Arrays

Bash · Reference cheat sheet

📋 Overview

Bash has indexed arrays (declare -a) and associative arrays (declare -A, bash 4+). Always expand with "$\{arr[@]\}" to preserve elements. Arrays are the safe alternative to storing lists in a single string.

🔧 Core concepts

Sparse indexed arrays keep original indices after unset. Iterate keys with "$\{!arr[@]\}".

💡 Examples

Indexed:

files=("a b.txt" "c.txt")
files+=("d.txt")
echo "${#files[@]}"
for f in "${files[@]}"; do
  echo "$f"
done
echo "${files[0]}"

Slice and copy:

slice=("${files[@]:1:2}")     # 2 elems from index 1
copy=("${files[@]}")

Associative:

declare -A ports=([web]=80 [db]=5432)
ports[cache]=6379
for k in "${!ports[@]}"; do
  echo "$k -> ${ports[$k]}"
done

From command (mapfile):

mapfile -t lines < file.txt
mapfile -d '' -t paths < <(find . -print0)

⚠️ Pitfalls

• "$\{arr[@]\}" vs $\{arr[@]\}: always quote.
• "$\{arr[*]\}" is one string when quoted—wrong for iterating items with spaces.
• Associative arrays need bash 4+; macOS system bash may be 3.2—use Homebrew bash or avoid.
• arr=($string) splits on IFS; prefer read -a or mapfile.
• unset arr removes whole array; unset 'arr[0]' one element.
• Indices are not re-packed after delete; don’t assume contiguous 0..n-1.

🔗 Related

• variables.md
• quoting.md
• loops.md
• string_ops.md
• globbing.md