Single UNIX Specification
The Single UNIX Specification (SUS) is the collective name of a family of standards for computer operating systems to qualify for the name "Unix". The SUS is developed and maintained by the Austin Group, based on earlier work by the IEEE and The Open Group.
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 
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 
This specification consisted of:
- the Base Definitions, Issue 5,
- the System Interfaces and Headers, Issue 5,
- the Commands and Utilities, Issue 5,
- the Networking Services, Issue 5,
- the X/Open Curses, Issue 4, Version 2,
and was at the core of the UNIX 98 brand.
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.
This standard consisted of:
- the Base Definitions, Issue 6,
- the System Interfaces and Headers, Issue 6,
- the Commands and Utilities, Issue 6,
and is at the core of the UNIX 03 brand.
2004: POSIX:2004 
2008: POSIX:2008 
This standard consists of:
- the Base Definitions, Issue 7,
- the System Interfaces and Headers, Issue 7,
- the Commands and Utilities, Issue 7.
SUSv3 totals some 3700 pages, which are thematically divided into four main parts:
- Base Definitions (XBD) - a list of definitions and conventions used in the specifications and a list of C header files which must be provided by compliant systems. 84 header files in total are provided.
- Shell and Utilities (XCU) - a list of utilities and a description of the shell, sh. 160 utilities in total are specified.
- System Interfaces (XSH) - contains the specification of various functions which are implemented as system calls or library functions. 1123 system interfaces in total are specified.
- Rationale (XRAT) - the explanation behind the standard.
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
- UNIX 98 - the mark for systems conforming to version 2 of the SUS (partial compliance)
- UNIX 03 - the mark for systems conforming to version 3 of the SUS (full compliance)
Older UNIX standards (superseded)
- UNIX 93 (completely superseded)
- UNIX 95 (compliance still acceptable for some simpler software subsystems)
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.
HP-UX 11i V3 Release B.11.31 is registered as UNIX 03 compliant. Previous releases are registered as UNIX 95.
OS X 
Apple's OS X is an Open Brand UNIX 03 registered product, first becoming registered with Mac OS X v10.5 "Leopard" on October 26, 2007 (when run on Intel processors). "X/Open Brand" is awarded to software which demonstrates compliance with an X/Open specification, in this case "UNIX 03".
Reliant UNIX 
The last Reliant UNIX versions were registered as UNIX 95 compliant (XPG4 hard branding).
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.
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 
Vendors of Unix-like systems such as Linux and FreeBSD do not typically certify their distributions, as the cost of certification and the rapidly changing nature of such distributions make the process too expensive to sustain.
BSD descendants 
- FreeBSD has a "C99 and POSIX Conformance Project" which aims for full compliance with a large subset of the SUS.
- Darwin is an open source operating system: it is essentially the open source subset of Mac OS X. Darwin is compliant with the SUS 03.
Linux aims to be compliant, but as certification is expensive, no Linux distribution has been registered as SUS compliant.
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, and also extends in several areas, the POSIX specifications, the Single UNIX Specification and other open standards. It is de facto accepted and followed by many Linux distributions.
See also 
- Andrew Josey (2004-10-28). Single UNIX Specification Frequently Asked Questions. 1.9.
- "The Open Group Announces Enhanced Single UNIX Specification" (Press release). The Open Group. March 12, 1997. Retrieved 2009-07-26.
- "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.
- "IEEE Std 1003.1, 2004 Edition". Retrieved 2009-07-26.
- "Base Specifications, Issue 7". The Open Group. Retrieved 2009-07-26.
- "The Austin Common Standards Revision Group". The Open Group. Retrieved 2009-07-26.
- Single UNIX Specification Version 4
- "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."
- The Open Group. "Mac OS X Version 10.5 Leopard on Intel-based Macintosh computers certification". Retrieved 2007-06-12.
- Solaris 2.5.1
- "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.
- The Open Group. "Register of Certified Products". Retrieved 2009-12-20.
- "The Open Brand Fee Schedule". The Open Group. Retrieved 2007-05-11. Lists fees required to use UNIX brand
- p. 7, Linux System Programming: Talking Directly to the Kernel and C Library
- About: The Single UNIX Specification
- Single UNIX Specifications:
- The Portable Application Standards Committee
- Register of products certified for the UNIX and other Open Group brands
- Unix-Wars (Living Internet)
- Unix Standards (Eric S. Raymond, The Art of Unix Programming)
- AIX Commands, Tools, Scripts and Explanations