POSIXsh

POSIXsh refers to one or more shell implementations that aim to be strictly compliant with the POSIX shell standard (POSIX.1). The primary goal of these projects is to provide a portable, predictable execution environment for shell scripts across Unix-like systems, serving as a reference point for POSIX sh semantics and behavior.

A POSIXsh implementation focuses on the features mandated by POSIX sh, including control structures such as if, case, for, while and until; command substitution with $(...) and backquotes; parameter expansion and quoting rules; redirections; and here documents. It typically supports arithmetic expansion using $((...)) as defined by POSIX. In keeping with the POSIX mandate, these shells generally do not expose non-POSIX extensions found in other shells, such as advanced arrays or interactive features.

Design and usage considerations emphasize portability and simplicity. POSIXsh projects are usually written in portable languages like C and are designed to be small, embeddable, and easy to verify against the standard. They are commonly used as /bin/sh or as a strictly POSIX-compliant interpreter for scripts that must run reliably on diverse systems. Because they adhere to a formal standard, POSIXsh implementations are often used to test scripts for portability and to identify script constructs that rely on non-POSIX features.

Relation to other shells: POSIXsh differs from shells such as Bash or Dash in its emphasis on strict POSIX compliance and avoidance of extensions. Script authors targeting portability may prefer POSIXsh to ensure predictable behavior across environments.

See also: POSIX, Unix shell, dash, BusyBox ash, Almquist shell.