System V, previously known as AT&T System V, was one of the versions of the Unix computer 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 the most successful version, and the source of several common Unix features, such as "SysV init scripts" (/etc/init.d), used to control system startup and shutdown, and the System V Interface Definition (SVID), a standard defining how System V systems should work.

While AT&T sold their own hardware which ran System V, many (perhaps most) customers ran a version from a reseller, based on AT&T's reference implementation. Popular SysV derivatives include Dell SVR4 and Bull SVR4. The most widely used versions of System V today are SCO OpenServer, based on System V Release 3, and Sun Microsystems Solaris Operating Environment and SCO UnixWare, both based on System V Release 4.

System V was an enhancement over AT&T's first commercial UNIX called System III. Traditionally, System V has been considered one of the two major "flavors" of UNIX, the other being BSD. However, with the advent of UNIX implementations developed from neither code base, such as Linux and QNX, this generalisation is not as accurate as it once was, and in any case standardisation efforts such as POSIX are tending to reduce the differences between implementations.

SVR1

The first version of System V was released in 1983. It introduced features such as the vi editor and curses from the Berkeley Software Distribution of UNIX developed at the University of California, Berkeley (UCB). This also included support for the DEC VAX machine. It also added support for inter-process communication using messages, semaphores, and shared memory.

SVR2

System V Release 2 was released in 1984. It added UNIX shell functions and the SVID.

SVR3

System V Release 3 was released in 1987. It included STREAMS, remote file sharing (RFS), shared libraries, and the Transport Layer Interface (TLI).

SVR4

System V Release 4.0 was announced on November 1, 1989 and was released in 1990. A joint project of UNIX Systems Laboratories and Sun Microsystems, it combined technology from Release 3 as well as 4.3BSD, Xenix, and SunOS:

• From BSD: TCP/IP support, csh
• From SunOS: the Network File System (NFS), memory mapped files, a new shared library system
• Other improvements:
  • ksh
  • ANSI C compatability
  • better internationalization support
  • an application binary interface (ABI)
  • support for standards such as POSIX, X/Open, and SVID3

SVR4.1

Release 4.1 added asynchronous I/O.

SVR4.2

Release 4.2 added support for the Veritas filesystem, access control lists (ACLs), and dynamically loadable kernel modules.

External link

• Unix FAQ - history