writing-robust-bash

name	writing-robust-bash
description	Use when writing or reviewing bash/shell scripts. Covers strict mode, quoting, error handling and cleanup traps, safe command usage, and bash-vs-POSIX portability so scripts fail loudly instead of silently doing the wrong thing.

Writing robust bash

A shell script's default behavior is to plow ahead after an error. Robust scripts opt out of that: fail fast, quote everything, and clean up after themselves.

Strict mode

Start non-trivial bash scripts with:

#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

• -e exit on error, -u error on unset variables, -o pipefail so a failure mid-pipe isn't masked by a later success.
• Narrowing IFS avoids surprise word-splitting on spaces.

Quoting

• Quote every expansion: "$var", "$@", "${arr[@]}". Unquoted expansions split on whitespace and glob.
• Use "$@" (not $*) to forward arguments intact.
• Prefer printf '%s\n' "$x" over echo "$x" for values that may start with - or contain escapes.

Errors & cleanup

• Use a trap for cleanup so temp files go away on any exit: tmp=$(mktemp); trap 'rm -f "$tmp"' EXIT.
• Check that required commands exist: command -v jq >/dev/null || { echo "need jq" >&2; exit 1; }.
• Send diagnostics to stderr (>&2) and return meaningful exit codes.

Safe patterns

• Prefer [[ … ]] over [ … ] in bash; it's safer with empty vars and supports =~.
• Don't parse ls; use globs or find … -print0 with read -d ''.
• Use local for function variables so they don't leak into the global scope.
• Quote command substitutions: count="$(wc -l < "$file")".

Portability

• If the shebang is #!/bin/sh, you're in POSIX territory: no [[ ]], no arrays, no local (in strict POSIX). Either target bash explicitly or stay within POSIX.
• set -euo pipefail is bash; plain POSIX sh supports only set -eu.

American English in comments and messages.