The Open Group publishes a document called the Single UNIX Specification that defines how a UNIX system should behave. There have been two versions of this document. Its content derives heavily from the POSIX specification, but the Single UNIX Specification has as of 2001 become more popular among operating system developers than the POSIX specification, largely because the Open Group makes its document available at a much lower price.
As of version 3 the specification contains four sections:
- Base Definitions and Headers
(a list of definitions and conventions used in the specifications and a list of C header files which must be provided by compliant systems) - System interfaces
(a list of available C system calls which must be provided) - Shell & Utilities
(a list of utilities and a description of the shell, sh) - Rationale
(the explanation behind the standard)