Array

Define an indexed array by listing elements inside parentheses, separated by spaces.

arr=(element1 element2 element3)

You can also assign elements individually by specifying an index.

arr[0]=element1
arr[1]=element2

To define an associative array, first declare it with declare -A, then assign key-value pairs.

declare -A assoc
assoc[key]=value

Reference a specific element with ${arr[index]}. Expand all elements with ${arr[@]} or ${arr[*]}.

${arr[0]}               # Element at index 0
${arr[@]}               # All elements (each as a separate argument)
${arr[*]}               # All elements (as one IFS-joined string)
${#arr[@]}              # Number of elements
${!arr[@]}              # All indices (keys)
${arr[@]:start:length}  # Slice: length elements starting at start

Append elements with +=. Remove an element with unset.

arr+=(new_element)        # Append to the end
unset arr[index]          # Delete the element at the given index

A basic example of defining an indexed array and referencing its elements. A list of members is stored in an array.

members=("user1" "user2" "user3" "user4")
echo ${members[0]}
echo ${members[2]}
echo ${#members[@]}
user1
user3
4

To iterate over all elements, use ${arr[@]} inside double quotes.

list_members.sh

#!/bin/bash

members=("user1" "user2" "user3" "user4" "user5")

for member in "${members[@]}"; do
    echo "Member: $member"
done

Run the following command:

bash list_members.sh
Member: user1
Member: user2
Member: user3
Member: user4
Member: user5

An example of declaring an associative array with declare -A and mapping user names to their team affiliations.

user_roles.sh

#!/bin/bash

declare -A affiliation

affiliation["user1"]="Team A"
affiliation["user2"]="Team B"
affiliation["user3"]="Team A"

for key in "${!affiliation[@]}"; do
    echo "$key => ${affiliation[$key]}"
done

Run the following command:

bash user_roles.sh
user3 => Team A
user2 => Team B
user1 => Team A

Appending and deleting elements. Use += to append and unset to delete.

methods=("method_a" "method_b" "method_c")
methods+=("method_d")
echo ${methods[@]}
unset methods[1]
echo ${methods[@]}
echo ${#methods[@]}
method_a method_b method_c method_d
method_a method_c method_d
3

Use a slice to extract a portion of an array. ${arr[@]:start:length} returns length elements starting from index start.

items=("item_a" "item_b" "item_c" "item_d" "item_e")
echo ${items[@]:1:3}
item_b item_c item_d

task_log.sh

#!/bin/bash
# Task log processing script

declare -A email
email["user1"]="user1@example.com"
email["user2"]="user2@example.com"
email["user3"]="user3@example.com"
email["user4"]="user4@example.com"
email["user5"]="user5@example.com"

tasks=("Setup Environment" "Code Review" "Deploy Application" "Monitor System")

echo "=== Task List ==="
for i in "${!tasks[@]}"; do
    echo "[$i] ${tasks[$i]}"
done

echo ""
echo "=== Member Contacts ==="
for name in "${!email[@]}"; do
    echo "$name: ${email[$name]}"
done

echo ""
echo "Recent tasks: ${tasks[@]:1:2}"

Both ${arr[@]} and ${arr[*]} expand all elements, but they behave differently inside double quotes. "${arr[@]}" expands each element as a separate argument, so elements containing spaces are not split. "${arr[*]}" joins all elements into a single string using the IFS separator (a space by default). In loops and function calls, always use "${arr[@]}".

Deleting an element with unset does not reindex the array. If you delete index 1, the remaining indices are 0, 2, 3, ... with a gap. ${#arr[@]} returns the actual element count, which may not equal the highest index. To compact the indices, reassign with arr=("${arr[@]}").

Bash arrays are one-dimensional only — multidimensional arrays are not natively supported. Common workarounds include using naming conventions like matrix_1_2 for multiple variables, or encoding data as delimited strings.

Common Mistake 1: Elements with spaces are split without quotes

Omitting double quotes when expanding an array causes the shell to split elements on whitespace, breaking multi-word values into separate arguments.

items=("value one" "value two" "value three")
for t in ${items[@]}; do
    echo "$t"
done
value
one
value
two
value
three

Without quotes, ${items[@]} is split on whitespace, producing six tokens instead of three. Wrapping the expansion in double quotes preserves each element as a single unit.

for t in "${items[@]}"; do
    echo "$t"
done
value one
value two
value three

Common Mistake 2: Forgetting declare -A for associative arrays

If you use string keys without first declaring the array with declare -A, Bash treats it as an indexed array. String keys are converted to the number 0, so each assignment overwrites the same slot and only the last value survives.

affiliation["user1"]="Team A"
affiliation["user2"]="Team B"
affiliation["user3"]="Team A"
echo ${affiliation[@]}
Team A

Three assignments were made, but only one value remains. Adding declare -A at the top fixes the problem.

declare -A affiliation
affiliation["user1"]="Team A"
affiliation["user2"]="Team B"
affiliation["user3"]="Team A"
echo ${#affiliation[@]}
3