Shell Scripting

Shell Scripting

Write scripts that fail loudly and safely, not ones that silently do the wrong thing.

Start every script with a strict header

#!/usr/bin/env bash
set -euo pipefail   # exit on error, error on unset var, fail a pipeline if any stage fails
IFS=$'\n\t'

Quoting & safety (the top source of bugs)

• Always quote expansions: "$var", "${arr[@]}", "$(cmd)" — unquoted values word-split and glob.
• Use [[ ... ]] over [ ... ] for tests; (( ... )) for arithmetic.
• Prefer "$(cmd)" over backticks. Check mkdir -p, rm -rf paths twice.
• Handle filenames with spaces/newlines: find ... -print0 | xargs -0, or while IFS= read -r line.

Common patterns

# Args with defaults
name="${1:-world}"

# Loop over files safely
for f in ./*.log; do [[ -e "$f" ]] || continue; echo "$f"; done

# Trap cleanup on exit
tmp="$(mktemp)"; trap 'rm -f "$tmp"' EXIT

# Check a command exists
command -v jq >/dev/null 2>&1 || { echo "jq required" >&2; exit 1; }

Guidance

• Run shellcheck script.sh if available — it catches most real bugs.
• Make scripts idempotent where possible; print what they're doing.
• Don't parse ls output; glob or use find. Don't eval untrusted input.
• For anything beyond ~50 lines of logic, consider Python instead.