POSIX Shell Scripting

The Portable Operating System Interface (POSIX) family of standards are specified by the IEEE to maintain OS interoperability¹.

The standardized user command line and scripting interface were based on the UNIX System V shell. But not only these are standardized, also are common utility programs.

POSIX.1-2008 combined most parts of previous POSIX standards into one. Latest revision is POSIX.1-2017

Resources

POSIX.1-2017 Specification with search box and frames

POSIX.1-2017 Rationale TOC (e.g. to be strictly POSIX compliant there must be NO #! at the beginning of the file)

The Shell Command Language Documentation

POSIX Utility Conventions: Naming of utilities, specification of options, option-arguments, and operands (The getopt() function assists in this matter).

Shell Portability Guidelines: If scripts need to be portable, some of the BASH-specific syntax elements should be avoided. Others should be avoided for all scripts, e.g. if there is a corresponding POSIX®-compatible syntax.

Pure sh Bible: a collection of pure POSIX alternatives to external processes.

Shells

Evolution of Shells in Linux

Comparison of command shells

Hyperpolyglot: Unix Shells (bash, fish, ksh, tcsh, zsh)

Dash as default /bin/sh shell in Ubuntu: Dash is more efficient than Bash. For system scripts this matters. It was changed in Ubuntu 6.10 (long time ago). Includes a list of "bashisms"². It was Ubuntu who migrated first to Dash, and then Debian decided to adopt it.

ash, the original Kenneth Almquist shell. It's a clone of the Bourne Shell to avoid licensing issues. It's the default shell in BusyBox.

dash: The Debian Almquist Shell is a modern POSIX-compliant implementation of /bin/sh. It has a low memory footprint compared to Bash. It's used in Debian as the default non-interactive shell.

mrsh: a minimal POSIX shell.

Tooling

shellcheck: a static analysis tool for shell scripts. Use shellcheck -s sh (or if script starts with #!/bin/sh) to check for POSIX and warn on portability issues.

ts(1) -- test script: ts provides functions for writing tests in shell. The test scripts can be run individually or in a batch format using ts as a command. ts makes a test directory available on a per-test basis so it's easy to sandbox tests that write or manipulate files. ts tries to use POSIX exclusively and so should (hopefully) work with any POSIX-compliant shell.

checkbashisms

Examples

A shell script to demonstrate various POSIX features related to shell variables

KISS package manager: Tiny and straightforward package manager for KISS (a Linux distribution) written in 600 lines of POSIX sh. Related software. Script related guidestones:

• All shell code must be written in a safe way, pass the shellcheck linter and match the style of any existing code.
• All distribution tooling and shell code must be written in a portable way. Otherwise, the user will be locked into a single coreutils and shell.
• One exception is made for sed -i as it is too useful to let go of. The -i flag has rather good support across implementations regardless.

Static site generator (180LoC), written in shell.

RSS feed generator (148LoC), written in shell.

git-issue: Git-based decentralized issue management. Interesting shellchecks to support, e.g. local variables, which are not standard but supported by most modern shells.

asdf-vm: Extendable version manager. Supports many shells.

bashmarks: Save and jump to commonly used directories. Has tab completion embedded in the same script (zsh and bash).