Editing Single UNIX Specification (section)

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

* '''Base Definitions (XBD)''' - a list of definitions and conventions used in the specifications and a list of [[C header file]]s 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, [[Bourne shell|sh]]. 160 utilities in total are specified.
* '''System Interfaces (XSH)''' - contains the specification of various functions which are implemented as [[system call]]s or [[Library (computing)|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|awk]], [[Echo (command)|echo]], [[ed (text editor)|ed]], [[Vi (text editor)|vi]], and hundreds of others. Required program-level services include basic [[input/output|I/O]] ([[Computer file|file]], [[Computer terminal|terminal]], and [[computer network|network]]) services. A test suite accompanies the standard. It is called '''PCTS''' or the '''POSIX Certification Test Suite'''.

Additionally, SUS includes [[curses (programming library)|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 UNIX despite having no code in common.<ref>{{cite web|url=https://www.linux.com/news/can-gnu-ever-be-unix/|title=Can GNU ever be Unix?|first=Jem|last=Matzan|date=July 30, 2004|website=linux.com}}</ref>