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POSIX is the collective name of a family of related standards defined by the IEEE and formally designated IEEE 1003. The international standard name is ISO/IEC 9945. The standards emerged from a project, begun circa 1985, to standardise the application program interface for software designed to run on variants of the UNIX OS. The term POSIX was suggested by Richard Stallman in response to an IEEE request for a memorable name. It is a near acronym for Portable Operating System Interface, with the X signifying the UNIX heritage of the API.

POSIX specifies the user and software interfaces to the OS in some 15 different documents. The standard user command line and scripting interface is the Korn shell. Other user-level programs, services and utilities include awk, echo, ed, and numerous (hundreds) others. Required program-level services include basic I/O (file, terminal, and network) services.

A test suite for POSIX accompanies the standard. It is called PCTS or the Posix Conformance Test Suite.

Since the IEEE has been charging very high rates for POSIX documentation and not allowing on-line publication of the standards, there has been a tendency toward the "Single UNIX Specification" standard, which is open, accepts input from anyone, and is freely available on the Internet. It is made by The Open Group.

For Linux systems, several common extensions and complementary de facto-standards are provided by the Linux Standard Base.

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