The most important things POSIX 7 defines
Greatly extends ANSI C with things like:
- more file operations:
- process and threads:
sem_*, shared memory (
kill, scheduling parameters (
- memory management:
- utilities: regular expressions (
Those APIs also determine underlying system concepts on which they depend, e.g.
forkrequires a concept of a process.
Many Linux system calls exist to implement a specific POSIX C API function and make Linux compliant, e.g.
sys_read, … Many of those syscalls also have Linux-specific extensions however.
Major Linux desktop implementation: glibc, which in many cases just provides a shallow wrapper to system calls.
- more file operations:
Many utilities are direct shell front ends for a corresponding C API function, e.g.
Major Linux desktop implementation: GNU Coreutils for the small ones, separate GNU projects for the big ones:
awk, … Some CLI utilities are implemented by Bash as built-ins.
a=b; echo "$a"
Major Linux desktop implementation: GNU Bash.
PATHsearch semantics are specified, including how slashes prevent
Program exit status
ANSI C says
EXIT_FAILUREfor failure, and leaves the rest implementation defined.
See also: What is the meaning of
$?(dollar question mark) in shell scripts?
There are two types: BRE (Basic) and ERE (Extended). Basic is deprecated and only kept to not break APIs.
Those are implemented by C API functions, and used throughout CLI utilities, e.g.
grepaccepts BREs by default, and EREs with
echo 'a.1' | grep -E 'a.[[:digit:]]'
Major Linux implementation: glibc implements the functions under regex.h which programs like
grepcan use as backend.
The Linux FHS greatly extends POSIX.
/is the path separator
NULcannot be used
- portable filenames
- use at most max 14 chars and 256 for the full path
- can only contain:
See also: what is posix compliance for filesystem?
Command line utility API conventions
Not mandatory, used by POSIX, but almost nowhere else, notably not in GNU. But true, it is too restrictive, e.g. single letter flags only (e.g.
-a), no double hyphen long versions (e.g.
A few widely used conventions:
-means stdin where a file is expected
--terminates flags, e.g.
ls -- -lto list a directory named
See also: Are there standards for Linux command line switches and arguments?
“POSIX ACLs” (Access Control Lists), e.g. as used as backend for
This was withdrawn but it was implemented in several OSes, including in Linux with
Who conforms to POSIX?
Many systems follow POSIX closely, but few are actually certified by the Open Group which maintains the standard. Notable certified ones include:
- OS X (Apple) X stands for both 10 and UNIX. Was the first Apple POSIX system, released circa 2001. See also: Is OSX a POSIX OS?
- AIX (IBM)
- HP-UX (HP)
- Solaris (Oracle)
Most Linux distros are very compliant, but not certified because they don’t want to pay the compliance check. Inspur’s K-UX and Huawei’s EulerOS are two certified examples.
The official list of certified systems be found at: https://www.opengroup.org/openbrand/register/ and also at the wiki page.
Windows implemented POSIX on some of its professional distributions.
Since it was an optional feature, programmers could not rely on it for most end user applications.
Support was deprecated in Windows 8:
In 2016 a new official Linux-like API called “Windows Subsystem for Linux” was announced. It includes Linux system calls, ELF running, parts of the
/proc filesystem, Bash, GCC, (TODO likely glibc?),
apt-get and more: https://channel9.msdn.com/Events/Build/2016/P488 so I believe that it will allow Windows to run much, if not all, of POSIX. However, it is focused on developers / deployment instead of end users. In particular, there were no plans to allow access to the Windows GUI.
Historical overview of the official Microsoft POSIX compatibility: http://brianreiter.org/2010/08/24/the-sad-history-of-the-microsoft-posix-subsystem/
Cygwin is a well known GPL third-party project for that “provides substantial POSIX API functionality” for Windows, but requires that you “rebuild your application from source if you want it to run on Windows”. MSYS2 is a related project that seems to add more functionality on top of Cygwin.
If the only thing from POSIX that you need are the command line utilities, also consider: https://github.com/shelljs/shelljs which re-implements a bunch of CLI utilities in Node.js, which already essentially implements a portability layer for the simpler system calls like
mkdir etc. Many people use that project in the
package.json of their project to allow running the project on Windows as well. Of course, it requires users to install the Node.js runtime, but given the popularity of Node.js, I don’t expect that to break/be hard to satisfy anytime soon.
Android has its own C library (Bionic) which does not fully support POSIX as of Android O: Is Android POSIX-compatible?
The Linux Standard Base further extends POSIX.
Use the non-frames indexes, they are much more readable and searchable: http://pubs.opengroup.org/onlinepubs/9699919799/nfindex.html
Get a full zipped version of the HTML pages for grepping: Where is the list of the POSIX C API functions?