The Portable Operating System Interface POSIX is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems.
Originally, the name “POSIX” referred to IEEE Std 1003.1-1988, released in 1988. The family of POSIX standards is formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945.
The standards emerged from a project that began circa 1985. Richard Stallman suggested the name POSIX to the IEEE instead of former IEEE-IX. The committee found it more easily pronounceable and memorable, and thus adopted it.
Unix was selected as the basis for a standard system interface partly because it was “manufacturer-neutral.” However, several major versions of Unix existed—so there was a need to develop a common denominator system.
The POSIX specifications for Unix-like operating systems originally consisted of a single document for the core programming interface, but eventually grew to 19 separate documents (POSIX.1, POSIX.2, etc.).The standardized user command line and scripting interface were based on the UNIX System V shell.
Many user-level programs, services, and utilities (including awk, echo, ed) were also standardized, along with required program-level services (including basic I/O: file, terminal, and network).
POSIX also defines a standard threading library API which is supported by most modern operating systems. Nowadays, most parts of POSIX are combined into a single standard (IEEE Std 1003.1-2008, also known as POSIX.1-2008).