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.45
In 1985, AT&T published System V Interface Definition (SVID), a specification of UNIX System V programming interfaces.6
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: 19857
- XPG2: X/Open Portability Guide Issue 2: 19877
- XPG3: X/Open Portability Guide Issue 3: 19898
- XPG4: X/Open Portability Guide Issue 4: 19927
XPG4 Base included the following documents:
- System Interface Definitions, Issue 4, ISBN 1-872630-46-4
- System Interfaces and Headers, Issue 4, ISBN 1-872630-47-2
- Commands and Utilities, Issue 4, ISBN 1-872630-48-0
1990s: Spec 1170
In the early 1990s, a separate effort known as the Common API Specification or Spec 1170 was initiated by several major vendors,9 who formed the COSE alliance in the wake of the Unix wars. In 1993, Spec 1170 was assigned by COSE to X/Open for fasttrack.10 In October 1993, a planned transfer of UNIX trademark from Novell to X/Open was announced;11 it was finalized in 2nd quarter of 1994.12 Spec 1170 would eventually become the Single Unix Specification.9
1994: Single UNIX Specification
In 1994, the X/Open Company released the Single UNIX Specification.9 The SUS was made up of documents that were part of the X/Open Common Applications Environment (CAE):9
- System Interface Definitions, Issue 4, Version 213
- System Interfaces and Headers, Issue 4, Version 214
- Commands and Utilities, Issue 4, Version 215
- Networking Services, Issue 416
This was a repackaging of the X/Open Portability Guide (XPG), Issue 4, Version 2.
Sources differ on whether X/Open Curses, Issue 4, Version 2 was part of this SUS;917 its copyright date is given as 1996.18 X/Open Curses, Issue 4 was published in 1995.19
In October 1994, X/Open indicated they were going to refer to Spec 1170 as '"Single-Unix" specification'.20
The SUS was at the core of the UNIX 95 brand.21
This version had 1168 programming interfaces.22
This version of SUS was drawn from the following sources:9
- XPG4 Base by X/Open
- System V Interface Definition, (SVID) Edition 3, Level 1 calls by AT&T
- Application Environment Specification (AES) by the Open Software Foundation (OSF)
- Interfaces found in common use and not yet covered by a formal specification, drawn from a survey of major applications.
1997: Single UNIX Specification, version 2
In 1996, X/Open merged with Open Software Foundation (OSF) to form The Open Group.2324
In 1997, the Open Group released the Single UNIX Specification, Version 2.2526
This specification consisted of:27
- System Interface Definitions, Issue 528
- System Interfaces and Headers, Issue 529
- Commands and Utilities, Issue 530
- Networking Services, Issue 531
- X/Open Curses, Issue 4, Version 232
and was at the core of the UNIX 98 brand.33
This version had 1434 programming interfaces.22
2001: Single UNIX Specification, version 3, POSIX.1-2001
Beginning in 1998, a joint working group of IEEE, ISO JTC 1 SC22 and The Open Group known as the Austin Group began to develop the combined standard that would be known as the core of Single UNIX Specification, Version 3 and as POSIX.1-2001. It was released on January 30, 2002.34
This SUS consisted of:35
- Base Specifications, Issue 6
- Base Definitions, Issue 6
- System Interfaces, Issue 6
- Shell and Utilities, Issue 6
- Rationale (Informative)
- X/Open Curses, Issue 4, Version 2
and is at the core of the UNIX 03 brand.36
The Base Specifications are technically identical to POSIX.1-2001, which is IEEE Std 1003.1-2001.35
This version had 1742 programming interfaces.22
An authorized guide is available for the version.37
2004 Edition
In 2004, a new edition of the POSIX.1-2001 standard was released, incorporating two technical corrigenda.3839 It is called IEEE Std 1003.1, 2004 Edition.39 Some informally call it POSIX.1-2004,404142 but this is not an official identification.
2008: Single UNIX Specification, version 4, POSIX.1-2008
In December 2008, the Austin Group published a new major revision of SUS and POSIX.434445 This is the Single UNIX Specification, Version 4 (SUSv4).46
This SUS consists of:47
- Base Specifications, Issue 7
- Base Definitions, Issue 7
- System Interfaces, Issue 7
- Shell and Utilities, Issue 7
- Rationale, Issue 7, (Informative)
- X/Open Curses, Issue 7
The Base Specifications are technically identical to POSIX.1-2008, which is IEEE Std 1003.1-2008.47
This version had 1833 interfaces, of which 1191 were in the System Interfaces section.48
2013 Edition
Technical Corrigendum 1 mostly targeted internationalization, and also introduced a role-based access model. A trademark UNIX V7 (not to be confused with V7 UNIX, the version of Research Unix from 1979) was created to mark compliance with SUS Version 4.49
2016 Edition
Technical Corrigendum 2 was published in September 2016, leading into IEEE Std 1003.1-2008, 2016 Edition and Single UNIX Specification, Version 4, 2016 Edition.
2018 Edition, POSIX.1-2017
In January 2018, an "administrative rollup" edition was released. It incorporates Single UNIX Specification version 4 TC1 and TC2, and is technically identical to the 2016 edition.50
The Base Specifications are technically identical to POSIX.1-2017, which is IEEE Std 1003.1-2017.51
2024: Single UNIX Specification, version 5, POSIX.1-2024
In January 2025, the Single UNIX® Specification Version 5 was published as POSIX®.1-2024 also simultaneously IEEE Std 1003.1™-2024 Edition, the Open Group Base Specifications Issue 852, and in 2026 adopted as ISO®/IEC/IEEE 9945:2026 Information technology - Portable Operating System Interface (POSIX®) base specifications - Issue 8 (Edition 2, 2026)53.
