POSIX Shell
Standardized command-language subset defined by {{IEEE 1003.1}} (POSIX.1, formerly POSIX.2) that specifies a portable shell and utilities derived from the {{Bourne shell}}; modern shells like {{dash}}, {{ash}}, and the /bin/sh persona of {{Bash (Shell)}} aim to implement it.
The POSIX Shell is the command-language standard maintained jointly by IEEE and The Open Group as part of IEEE Std 1003.1. It was originally split out as IEEE 1003.2 in 1992 ("Shell and Utilities"), then folded into a single base specification in later revisions; the current edition is IEEE Std 1003.1-2024, Issue 8, published in June 2024. The shell-language chapter defines a portable scripting interface derived from the Bourne shell of UNIX System V, not from Bash (Shell) or the Korn shell, although both contributed extensions that later folded back into the standard. POSIX specifies the lexical structure (quoting, word splitting, tokens), the control-flow constructs (if, for, while, until, case), command substitution in $(...) form, the four basic parameter expansion operators :- :+ := :?, here-documents, pipelines, function definitions, and a set of utilities including awk, sed, grep, find, and test. It does not mandate arrays (those are Bash and ksh extensions), the conditional (Bash/ksh), brace expansion, or process substitution. In practice, "POSIX shell" today usually means a minimal /bin/sh implementation. Debian and Ubuntu ship dash (Debian Almquist Shell) as /bin/sh for fast script startup. Alpine Linux and BusyBox use ash. Bash provides a --posix mode and behaves more conservatively when invoked as sh. Writing portable POSIX shell rather than Bash is the norm for system init scripts, package manager hooks, and software that must run on systems where Bash is absent (notably some embedded Linux distributions and the BSDs).