Conditionals

Bash · Reference cheat sheet

📋 Overview

Bash branches with if/elif/else, case, and [[ ]] / [ ] / (( )) tests. Prefer [[ ]] for strings and files; (( )) for integers. Exit status 0 is true; non-zero is false.

🔧 Core concepts

[[ ]] operators: -eq -ne -lt -le -gt -ge (numbers as strings carefully), =, ==, !=, =~ regex, -z empty, -n non-empty, -f -d -e -L -r -w -x files, -nt / -ot newer/older.

💡 Examples

File and string checks:

if [[ -f "$config" && -r "$config" ]]; then
  source "$config"
elif [[ -z "${config:-}" ]]; then
  echo "No config" >&2
  exit 1
fi

Numeric:

if (( count > 10 )); then
  echo "too many"
fi

if [[ "$count" -gt 10 ]]; then …; fi   # integer compare in [[ ]]

Regex and case:

if [[ "$email" =~ ^[A-Za-z0-9._%+-]+@ ]]; then
  echo "looks like email"
fi

case "$1" in
  start|on)  start_svc ;;
  stop|off)  stop_svc ;;
  *)         echo "usage: $0 start|stop" >&2; exit 2 ;;
esac

One-liners:

[[ -d "$dir" ]] || mkdir -p "$dir"
command -v jq >/dev/null || { echo "jq required"; exit 1; }

⚠️ Pitfalls

• [ $a = $b ] breaks on empty/spaces; always quote: [ "$a" = "$b" ].
• == inside [ ] is non-POSIX; use = or switch to [[ ]].
• (( )) treats empty/unset as 0 with set -u risks—initialize first.
• =~ right-hand side should be unquoted to be a regex; quoted = literal.
• Prefer [[ ]] over [ ] in bash scripts; keep [ ] for sh portability.
• if cmd tests exit code—not stdout. Capture output separately.

🔗 Related

• exit_codes.md
• loops.md
• quoting.md
• variables.md
• debugging.md