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The Single UNIX Specification (SUS) is The Open Group’s standard for the core interfaces of a UNIX operating-system environment. It defines a shared set of interfaces and services intended to make software behavior more predictable across conforming systems. SUS is closely related to POSIX, but POSIX compatibility alone does not grant permission to use the UNIX trademark.
What the Single UNIX Specification defines
The Open Group describes SUS as the standard against which the core interfaces of a UNIX operating system are measured. Its scope includes the base operating-system environment, networking services, windowing-system services, internationalization, and programming-language aspects. The core volumes are also part of POSIX and ISO/IEC 9945. The Open Group’s UNIX Standard page provides its overview.
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In practical terms, SUS specifies interfaces that applications and developers can rely on in conforming environments. It does not prescribe one vendor’s complete operating-system implementation, nor does UNIX-like behavior by itself establish that a product has been certified.
How SUS relates to POSIX
POSIX.1 and SUS share a substantial base. The Austin Group develops and maintains the core open-systems interfaces for POSIX.1, ISO/IEC 9945, and the core of SUS. The standardized service interface uses the C programming language, a command interpreter, and common utility programs. The Open Group Base Specifications Issue 7 preface describes this relationship.
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The useful distinction is the XSI option: it identifies POSIX.1 portions addressing the extension added to support SUS. POSIX is therefore not simply another name for every SUS requirement. The POSIX.1-2017 preface also says it codifies the common existing definition of the UNIX system. The Issue 7 preface explains the XSI terminology.
Current SUS edition and UNIX certification standard
The Open Group identifies UNIX V7 as its latest certification standard and says it is aligned with SUS Version 4, 2018 Edition. Its UNIX standard page uses this certification wording.
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A newer standards publication should not be confused with that certification label. POSIX.1-2024, also called The Open Group Base Specifications Issue 8, is technically identical to Issue 8 and incorporates appropriate parts of SUS Version 5. That publication does not, by itself, establish a Version 5 UNIX certification release. The POSIX.1-2024 rationale describes the Issue 8 relationship.
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Technical conformance and trademark authorization are separate questions. The Open Group says a supplier is licensed to use the UNIX trademark on a product after the product demonstrates that it passes the applicable certification test suites and the supplier agrees to the Certification Program terms. Each registered certified product receives an Open Brand Certificate and an entry in the Directory of Registered Products. The Open Group’s UNIX Standard page describes the program.
To check whether a particular product is certified, consult the official Directory of Registered Products. Check the exact registered product and configuration rather than inferring certification from a broad operating-system family name, source compatibility, or a vendor’s informal description.
Does Linux comply with SUS?
“Linux” names a broad ecosystem, not one specific registered product and configuration. The sources cited here do not establish product-specific SUS conformance or UNIX certification for Linux distributions. Those claims require current evidence for the specific product and applicable tests; UNIX-like behavior or POSIX compatibility is not proof of trademark certification.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Further reading
The Open Group published The Single UNIX Specification, Authorized Guide to Version 4 in March 2010 (ISBN 1931624844). It is a historical companion guide, not a substitute for checking the current normative standard and certification information.
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