The standards emerged from a project that began in 1984 building on work from related activity in the /usr/group association.
[citation needed] Unix was selected as the basis for a standard system interface partly because it was "manufacturer-neutral".
[8] The standardized user command line and scripting interface were based on the UNIX System V shell.
POSIX also defines a standard threading library API which is supported by most modern operating systems.
POSIX.1-2001 (or IEEE Std 1003.1-2001) equates to the Single UNIX Specification, version 3 minus X/Open Curses.
Depending upon the degree of compliance with the standards, one can classify operating systems as fully or partly POSIX compatible.
Current versions of the following operating systems have been certified to conform to one or more of the various POSIX standards.
This means that they passed the automated conformance tests[20] and their certification has not expired and the operating system has not been discontinued.