Single UNIX Specification

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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 deemed to be compliant with a UNIX standard such as UNIX 98 or UNIX 03.

A computer is a machine that can be instructed to carry out sequences of arithmetic or logical operations automatically via computer programming. Modern computers have the ability to follow generalized sets of operations, called programs. These programs enable computers to perform an extremely wide range of tasks. A "complete" computer including the hardware, the operating system, and peripheral equipment required and used for "full" operation can be referred to as a computer system. This term may as well be used for a group of computers that are connected and work together, in particular a computer network or computer cluster.

Operating system Software that manages computer hardware resources

An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.

Unix family of computer operating systems that derive from the original AT&T Unix

Unix is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, development starting in the 1970s at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others.


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.

Linux Family of free and open-source software operating systems based on the Linux kernel

Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged in a Linux distribution.

The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines the application programming interface (API), along with command line shells and utility interfaces, for software compatibility with variants of Unix and other operating systems.


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.

International Organization for Standardization An international standard-setting body composed of representatives from national organizations for standards

The International Organization for Standardization is an international standard-setting body composed of representatives from various national standards organizations.

International Electrotechnical Commission organization

The International Electrotechnical Commission is an international standards organization that prepares and publishes international standards for all electrical, electronic and related technologies – collectively known as "electrotechnology". IEC standards cover a vast range of technologies from power generation, transmission and distribution to home appliances and office equipment, semiconductors, fibre optics, batteries, solar energy, nanotechnology and marine energy as well as many others. The IEC also manages four global conformity assessment systems that certify whether equipment, system or components conform to its international standards.

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.

Common Open Software Environment Operating system standards initiative

The Common Open Software Environment (COSE) was an initiative formed in March 1993 by the major Unix vendors of the time to create open, unified operating system (OS) standards.

Unix wars struggles between vendors of the Unix computer operating system in the late 1980s and early 1990s to set the standard for Unix thenceforth

The Unix wars were the struggles between vendors of the Unix computer operating system in the late 1980s and early 1990s to set the standard for Unix thenceforth.

X/Open Company, Ltd., originally the Open Group for Unix Systems, was a consortium founded by several European UNIX systems manufacturers in 1984 to identify and promote open standards in the field of information technology. More specifically, the original aim was to define a single specification for operating systems derived from UNIX, to increase the interoperability of applications and reduce the cost of porting software. Its original members were Bull, ICL, Siemens, Olivetti, and Nixdorf—a group sometimes referred to as BISON. Philips and Ericsson joined soon afterwards, at which point the name X/Open was adopted.

1994: Single UNIX Specification

This was a repackaging of the X/Open Portability Guide (XPG), Issue 4, Version 2.

1995 Edition

In 1995, the Open Group released the Single UNIX Specification Version 1, 1995 Edition. [1]

This specification consisted of:

  • System Interface Definitions, Issue 4, Version 2,
  • System Interfaces and Headers, Issue 4, Version 2,
  • Commands and Utilities, Issue 4, Version 2,
  • X/Open Curses, Issue 4 Version 2,
  • Networking Services, Issue 4,

and was at the core of the UNIX 95 brand. [2]

1997: Single UNIX Specification version 2

In 1997, the Open Group released the Single UNIX Specification Version 2. [3] [4]

This specification consisted of:

and was at the core of the UNIX 98 brand. [5]

2001: Single UNIX Specification version 3, POSIX:2001

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. [6]

This standard consisted of:

and is at the core of the UNIX 03 brand. [7]

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). [8] [9]

2008: Single UNIX Specification version 4, POSIX:2008

In December 2008, the Austin Group published a new major revision, known as POSIX:2008 (formally: IEEE Std 1003.1-2008). [10] [11] [12] This is the core of the Single UNIX Specification, Version 4 (SUSv4). [13]

This standard consists of:

2013 Edition

The Technical Corrigendum 1 is mostly targeting internationalization and it introduces a role-based access model. It was published in 2012 for the Unix Base specification and it is registered as the 2013 Edition of POSIX 2008. A trademark UNIX V7 (not to be confused with V7 UNIX, the version of Research Unix from 1979) has been created to mark compliance with SUS Version 4. [14]

2016 Edition

The Technical Corrigendum 2 has been published in September 2016, leading into IEEE Std 1003.1-2008, 2016 Edition and Single UNIX Specification, Version 4, 2016 Edition.

2018 Edition

In January 2018 an "administrative rollup" edition, susv4-2018, was released. It incorporates Single UNIX Specification version 4 TC1 and TC2, and is technically identical to the 2016 edition. [15]


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.[ citation needed ]

Marks for compliant systems

There are three official marks for conforming systems

Older UNIX standards (superseded)


ProductVendorArchitectureUNIX V7UNIX 03UNIX 98UNIX 95UNIX 93
AIX IBM Corporation PowerPC NoYesYesNoNo
EulerOS Huawei x86-64 NoYesNoNoNo
HP-UX Hewlett Packard Enterprise IA-64, PA-RISC NoYesNoYesNo
IRIX Silicon Graphics, Inc. MIPS NoNoNoYesNo
K-UX Inspur x86-64 NoYesNoNoNo
macOS (formerly OS X) Apple x86-64 NoYesNoNoNo
OpenServer Xinuos IA-32 NoNoNoNoYes
Solaris Oracle Corporation IA-32, x86-64, SPARC YesYesYesNoNo
Tru64 UNIX Digital Equipment Corporation Alpha NoNoYesNoNo
UnixWare Xinuos IA-32 NoNoNoYesNo
z/OS USS IBM Corporation z/Architecture NoNoNoYesNo

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 was certified as UNIX 03 compliant. [17] The UNIX 03 conformance statement shows that the standard C compiler is from the GNU Compiler Collection (gcc), and that the system is a Linux distribution of the Red Hat family. [18]


HP-UX 11i V3 Release B.11.31 is registered as UNIX 03 compliant. Previous releases are registered as UNIX 95. [19]

HP-UX 11i features also provide partial conformance to the UNIX 98 specification. [20]

Inspur K-UX

Inspur K-UX 2.0 and 3.0 for the x86-64 architecture were certified as UNIX 03 compliant. [21] [22] 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 Linux distribution of the Red Hat family. [23]


Apple's macOS (previously known as Mac OS X or OS X) is a UNIX 03 registered product, [24] first becoming registered with Mac OS X 10.5 "Leopard" on October 26, 2007 (when run on Macs with Intel processors). [25] [26] All newer versions of macOS (except Mac OS X 10.7 "Lion") have been registered. [27]


Solaris 11 complies with the Single UNIX Specification. [28] 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. [29]


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. [30]

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 Linux, FreeBSD, and MINIX, typically do not certify their distributions and do not install full POSIX utilities by default. [32] [33] Sometimes, SUS compliance can be improved by installing additional packages, but very few Linux systems can be configured to be completely conformant.

Darwin, the open source subset of macOS, has behavior that can be set to comply with UNIX 03. [34] [35]

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. [36]

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; but there are some conflicts between the LSB and The POSIX standards. [37] However, although these standards are commonly accepted, few Linux distributions actually go through certification as LSB compliant. [38]

See also

Related Research Articles

Darwin is an open-source Unix-like operating system first released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, Mach, and other free software projects.

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uname standard UNIX utility

uname is a computer program in Unix and Unix-like computer operating systems that prints the name, version and other details about the current machine and the operating system running on it.

Linux Standard Base

The Linux Standard Base (LSB) is a joint project by several Linux distributions under the organizational structure of the Linux Foundation to standardize the software system structure, including the Filesystem Hierarchy Standard used in the Linux kernel. The LSB is based on the POSIX specification, the Single UNIX Specification (SUS), and several other open standards, but extends them in certain areas.

The ln command is a standard Unix command utility used to create a hard link or a symbolic link (symlink) to an existing file. The use of a hard link allows multiple filenames to be associated with the same file since a hard link points to the inode of a given file, the data of which is stored on disk. On the other hand, symbolic links are special files that refer to other files by name.

UNIX System V early commercial UNIX operating system

UNIX System V is one of the first commercial versions of the Unix operating system. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, numbered 1, 2, 3, and 4. System V Release 4 (SVR4) was commercially the most successful version, being the result of an effort, marketed as Unix System Unification, which solicited the collaboration of the major Unix vendors. It was the source of several common commercial Unix features. System V is sometimes abbreviated to SysV.

In Unix and related computer operating systems, a file descriptor is an abstract indicator (handle) used to access a file or other input/output resource, such as a pipe or network socket. File descriptors form part of the POSIX application programming interface. A file descriptor is a non-negative integer, generally represented in the C programming language as the type int.

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