Portable Operating System Interface (POSIX)

• is an IEEE standard designed to facilitate application portability
• is an attempt by a consortium of vendors to create a single standard version of UNIX
• If they are successful, it will make it easier to port applications between hardware platforms

POSIX - Standards

C API

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Greatly extends ANSI C with things like:

• more file operations: mkdir, dirname, symlink, readlink, link (hardlinks), poll(), stat, sync, nftw()
• process and threads: fork, execl, wait, pipe, semaphores sem_*, shared memory (shm_*), kill, scheduling parameters (nice, sched_*), sleep, mkfifo, setpgid()
• networking: socket()
• memory management: mmap, mlock, mprotect, madvise, brk()
• utilities: regular expressions (reg*)

Those APIs also determine underlying system concepts on which they depend, e.g. fork requires 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_write, 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.

CLI Utilities

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e.g.: cd, ls, echo, …

Many utilities are direct shell front ends for a corresponding C API function, e.g. mkdir.

Major Linux desktop implementation: GNU Coreutils for the small ones, separate GNU projects for the big ones: sed, grep, awk, … Some CLI utilities are implemented by Bash as built-ins.

Shell Language

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e.g. a=b; echo “$a”

Major Linux desktop implementation: GNU Bash.

Environment Variables

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e.g. HOME, PATH.

PATHsearch semantics are specified, including how slashes prevent PATH search.

Program Exit Status

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ANSI C says 0 or EXIT_SUCCESS for success, EXIT_FAILURE for failure, and leaves the rest implementation defined.

POSIX adds:

• 126: command found but not executable
• 127: command not found
• >128: terminated by a signal

But POSIX does not seem to specify the 128 + SIGNAL_ID rule used by Bash: https://unix.stackexchange.com/questions/99112/default-exit-code-when-process-is-terminated

Regular Expression

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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. grep accepts BREs by default, and EREs with -E.

e.g. echo ‘a.1’ | grep -E ‘a.[[:digit:]]’

Major Linux implementation: glibc implements the functions under regex.h which programs like grep can use as backend.

Directory Structure

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e.g. /dev/null, /tmp

The Linux FHS greatly extends POSIX

Filenames

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• / is the path separator
• NUL cannot be used
• . is cwd, .. parent
• portable filenames
  • use at most max 14 chars and 256 for the full path
  • can only contain: a-zA-Z0-9._-

See also: what is posix compliance for filesystem?

Command Line Utility API Conventions

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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. —all).

A few widely used conventions:

• - means stdin where a file is expected
• — terminates flags, e.g. ls — -l to list a directory named -l

See also: Are there standards for Linux command line switches and arguments?

”POSIX ACLs” (Access Control Lists)

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e.g. as used as backend for setfacl

This was withdrawn but it was implemented in several OSes, including in Linux with setxattr

Subpages

• UNIX - POSIX Signals