Single UNIX Specification

The Single UNIX Specification (SUS) is the collective name of a family of standards for computer operating systems, compliance with which is required to qualify for using the "UNIX" trademark. The core specifications of the SUS are developed and maintained by the Austin Group, which is a joint working group of IEEE, ISO JTC 1 SC22 and The Open Group. If an operating system is submitted to The Open Group for certification, and passes conformance tests, then it is termed to be compliant with a UNIX standard such as UNIX 98 or UNIX 03.

Very few BSD and GNU/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.


1980s: Motivation

The SUS emerged from a mid-1980s project 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.

1988: POSIX

In 1988, these standards became IEEE 1003 (also registered as ISO/IEC 9945), or POSIX, which loosely stands for Portable Operating System Interface.

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, who formed the COSE alliance in the wake of the Unix wars. This specification became more popular because it was available at no cost, whereas the IEEE charged a substantial fee for access to the POSIX specification. Management over these specifications was assigned to X/Open who also received the Unix trademark from Novell in 1993. Unix International (UI) merged into Open Software Foundation (OSF) in 1994 only to merge with X/Open to form The Open Group in 1996.

1997: Single UNIX Specification version 2

In 1997, the Open Group released the Single UNIX Specification Version 2.[1][2]

This specification consisted of:

and was at the core of the UNIX 98 brand.[3]

2001: POSIX:2001, Single UNIX Specification version 3

Beginning in 1998, a joint working group known as the Austin Group began to develop the combined standard that would be known as the Single UNIX Specification Version 3 and as POSIX:2001 (formally: IEEE Std 1003.1-2001). It was released on January 30, 2002.[4]

This standard consisted of:

and is at the core of the UNIX 03 brand.[5]

2004: POSIX:2004

In 2004, a new edition of the POSIX:2001 standard was released, incorporating two technical corrigenda. It is called POSIX:2004 (formally: IEEE Std 1003.1-2004).[6][7]

2008: POSIX:2008

In December 2008, the Austin Group published a new major revision, known as POSIX:2008 (formally: IEEE Std 1003.1-2008).[8][9][10] This is the core of the Single UNIX Specification, Version 4 (SUSv4).[11]

This standard consists of:


SUSv3 totals some 3700 pages, which are thematically divided into four main parts:

The standard user command line and scripting interface is the POSIX shell, an extension of the Bourne Shell based on an early version of the Korn Shell. Other user-level programs, services and utilities include awk, echo, ed, vi, and hundreds of others. Required program-level services include basic I/O (file, terminal, and network) services. A test suite accompanies the standard. It is called PCTS or the POSIX Certification Test Suite.

Additionally, SUS includes CURSES (XCURSES) specification, which specifies 372 functions and 3 header files. All in all, SUSv3 specifies 1742 interfaces.

Note that a system need not include source code derived in any way from AT&T Unix to meet the specification. For instance, IBM OS/390, now z/OS, qualifies as a "Unix" despite having no code in common.

Marks for compliant systems

There are two official marks for conforming systems

Older UNIX standards (superseded)


Product Vendor Architecture UNIX 03 UNIX 98 UNIX 95 UNIX 93
AIX IBM Corporation PowerPC Yes Yes No No
FTX Cemprus LLC PA-RISC No No No Yes
EulerOS Huawei x86-64 Yes No No No
HP-UX Hewlett-Packard Company IA-64, PA-RISC Yes No Yes No
IRIX Silicon Graphics, Inc. MIPS No No Yes No
K-UX Inspur x86-64 Yes No No No
macOS (formerly OS X) Apple x86-64 Yes No No No
OpenServer Xinuos IA-32 No No No Yes
Solaris Oracle Corporation IA-32, x86-64, SPARC Yes Yes No No
Tru64 UNIX Digital Equipment Corporation Alpha No Yes No No
UnixWare Xinuos IA-32 No No Yes No
z/OS USS IBM Corporation z/Architecture No No Yes No

Currently Registered UNIX systems


AIX 5L V5.2 with some updates, AIX 5L V5.3 and AIX 6.1, are registered as UNIX 03 compliant. AIX 5L V5.2 is registered as UNIX 98 compliant.


EulerOS 2.0 for the x86-64 architecture were certified as UNIX 03 compliant. [12]The UNIX 03 conformance statement shows that the standard C compiler is from the GNU Compiler Collection (gcc), and that the system is a GNU/Linux distribution of the Red Hat family.[13]


HP-UX 11i V3 Release B.11.31 is registered as UNIX 03 compliant. Previous releases are registered as UNIX 95.[14]

HP-UX 11i features also provide partial conformance to the UNIX 98 specification.[15]

Inspur K-UX

Inspur K-UX 2.0 and 3.0 for the x86-64 architecture were certified as UNIX 03 compliant.[16][17] The UNIX 03 conformance statement for Inspur K-UX 2.0 and 3.0 shows that the standard C compiler is from the GNU Compiler Collection (gcc), and that the system is a GNU/Linux distribution of the Red Hat family.[18]


Apple's macOS (previously known as OS X) is a UNIX 03 registered product,[19] first becoming registered with Mac OS X v10.5 "Leopard" on October 26, 2007 (when run on Macs with Intel processors).[20][21] All newer versions of macOS (except Mac OS X Lion) have been registered.[22]


Solaris 11 complies with the Single UNIX Specification.[23] Solaris 10 is registered as UNIX 03 compliant on 32-bit and 64-bit x86 (X86-64) and SPARC systems. Solaris 8 and 9 are registered as UNIX 98 compliant on 32-bit x86 and SPARC systems; 64-bit x86 systems are not supported.

Solaris 2.5.1 was also registered as UNIX 95 compliant on the PReP PowerPC platform in 1996, but the product was withdrawn before more than a few dozen copies had been sold.[24]


IBM z/OS 1.2 and higher is registered as UNIX 95 compliant. z/OS 1.9, released on September 28, 2007, and subsequent releases "better align" with UNIX 03.[25]

Previously Registered UNIX systems

Reliant UNIX

The last Reliant UNIX versions were registered as UNIX 95 compliant (XPG4 hard branding).


UnixWare 7.1.3 is registered as UNIX 95 compliant. SCO OpenServer 5 is registered as UNIX 93 compliant.

Tru64 UNIX

Tru64 UNIX V5.1A and later are registered as UNIX 98 compliant.


Other operating systems registered as UNIX 95 or UNIX 93 compliant:

Non-registered Unix-like systems

Developers and vendors of Unix-like operating systems such as GNU/Linux, FreeBSD, and MINIX, typically do not certify their distributions and do not install full POSIX utilities by default.[27][28] Sometimes, SUS compliance can be improved by installing additional packages, but very few Linux systems can be configured completely conformant.

Darwin, the open source subset of OS X, has behavior that can be set to comply with UNIX 03.[29]

FreeBSD previously had a "C99 and POSIX Conformance Project" which aimed for compliance with a subset of the Single UNIX Specification, and documentation where there were differences.[30]

The FreeBSD C99 & POSIX® Conformance Project aims to implement all requirements of the ISO 9899:1999 (C99) and IEEE 1003.1-2001 (POSIX) standards. In cases where aspects of these standards cannot be followed, those aspects will be documented in the c99(7) or posix(7) manuals. It is also an aim of this project to implement regression tests to ensure correctness whenever possible.

For Linux, the Linux Standard Base was formed in 2001 as an attempt to standardize the internal structures of Linux-based systems for increased compatibility. It is based on the POSIX specifications, the Single UNIX Specification, and other open standards, and also extends them in several areas. Although these standards are commonly accepted, few Linux distributions actually go through certification as LSB compliant.[31] There are conflicts between the LSB and The POSIX standards.[32]

See also



  2. "The Open Group Announces Enhanced Single UNIX Specification" (Press release). The Open Group. March 12, 1997. Retrieved 2009-07-26.
  4. "The Open Group announces completion of the joint revision to POSIX and the Single UNIX Specification" (Press release). The Open Group. January 30, 2002. Retrieved 2009-07-26.
  7. "IEEE Std 1003.1, 2004 Edition". Retrieved 2009-07-26.
  9. "Base Specifications, Issue 7". The Open Group. Retrieved 2009-07-26.
  10. "The Austin Common Standards Revision Group". The Open Group. Retrieved 2009-07-26.
  11. Single UNIX Specification Version 4
  12. "Huawei KunLun EulerOS 2.0 Operating System Passes UNIX Certification". Huawei. Retrieved 2016-10-29.
  13. Peng Shen. "Huawei Conformance Statement: Commands and Utilities V4". The Open Group. Retrieved 2016-10-29.
  14. "UNIX 2003 Standard Profile conformance". Hewlett Packard. Retrieved 2014-07-22.
  15. "HP-UX Software Transition Kit". Hewlett Packard. Retrieved 2014-07-22.
  16. "The Open Brand Register of Certified Products". The Open Group. 2014-05-29. Retrieved 2014-05-29.
  17. Xie Ruohong. "Inspur Conformance Statement". The Open Group. Retrieved 2015-12-08.
  18. Xie Ruohong. "Inspur Conformance Statement: Commands and Utilities V4". The Open Group. Retrieved 2015-12-08.
  20. "Mac OS X Leopard - Technology - UNIX". Leopard Technology Overview. Apple Inc. Archived from the original on 2007-08-23. Retrieved 2007-06-11. Leopard is now an Open Brand UNIX 03 Registered Product, conforming to the SUSv3 and POSIX 1003.1 specifications for the C API, Shell Utilities, and Threads.
  21. The Open Group. "Mac OS X Version 10.5 Leopard on Intel-based Macintosh computers certification". Retrieved 2007-06-12.
  24. Solaris 2.5.1
  25. "Preview: IBM z/OS V1.9 advanced infrastructure solutions for your business needs" (PDF). IBM. February 6, 2007. pp. 4, 15. Retrieved 2007-06-11.
  26. The Open Group. "Register of Certified Products". Retrieved 2009-12-20.
  27. For example pax and sccs are usually not installed on Linux, as they are not commonly used.
  28. Ubuntu bug tracker: No UNIX compatible pax implementation
  30. "FreeBSD C99 and POSIX conformance project". FreeBSD Project. Archived from the original on December 23, 2013.
  31. "Certified Products Product Directory". The Linux Foundation. Retrieved 2015-12-09.
  32. "ISO/IEC TR 24715:2006 - Information technology -- Programming languages, their environments and system software interfaces -- Technical Report on the Conflicts between the ISO/IEC 9945 (POSIX) and the Linux Standard Base (ISO/IEC 23360)". Retrieved 2011-10-15.

External links

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