Definition of System v release 4

Unix System V, commonly abbreviated SysV and rarely called System 5, was 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 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. The system also forms the basis of the System V Interface Definition (SVID), a standard defining how System V systems should work. The other one of the two major branches of the Unix system is Berkeley Software Distribution.