Single Unix Specification

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What Does Single Unix Specification Mean?

Single Unix Specification is a family of standards required to be followed by the OS to qualify for the name UNIX. It is developed and maintained by the Austin group, based on earlier work by Open group and IEEE. It was created in the mid 1980s to standardize OS interfaces for variants of the UNIX OS.


The single Unix specification ensures that program developed in one unix system would run in somewhat different (different flavor) Unix OS.

Techopedia Explains Single Unix Specification

Initially, the UNIX OS had four elements; the specification, the technology, the registered trade mark (e.g. UNIX) and the product (e.g. UNIXWare). With the advent of the Single UNIX specification, there arose a single open consensus specification to define the product. The specification and the trade mark are managed and held in trust for the industry by the X/Open Company.

The official marks for conforming systems are UNIX 98, UNIX 05, UNIX 93 and UNIX 95. The user and software interfaces to the OS are specified in four sections:

  • Base Definitions: Definitions and conventions used in the specifications together with a list of C header files provided by compliant systems
  • Shell and Utilities: Utilities and a description of the shell
  • System Interfaces: Including the C system calls, which must be provided
  • Rationale: Including the explanation behind the standard

The registered UNIX like systems include such systems as:

  • AIX: UNIX 03 compliant
  • HP/UX: UNIX 03 compliant
  • Mac OS X and Mac OS X Server: UNIX 03 compliant
  • Z/OS: UNIX 95 compliant
  • SCO: UNIX 95 compliant
  • Solaris 8 & 9: UNIX 98 compliant
  • Solaris 10: UNIX 03 compliant
  • Tru64 UNIX: UNIX 98 compliant

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Margaret Rouse
Technology Expert
Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.