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.
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