The Single UNIX Specification (SUS) is a standard for computer operating systems, compliance with which is required to qualify for using the "UNIX" trademark. The standard specifies programming interfaces for the C language, a command-line shell, and user commands. The core specifications of the SUS known as Base Specifications are developed and maintained by the Austin Group, which is a joint working group of IEEE, ISO/IEC JTC 1/SC 22/WG 15 and The Open Group. If an operating system is submitted to The Open Group for certification, and passes conformance tests, then it is deemed to be compliant with a UNIX standard such as UNIX 98 or UNIX 03. Very few BSD and Linux-based operating systems are submitted for compliance with the Single UNIX Specification, although system developers generally aim for compliance with POSIX standards, which form the core of the Single UNIX Specification. The latest SUS consists of two parts: the base specifications technically identical to POSIX, and X/Open Curses specification. Some parts of the SUS are optional. History 1980s: Motivation The SUS emerged from multiple 1980s efforts to standardize operating system interfaces for software designed for variants of the Unix operating system. The need for standardization arose because enterprises using computers wanted to be able to develop programs that could be used on the computer systems of different manufacturers without reimplementing the programs. Unix was selected as the basis for a standard system interface partly because it was manufacturer-neutral. In 1984, the UNIX user group called /usr/group published the results of their standardization effort for programming interfaces in their 1984 /usr/group standard, which became basis for what would become the POSIX.1-1988 standard. In 1985, AT&T published System V Interface Definition (SVID), a specification of UNIX System V programming interfaces. 1988: POSIX In 1988, standardization efforts resulted in IEEE 1003 (also registered as ISO/IEC 9945), or POSIX.1-1988, which loosely stands for Portable Operating System Interface. 1980s and 1990s: X/Open Portability Guide The X/Open Portability Guide (XPG) was a precursor to the SUS, published by the X/Open Company, a consortium of companies established in 1984. The guides were published in the following years. XPG1: X/Open Portability Guide Issue 1: 1985 XPG2: X/Open Portability Guide Issue 2: 1987 XPG3: X/Open Portability Guide Issue 3: 1989 XPG4: X/Open Portability Guide Issue 4: 1992 XPG4 Base included the following documents: System Interface Definitions, Issue 4, System Interfaces and Headers, Issue 4, Commands and Utilities,