Variables

Bash · Reference cheat sheet

📋 Overview

Bash variables hold strings (and, with care, integers/arrays). Assignment has no spaces: name=value. Expand with $name or $\{name\}. Prefer quoting expansions to avoid word-splitting and globbing.

🔧 Core concepts

Special: $0 script name, $1…$n args, $# count, $@ / $* all args, $? last exit, $$ PID, $! last background PID, $_ last arg of previous command.

💡 Examples

Basics and defaults:

name="Ada"
echo "Hello, ${name}"
port="${PORT:-8080}"
: "${API_KEY:?API_KEY is required}"

Export and env:

export DATABASE_URL="postgres://localhost/app"
env | grep DATABASE
printenv DATABASE_URL

Arithmetic:

declare -i count=0
count+=1
((count++))
echo $((count * 2))

Safe empty check:

if [[ -z "${var:-}" ]]; then
  echo "unset or empty"
fi

⚠️ Pitfalls

• var = value runs command var (spaces break assignment).
• Unquoted $var splits on IFS and expands globs.
• $1 vs "$1": always quote unless you intentionally want splitting.
• $* vs $@: use "$@" to preserve argument boundaries.
• Environment variables are always strings; (( )) / declare -i for math.
• local only works inside functions; using it at top level errors.

🔗 Related

• quoting.md
• arrays.md
• string_ops.md
• functions.md
• conditionals.md