The Single UNIX Specification is the collective name of a family of standards developed and maintained by the Austin Group. It is based on earlier work by the IEEE and The Open Group and currently published as the Single UNIX Specification Version 3, and also as The Open Group Base Specifications, IEEE Standard 1003.1 (POSIX) and 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 operating system.
Previously, The Open Group's Single Unix Specification was separate from the official IEEE POSIX. The near-equivalent Single UNIX Specification became more popular because it was available for free, whereas the IEEE charged a substantial fee for access to the POSIX specification. Beginning in 1998 a joint working group, the Austin Group, began to develop the combined standard that would be known as the Single UNIX Specification.
The user and software interfaces to the OS are specified in four main sections:
- Base Definitions - A list of definitions and conventions used in the specifications and a list of C header files which must be provided by compliant systems.
- Shell and Utilities - A list of utilities and a description of the shell, sh.
- System Interfaces - A list of available C system calls which must be provided.
- Rationale - The explanation behind the standard.
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, computer terminal, and network) services.
A test suite accompanies the standard. It is called PCTS or the Posix Certification Test Suite.
For Linux systems, several common extensions and complementary de facto-standards are provided by the Linux Standard Base.