POSIXstandard

POSIX, or the Portable Operating System Interface, is a family of standards designed to promote compatibility among operating systems. The standards specify a consistent application programming interface (API), shell command language, and utility interfaces, enabling software to be ported more easily across systems that conform to POSIX. The standards originated in the 1980s as part of an effort to unify UNIX interfaces and are now maintained by The Open Group in collaboration with ISO/IEC. The ISO version is published as ISO/IEC 9945.

POSIX is organized into multiple parts. The core is POSIX.1 (System Interfaces), which defines interfaces for process control, file and I/O, signals, threading, and other runtime services. POSIX.2 covers the Shell and Utilities, specifying standard command syntax and behavior for common utilities. Subsequent amendments and editions address real-time extensions, threading models (such as pthreads), timers, and internationalization. Notable historical editions include POSIX.1-2001, POSIX.1-2008, and POSIX.1-2017.

Adoption and conformance: Most major Unix-like systems implement substantial portions of POSIX, including Linux, macOS, BSD variants, and Solaris. Windows environments offer partial POSIX compatibility through projects like Cygwin and Windows Subsystem for Linux. The Open Group administers conformance programs and maintains the POSIX Testsuite, used by vendors to certify compliance. The standard is closely linked to the Single UNIX Specification, and vendor claims of POSIX conformance are commonly used as an indicator of portability across platforms.