Associative Arrays

An associative array maps keys to values — like a dictionary. It must be declared with declare -A (Bash 4 and later).

declare -A capitals=(
  [France]="Paris"
  [Japan]="Tokyo"
  [Peru]="Lima"
)

capitals[Kenya]="Nairobi"

echo "${capitals[Japan]}"
echo "Countries: ${#capitals[@]}"

for country in $(printf '%s\n' "${!capitals[@]}" | sort); do
  echo "$country → ${capitals[$country]}"
done

Tokyo
Countries: 4
France → Paris
Japan → Tokyo
Kenya → Nairobi
Peru → Lima

• ${capitals[key]} reads a value; ${!capitals[@]} lists the keys.
• Keys come back in no particular order, so sort them when order matters. (Here the keys have no spaces, so looping over the sorted output is safe.)

Counting things

declare -A count
for word in red blue red green blue red; do
  (( count[$word]++ ))
done

for word in "${!count[@]}"; do
  echo "${count[$word]} $word"
done | sort -rn

3 red
2 blue
1 green

Checking whether a key exists

declare -A config=([port]=8080 [debug]=false)

if [[ -v config[port] ]]; then
  echo "port is set to ${config[port]}"
fi
echo "timeout: ${config[timeout]:-30 (default)}"

port is set to 8080
timeout: 30 (default)

-v tests whether a key exists (Bash 4.3+); ${config[key]:-default} supplies a default.

Common mistakes

• Forgetting declare -A. Without it, Bash creates an ordinary indexed array and treats every key as index 0.
• Using associative arrays in scripts that must run on macOS's built-in Bash 3.2, which doesn't support them.