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Arrays in Shell Scripting

An array is a variable that can hold multiple values under a single name. Bash supports both one-dimensional indexed arrays (using numbers as keys) and associative arrays (using strings as keys).

Indexed Arrays​

Indexed arrays use integers as their indices, starting at 0.

Declaring an Array​

# Declare and assign
FRUITS=("Apple" "Banana" "Cherry")

# Assign individually
FRUITS[3]="Date"

Accessing Elements​

To access an array element, you must use curly braces around the variable name and index inside your code.

echo "First fruit: ${FRUITS[0]}"

Accessing All Elements​

To print all elements, use the @ or * symbol as the index.

echo "All fruits: ${FRUITS[@]}"

Array Length​

Prefix the array variable with a # to get the number of elements.

echo "Number of fruits: ${#FRUITS[@]}"

Iterating Over an Array​

for FRUIT in "${FRUITS[@]}"; do
    echo "I like $FRUIT"
done

Associative Arrays​

Introduced in Bash 4.0, associative arrays use strings (keys) instead of numbers as indices. You must declare them explicitly using the declare -A command.

Declaring and Using Associative Arrays​

# Declare an associative array
declare -A USER_INFO

# Assign values
USER_INFO["name"]="John Doe"
USER_INFO["age"]="30"
USER_INFO["city"]="New York"

# Access values
echo "User Name: ${USER_INFO["name"]}"

Iterating Over Keys and Values​

# Iterate over keys (use !)
for KEY in "${!USER_INFO[@]}"; do
    echo "Key: $KEY, Value: ${USER_INFO[$KEY]}"
done

Best Practices​

  • Remember that Bash indexing starts at 0.
  • Always quote your array expansions "${ARRAY[@]}" to prevent word splitting if elements contain spaces.
  • Note that macOS ships with an older version of Bash by default (v3.2), which does not support associative arrays.