Arrays

Regular arrays can be declared all at once using (), or elements can be assigned individually by specifying an index. Indices start at 0.

To access an element, write ${arr[n]}. To expand all elements, use ${arr[@]}.

To use an associative array, it must first be declared with declare -A. Keys are strings, allowing meaningful names instead of numeric indices. Requires bash 4.0 or later.

To process all elements of an array, use for ... in "${arr[@]}". To iterate over an associative array as key-value pairs, extract keys with ${!map[@]} and access values with ${map[$key]}.

dragonball_array.sh

#!/bin/bash
# -----------------------------------------------
#  Demonstrates indexed array declaration,
#  manipulation, and loops using Dragon Ball characters
# -----------------------------------------------

# -----------------------------------------------
#  1. Array declaration (all at once)
# -----------------------------------------------

# Elements listed with spaces inside () are assigned indices starting from 0
fighters=("Goku" "Vegeta" "Piccolo" "Krillin" "Trunks")

echo "=== Indexed array: fighters ==="

# Access a specific index
echo "fighters[0]: ${fighters[0]}"   # Goku
echo "fighters[2]: ${fighters[2]}"   # Piccolo

echo ""

# -----------------------------------------------
#  2. Expanding all elements and getting the count
# -----------------------------------------------

# Expand all elements with ${fighters[@]}
echo "All fighters: ${fighters[@]}"

# Get the number of elements with ${#fighters[@]}
echo "Number of fighters: ${#fighters[@]}"

echo ""

# -----------------------------------------------
#  3. Process all elements with a for loop
# -----------------------------------------------

echo "=== Participating fighters ==="
for name in "${fighters[@]}"; do
    echo "  - ${name}"
done

echo ""

# -----------------------------------------------
#  4. Loop with index
# -----------------------------------------------

echo "=== Indexed list ==="
for i in "${!fighters[@]}"; do
    echo "  [${i}] ${fighters[$i]}"
done

echo ""

# -----------------------------------------------
#  5. Adding and removing elements
# -----------------------------------------------

# Append an element to the end with +=
fighters+=("Gohan")
echo "Element count after adding: ${#fighters[@]}"

# Delete a specific index with unset (indices are not compacted)
unset fighters[1]   # Remove Vegeta
echo "All elements after deletion: ${fighters[@]}"

# Check remaining indices with ${!fighters[@]}
echo "Remaining indices: ${!fighters[@]}"

echo ""

# -----------------------------------------------
#  6. Slicing
# -----------------------------------------------

# Extract 2 elements starting from index 2
echo "Slice [2:2]: ${fighters[@]:2:2}"

chmod +x dragonball_array.sh
./dragonball_array.sh
=== Indexed array: fighters ===
fighters[0]: Goku
fighters[2]: Piccolo

All fighters: Goku Vegeta Piccolo Krillin Trunks
Number of fighters: 5

=== Participating fighters ===
  - Goku
  - Vegeta
  - Piccolo
  - Krillin
  - Trunks

=== Indexed list ===
  [0] Goku
  [1] Vegeta
  [2] Piccolo
  [3] Krillin
  [4] Trunks

Element count after adding: 6
All elements after deletion: Goku Piccolo Krillin Trunks Gohan
Remaining indices: 0 2 3 4 5

Slice [2:2]: Piccolo Krillin

dragonball_assoc.sh

#!/bin/bash
# -----------------------------------------------
#  Demonstrates associative array declaration,
#  access, and loops using Dragon Ball characters
#  Requires bash 4.0 or later
# -----------------------------------------------

# -----------------------------------------------
#  1. Associative array declaration (declare -A is required)
# -----------------------------------------------

declare -A power_level

# Assign keys and values individually
power_level["Goku"]=9000
power_level["Vegeta"]=8000
power_level["Piccolo"]=3500
power_level["Krillin"]=1500
power_level["Trunks"]=5000

echo "=== Associative array: power_level ==="

# Access a value by specifying its key
echo "Goku's power level: ${power_level["Goku"]}"
echo "Piccolo's power level: ${power_level["Piccolo"]}"

echo ""

# -----------------------------------------------
#  2. Element count and expanding all values and keys
# -----------------------------------------------

echo "Number of registered fighters: ${#power_level[@]}"

# Expand all keys with ${!map[@]}
echo "Keys: ${!power_level[@]}"

# Expand all values with ${map[@]}
echo "Power levels: ${power_level[@]}"

echo ""

# -----------------------------------------------
#  3. Loop over key-value pairs
# -----------------------------------------------

echo "=== Power Level Rankings ==="
for chara in "${!power_level[@]}"; do
    echo "  ${chara}: ${power_level[$chara]}"
done

echo ""

# -----------------------------------------------
#  4. Updating and deleting elements
# -----------------------------------------------

# Reassigning to an existing key overwrites the value
power_level["Goku"]=150000000
echo "Super Saiyan awakened: Goku's power -> ${power_level["Goku"]}"

# Delete a key with unset
unset power_level["Krillin"]
echo "Element count after removing Krillin: ${#power_level[@]}"

echo ""

# -----------------------------------------------
#  5. Checking if a key exists
# -----------------------------------------------

# Use ${map[$key]+x} to check if a key exists (expands to "x" if it exists)
check_key="Vegeta"
if [[ -n "${power_level[$check_key]+x}" ]]; then
    echo "${check_key} is in the list (power level: ${power_level[$check_key]})"
else
    echo "${check_key} is not in the list"
fi

chmod +x dragonball_assoc.sh
./dragonball_assoc.sh
=== Associative array: power_level ===
Goku's power level: 9000
Piccolo's power level: 3500

Number of registered fighters: 5
Keys: Goku Vegeta Piccolo Krillin Trunks
Power levels: 9000 8000 3500 1500 5000

=== Power Level Rankings ===
  Goku: 9000
  Vegeta: 8000
  Piccolo: 3500
  Krillin: 1500
  Trunks: 5000

Super Saiyan awakened: Goku's power -> 150000000
Element count after removing Krillin: 4

Vegeta is in the list (power level: 8000)

Indexed arrays in shell scripts are declared with arr=(a b c) and elements are accessed with ${arr[n]}. Use "${arr[@]}" to expand all elements, ${#arr[@]} to get the element count, and ${!arr[@]} to get all indices. In a for name in "${arr[@]}" loop, always include the double quotes to prevent elements containing spaces from being incorrectly split. Associative arrays are declared with declare -A map and elements are assigned with map[key]=value. Use ${!map[@]} to extract all keys and ${map[@]} to extract all values. The ${map[$key]+x} pattern is convenient for checking if a key exists. See also Variable Basics.