Bash Scripting

Bash scripting — scripts that survive a stranger's machine

You are the shell author who has been burned: by an unquoted "$@" that exploded a path with spaces, by rm -rf "$DIR/" where $DIR was empty, by a trap that fired twice, by a set -e that swore it caught errors and didn't. Write every script as if it will run in CI, in a container, and on a 2014 Mac with bash 3.2 — because eventually it will.

The first decision is not a line of code. It is: which shell am I targeting? That choice decides what you are allowed to write. Make it before the shebang.

The header you start every bash script with

#!/usr/bin/env bash
set -euo pipefail        # see "Strict mode, honestly" — this is a baseline, not a force field
IFS=$'\n\t'              # split words on newline/tab only, never on spaces

• #!/usr/bin/env bash — find bash on PATH; do not hardcode /bin/bash, which is 3.2 on macOS and may not exist on some images.
• set -e — exit on an uncaught non-zero command. Leaky (below), still worth having.
• set -u — error on an unset variable, so a typo'd $OUPUT fails loud instead of expanding to empty.
• set -o pipefail — a pipeline fails if any stage fails, not just the last. Bashism — not in POSIX sh.
• IFS=$'\n\t' — stops the classic "unquoted expansion splits on every space" bug at the source.

Pick your shell first

set -o pipefail, arrays, [[ ]], and local are bashisms. If your shebang is #!/bin/sh you may not get bash — on Debian/Alpine sh is dash/busybox.

Two traps to internalize: macOS /bin/bash is 3.2 (no declare -A, no ${var,,}, no mapfile); and bash 5.3 added ${ cmd; } / GLOBSORT / read -E that will not run on either of the above. Pick a floor and stay above it — the full bash-vs-POSIX feature matrix, bash 3.2 workarounds, dash/busybox gotchas, the 5.3 features to guard, and how to test one script under several shells are in references/portability.md.

Strict mode, honestly

set -e is not a force field. It is silently suppressed in three places, and people ship broken scripts because they trusted it:

1. In an assignment with command substitution. local x=$(failing) — the assignment succeeds (exit status is the local/assignment, not the substitution).
2. In a condition. Anything in if, while, &&, ||, or after ! is exempt by design — set -e would make if grep … unusable otherwise.
3. Inside functions called in a condition: a failing line won't abort.

So: do not lean on set -e for control flow. Check what matters explicitly.

# Bad — set -e will NOT catch this; x is empty, script sails on
local x=$(curl -fsS "$url")

# Good — split declaration from assignment so the substitution's status is seen
local x
x=$(curl -fsS "$url") || { echo "fetch failed" >&2; return 1; }

trap 'echo "failed at line $LINENO" >&2' ERR gives you a breadcrumb on the uncaught failures set -e does catch. To deliberately ignore a non-zero exit, say so: cmd || true (and a comment why), never a bare unchecked cmd.

Quoting — the highest-value section

Unquoted expansions are the №1 cause of shell bugs and of the data-loss story everyone has heard. Quote every expansion unless you have a specific reason not to.

The rm -rf disaster: if $DIR is unset/empty, rm -rf $DIR/ becomes rm -rf /. Guard the variable and quote it:

: "${DIR:?DIR must be set}"   # abort with a message if unset or empty
rm -rf "${DIR:?}"/           # belt and braces: fail rather than expand to /

Use [[ … ]] in bash (no word-splitting inside, supports =~, &&); use [ … ] in POSIX sh. [ a == b ] is a bashism — POSIX [ uses =.

Arrays, not space-split strings

The instant an argument list is built from a string, spaces betray you. Build it as an array and expand "${arr[@]}" (each element stays one argument).

# Bad — flags string splits wrong if any value contains a space
flags="--name my project --force"
docker run $flags image          # 5 args, "my" and "project" split apart

# Good — array; "${flags[@]}" expands to exactly 4 arguments
flags=(--name "my project" --force)
docker run "${flags[@]}" image

Iterating files: never parse ls. Use a glob, or find -print0 with a NUL-delimited read so even newlines in names are safe.

# Good — glob; the ./ prefix protects files named like "-rf"
for f in ./*.txt; do
  [ -e "$f" ] || continue       # guard the no-match case (glob stays literal)
  process "$f"
done

# Good — robust against spaces AND newlines in filenames
find . -name '*.log' -print0 | while IFS= read -r -d '' f; do
  process "$f"
done

macOS bash 3.2 has no mapfile/readarray; the find -print0 loop above is the portable way to collect names.

Cleanup with traps

A script that creates temp state must remove it even when interrupted. Register one EXIT trap right after you create the resource — EXIT fires on normal exit, on set -e abort, and after INT/TERM, so you don't need per-signal traps.

tmp=$(mktemp -d)                          # never a fixed /tmp/foo path (race + collision)
trap 'rm -rf "$tmp"' EXIT                 # one trap, covers every exit path

work_in "$tmp"

Make cleanup idempotent (rm -rf tolerates a missing dir) so a double-fire or a re-entry is harmless. Track background PIDs and reap them in the same trap:

server & srv_pid=$!
trap 'kill "$srv_pid" 2>/dev/null; rm -rf "$tmp"' EXIT

Input & variables

Option parsing uses getopts (POSIX, single-dash flags):

verbose=0; out=""
while getopts ":vo:" opt; do
  case "$opt" in
    v) verbose=1 ;;
    o) out="$OPTARG" ;;
    *) echo "usage: $0 [-v] [-o FILE]" >&2; exit 2 ;;
  esac
done
shift $((OPTIND - 1))

Use printf '%s\n' "$x" instead of echo "$x" for arbitrary data — echo's handling of -n/-e and backslashes is not portable.

Run ShellCheck — and fix, don't silence

ShellCheck v0.11.0 (2025-08) is the canonical static analyzer. Run it on every script; it catches most of the above before runtime.

shellcheck script.sh             # bash target
shellcheck -s sh script.sh       # verify POSIX-sh compliance

Codes worth memorizing: SC2086 (unquoted expansion → quote it), SC2046 (unquoted $(…) word-splits), SC2164 (cd without || exit), SC2155 (local x=$(cmd) masks the command's exit status — declare then assign).

Silence a finding only with a justified directive on the line directly above it, never project-wide:

# shellcheck disable=SC2086  # word-splitting is intentional: $flags is a flag list we control
some_cmd $flags
# shellcheck source=lib/common.sh   # resolve a dynamic `source` for cross-file analysis
. "$dir/common.sh"

Wire shellcheck into CI as a run: step — the workflow scaffolding around it belongs to ../github-actions/SKILL.md, the shell inside the step belongs here.

Anti-patterns