Updated: 2021-11-25


SUS extends POSIX; LSB extends POSIX and SUS, with certain conflicts.

Only SUS certified OS can be called "Unix" (because the owner of SUS, the Open Group, owns the Unix trademark); NOT all Linux distros are LSB compliant, e.g. Debian and Ubuntu are not.


Portable Operating System Interface (POSIX): a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems.

POSIX is not UNIX officially simply because IEEE does not own the trademark

POSIX standardizes:

  • C headers
  • shell and unitilities: ls, awk, echo, vi, etc.
  • System Interfaces: system calls and library functions


  • POSIX-certified (passed the automated conformance tests.): macOS, Oracle Solaris, Huawei EulerOS
  • Mostly POSIX-compliant: Linux, FreeBSD, Android, Darwin

Note that Darwin forms the Unix-based core set of components for macOS. Darwin is mostly POSIX-compatible but has never been certified, macOS is certified.

Zsh is able to emulate POSIX shells, but its default mode is not POSIX compatible

Invoking Bash with the --posix option or stating set -o posix in a script causes Bash to conform very closely to the POSIX


Single UNIX Specification

POSIX standards form the core of the Single UNIX Specification. Sometimes we use "SUS" to refer to the full standard, and "POSIX" to refer to the base standard and possibly the non-Unix-centric option groups. (which means POSIX is a subset of SUS)

Only the compliant operating systems can use the "UNIX" trademark. Very few BSD and Linux-based operating systems are submitted for compliance with the Single UNIX Specification

macOS is compliant.

Now the Open Group certifies both POSIX and SUS. POSIX and SUS started to converge after 2001.

Linux Standard Base (LSB)


Linux is NOT POSIX-certified due to high costs.

LSB is maintained by a working group at the Linux Foundation.

The LSB is based on the POSIX, SUS, and several other open standards, but extends them in certain areas.

The LSB specifies for example: standard libraries, a number of commands and utilities that extend the POSIX standard, the layout of the file system hierarchy, run levels, the printing system, including spoolers such as CUPS and tools like Foomatic and several extensions to the X Window System.

  • ABI: application binary interface (The LSB is designed to be binary-compatible)

In September 2015, the Debian project confirmed that while support for Filesystem Hierarchy Standard (FHS) would continue, support for LSB had been dropped. Ubuntu followed Debian in November 2015.

To get LSB version:

$ lsb_release -a

There are conflicts between the LSB and The POSIX standards