developed at Bell Labs by Dennis Ritchie, thereby greatly increasing its portability (i.e., the ability to be used on a variety of computer types with relatively little modification). This version was named Version 3.

AT&T was required under the terms of a 1958 court order in an antitrust (i.e., anti-monopoly) case to license its non-telephone-related technology to anyone who requested it. When Thompson and Ritchie published an article about UNIX in a technical journal in 1973, several universities requested and received the UNIX tapes (the standard method of software distribution at that time) and manuals for educational purposes for only a nominal fee. Subsequently, many universities began using UNIX in their curricula, which led to computer science faculty and students making numerous improvements.

Version 6, introduced in 1975, was the first UNIX version that was widely available outside of AT&T; academic and research institutions could obtain its C source code free of charge. Source code (also referred to as source or code) is the version of software as it is originally written (i.e., typed into a computer) by a human in any of numerous programming languages, among the most popular today of which are C, C++ and Java.

Two major centers of UNIX activity emerged: the University of California at Berkeley (UCB) and the Massachusetts Institute of Technology (MIT). UCB developed BSD (Berkeley Software Distribution), a version of UNIX whose innovations were incorporated back into AT&T's UNIX and all subsequent Unix-like operating systems. MIT spawned the Free Software Foundation (FSF) and its GNU project, which has a goal of developing a complete, high performance and entirely free Unix-like operating system and which has developed some of the best and most widely used utility programs for such systems. MIT also gave birth to the X Window System, a free, cross-platform, complete and extremely successful system for managing GUIs (graphical user interfaces) on both single computers and computer networks.

Version 7, which followed in 1979, incorporated some of UCB's improvements and was the first UNIX to emphasize portability. The names Version 6 and Version 7 refer to editions of internal documentation at Bell Labs, and the two releases are also sometimes referred to as the Sixth and Seventh Editions. Neither were commercial products, nor were they officially supported by AT&T. However, they became the foundations upon which both major UNIX lineages, i.e., System V and BSD, and all modern Unix-like operating systems are based.

Commercial UNIX

A major change in AT&T's policy with regard to UNIX occurred in 1982 as a result of the U.S. Department of Justice permitting the company to to enter the computer business, and thus sell UNIX commercially, in return for divesting its local telephone company business.

Later in the same year, AT&T announced official support for UNIX and released its first commercial version, System III, which was a somewhat spartan implementation based on Version 7. Meanwhile, Western Electric, AT&T's manufacturing subsidiary, to whom AT&T had earlier transferred UNIX research and development, continued to offer older versions.

The new, commercial UNIX releases no longer included the source code. Without the source code, it is extremely difficult to study, customize or improve software. Consequently, UCB continued to develop its own version of UNIX, then called BSD UNIX, which was based on Version 7, as an alternative to System III and subsequent AT&T versions. Perhaps the most important result of the BSD development effort was the addition of TCP/IP (Transmission Control Protocol/Internet Protocol) network code to the mainstream UNIX kernel. TCP/IP has become the standard set of protocols that computers and other devices use to communicate over networks, including the Internet.

Release 1

In order to end the confusion arising from the proliferation of versions, AT&T integrated features developed at universities and other companies to create UNIX System V Release 1 and began offering it in 1983. Although this version by itself lacked networking capabilities and a number of other features that have long since become standard in Unix-like operating systems, it represented a significant improvement over System III.

This improvement was largely a result of the packaging together with additional software from BSD, enhancements which were known collectively as the Berkeley Extensions. They included TCP/IP and utilities such as the csh shell (i.e., a program that provides the traditional, text-only user interface for Unix-like operating systems) and the now ubiquitous vi text editor. System V also included support for the VAX, a high performance, 32-bit computer that was developed by Digital Equipment Corporation (DEC) in the mid-1970s.

Moreover, it introduced additional types of interprocess communications (IPC) in user mode, the mode in which each process (i.e., an executing instance of a program) starts out. IPC is the exchange of information between processes on one or more computers. The additional types, collectively referred to as the System V IPC, included semaphores, message queues and shared memory.

Release 4

Four major versions of System V were released, termed Releases 1, 2, 3 and 4. System V Release 4, also called SVR4,was introduced in 1989 and it was the most successful and influential. It was the source of several common features in modern Unix-like operating systems, including the SysV init scripts (which are used to control system startup and shutdown) and the System V Interface Definition (a standard defining how System V systems should work).

As a result of the continued improvement in performance and growth in popularity of UNIX, other companies began to offer commercial version of it for their own hardware systems. Most of these new flavors (i.e., versions) of UNIX were developed from the System V base under license from AT&T, although some some were developed from BSD.

One of the leading developers of BSD, Bill Joy, went on to create SunOS, the ancestor of Solaris, and to co-found Sun Microsystems for the purpose of distributing the operating system. Solaris, a SRV4 descendant, is the most widely used proprietary (i.e., commercial) Unix-like operating system.

In 1993 AT&T sold all of its UNIX rights to Novell, Inc., a Provo, Utah-based software company. Novell then developed its own version, UNIXware, which combined its Netware with SVR4, and attempted, with little success, to use this to compete with Microsoft's new Windows NT.

Although System V and BSD UNIX have traditionally been considered the two major branches of Unix-like operating systems, this classification has been becoming less relevant with the advent of Unix-like systems that are derived from neither source code base, such as Linux and QNX. Moreover, operating system standardization efforts such as POSIX (Portable Operating System Interface) have tended to reduce the differences between implementations.

Created May 23, 2005.
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