Bash Scripting Defaults

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tagsdev, bash, cli, shell

2 min read

A running collection of small Bash idioms worth reaching for by default, not a full guide.

Strict Mode

set -euo pipefail

Put at the top of every non-trivial script. Each flag:

  • -e — exit immediately if a command exits non-zero. Exceptions: commands in an if/while condition, negated with !, or part of &&/||.
  • -u — treat unset variables as an error instead of expanding to an empty string. Catches typos like $fiel instead of $file.
  • -o pipefail — a pipeline’s exit status is the last non-zero command in it, not just the last command. Without it, false | true exits 0.

-o pipefail has no short flag, hence the separate -o form; -eu are combinable short flags.

Trap

trap <action> <signal...> runs <action> when the shell receives a signal or hits a pseudo-signal. Not just for errors — it’s the general hook into a script’s lifecycle.

Common signals/pseudo-signals:

  • EXIT — fires on any script exit (normal, exit N, or error). Use for cleanup that must always run.
  • ERR — fires when a command fails (respects set -e semantics).
  • INT — Ctrl-C (SIGINT).
  • TERM — normal kill signal.
  • DEBUG — fires before every command; useful for tracing.
  • RETURN — fires when a function or sourced script returns.

Error trap, prints where the script died:

trap 'echo "Error on line $LINENO" >&2' ERR

Cleanup regardless of exit status — the most common use, since EXIT fires even after set -e kills the script:

tmpdir=$(mktemp -d)
trap 'rm -rf "$tmpdir"' EXIT

Catch multiple signals with one trap (space-separated):

trap 'echo "interrupted" >&2; exit 1' INT TERM

Chain cleanup with the original exit code preserved:

trap 'ec=$?; rm -rf "$tmpdir"; exit $ec' EXIT

Reset a trap to default behavior:

trap - INT

List currently registered traps:

trap -p

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