Arrays¶

Arrays let you store and manipulate collections of values. Bash supports indexed (numeric) and associative (key-value) arrays.

Indexed Arrays¶

Creating Arrays¶

# Direct assignment
fruits=("apple" "banana" "cherry")

# Index assignment
colors[0]="red"
colors[1]="green"
colors[2]="blue"

# From command output
files=($(ls *.txt))

# Empty array
empty=()

Accessing Elements¶

fruits=("apple" "banana" "cherry")

echo "${fruits[0]}"      # apple (first element)
echo "${fruits[1]}"      # banana
echo "${fruits[-1]}"     # cherry (last element, bash 4.3+)
echo "${fruits[@]}"      # All elements: apple banana cherry
echo "${fruits[*]}"      # All as single string

Braces Required

Always use braces for array access:

echo $fruits[0]      # Wrong - prints "apple[0]"
echo ${fruits[0]}    # Correct - prints "apple"

Array Length¶

fruits=("apple" "banana" "cherry")
echo "${#fruits[@]}"     # 3 (number of elements)
echo "${#fruits[0]}"     # 5 (length of first element)

Array Indices¶

fruits=("apple" "banana" "cherry")
echo "${!fruits[@]}"     # 0 1 2 (all indices)

Iterating¶

fruits=("apple" "banana" "cherry")

# Iterate over values
for fruit in "${fruits[@]}"; do
    echo "$fruit"
done

# Iterate with indices
for i in "${!fruits[@]}"; do
    echo "$i: ${fruits[$i]}"
done

Output:

apple
banana
cherry
0: apple
1: banana
2: cherry

Modifying Arrays¶

Adding Elements¶

fruits=("apple" "banana")

# Append
fruits+=("cherry")

# Append multiple
fruits+=("date" "elderberry")

# Insert at index
fruits[5]="fig"    # Sparse array - index 3,4 are unset

echo "${fruits[@]}"
# apple banana cherry date elderberry fig

Removing Elements¶

fruits=("apple" "banana" "cherry")

# Remove by index
unset fruits[1]
echo "${fruits[@]}"    # apple cherry

# Note: doesn't reindex - index 1 is now empty
echo "${!fruits[@]}"   # 0 2

Replacing Elements¶

fruits=("apple" "banana" "cherry")
fruits[1]="blueberry"
echo "${fruits[@]}"    # apple blueberry cherry

Array Slice¶

arr=(a b c d e f)
echo "${arr[@]:2:3}"   # c d e (from index 2, length 3)
echo "${arr[@]:2}"     # c d e f (from index 2 to end)
echo "${arr[@]::3}"    # a b c (first 3 elements)

Copying Arrays¶

original=("a" "b" "c")
copy=("${original[@]}")

Associative Arrays¶

Associative arrays use strings as keys. Requires Bash 4.0+.

Creating¶

# Must declare first
declare -A user

user[name]="Alice"
user[age]=30
user[email]="alice@example.com"

# Or inline
declare -A person=(
    [name]="Bob"
    [age]=25
    [city]="NYC"
)

Accessing¶

declare -A user=([name]="Alice" [age]=30)

echo "${user[name]}"      # Alice
echo "${user[@]}"         # All values
echo "${!user[@]}"        # All keys: name age
echo "${#user[@]}"        # Number of elements: 2

Iterating¶

declare -A user=([name]="Alice" [age]=30 [city]="NYC")

# Iterate over keys
for key in "${!user[@]}"; do
    echo "$key: ${user[$key]}"
done

Checking Key Exists¶

declare -A user=([name]="Alice")

if [[ -v user[name] ]]; then
    echo "Name is set"
fi

if [[ -v user[age] ]]; then
    echo "Age is set"
else
    echo "Age is not set"
fi

Common Patterns¶

Array from String¶

str="one two three"
read -ra arr <<< "$str"
echo "${arr[1]}"    # two

With custom delimiter:

str="one,two,three"
IFS=',' read -ra arr <<< "$str"
echo "${arr[1]}"    # two

Array to String¶

arr=("one" "two" "three")
str="${arr[*]}"
echo "$str"    # one two three

With custom delimiter:

arr=("one" "two" "three")
IFS=','
str="${arr[*]}"
IFS=' '
echo "$str"    # one,two,three

Or use printf:

arr=("one" "two" "three")
printf -v str '%s,' "${arr[@]}"
str="${str%,}"    # Remove trailing comma
echo "$str"       # one,two,three

Check if Array Contains Value¶

arr=("apple" "banana" "cherry")

contains() {
    local item="$1"
    shift
    local arr=("$@")
    for elem in "${arr[@]}"; do
        [[ "$elem" == "$item" ]] && return 0
    done
    return 1
}

if contains "banana" "${arr[@]}"; then
    echo "Found banana"
fi

Using pattern matching:

arr=("apple" "banana" "cherry")
[[ " ${arr[*]} " =~ " banana " ]] && echo "Found"

Remove Duplicates¶

arr=("a" "b" "a" "c" "b")

declare -A seen
unique=()
for item in "${arr[@]}"; do
    if [[ ! -v seen[$item] ]]; then
        seen[$item]=1
        unique+=("$item")
    fi
done

echo "${unique[@]}"    # a b c

Sort Array¶

arr=("banana" "apple" "cherry")

# Using printf and sort
IFS=$'\n' sorted=($(printf '%s\n' "${arr[@]}" | sort))
echo "${sorted[@]}"    # apple banana cherry

# Numeric sort
nums=(10 2 30 1)
IFS=$'\n' sorted=($(printf '%s\n' "${nums[@]}" | sort -n))
echo "${sorted[@]}"    # 1 2 10 30

Reverse Array¶

arr=("a" "b" "c" "d")
reversed=()
for ((i=${#arr[@]}-1; i>=0; i--)); do
    reversed+=("${arr[$i]}")
done
echo "${reversed[@]}"    # d c b a

Filter Array¶

arr=(1 2 3 4 5 6 7 8 9 10)
evens=()
for num in "${arr[@]}"; do
    ((num % 2 == 0)) && evens+=("$num")
done
echo "${evens[@]}"    # 2 4 6 8 10

Map Function Over Array¶

arr=(1 2 3 4 5)
doubled=()
for num in "${arr[@]}"; do
    doubled+=($((num * 2)))
done
echo "${doubled[@]}"    # 2 4 6 8 10

Reading File into Array¶

Using mapfile (Bash 4+)¶

mapfile -t lines < file.txt
echo "Lines: ${#lines[@]}"
echo "First: ${lines[0]}"

Options:

-t - Remove trailing newlines
-n count - Read at most count lines
-s skip - Skip first skip lines

Using While Loop¶

lines=()
while IFS= read -r line; do
    lines+=("$line")
done < file.txt

Passing Arrays to Functions¶

By Value (Copy)¶

process_array() {
    local arr=("$@")
    for item in "${arr[@]}"; do
        echo "Item: $item"
    done
}

fruits=("apple" "banana" "cherry")
process_array "${fruits[@]}"

By Reference (Bash 4.3+)¶

process_array() {
    local -n arr=$1    # nameref
    for item in "${arr[@]}"; do
        echo "Item: $item"
    done
}

fruits=("apple" "banana" "cherry")
process_array fruits    # Pass array name, not contents

Stack and Queue¶

Stack (LIFO)¶

stack=()

push() { stack+=("$1"); }
pop() {
    local top="${stack[-1]}"
    unset 'stack[-1]'
    echo "$top"
}

push "a"
push "b"
push "c"
echo $(pop)    # c
echo $(pop)    # b

Queue (FIFO)¶

queue=()

enqueue() { queue+=("$1"); }
dequeue() {
    local front="${queue[0]}"
    queue=("${queue[@]:1}")
    echo "$front"
}

enqueue "a"
enqueue "b"
enqueue "c"
echo $(dequeue)    # a
echo $(dequeue)    # b

Try It¶

Basic array operations:

fruits=("apple" "banana" "cherry")
echo "Count: ${#fruits[@]}"
echo "First: ${fruits[0]}"
echo "All: ${fruits[@]}"

Iterate with index:

colors=("red" "green" "blue")
for i in "${!colors[@]}"; do
    echo "$i: ${colors[$i]}"
done

Associative array:

declare -A user=([name]="Alice" [city]="NYC")
echo "Name: ${user[name]}"
for key in "${!user[@]}"; do
    echo "$key = ${user[$key]}"
done

Array from command:

mapfile -t lines < <(echo -e "one\ntwo\nthree")
echo "${lines[@]}"

Summary¶

Operation	Indexed Array	Associative Array
Declare	`arr=()`	`declare -A arr`
Assign	`arr[0]="val"`	`arr[key]="val"`
Access	`${arr[0]}`	`${arr[key]}`
All values	`${arr[@]}`	`${arr[@]}`
All keys	`${!arr[@]}`	`${!arr[@]}`
Length	`${#arr[@]}`	`${#arr[@]}`
Append	`arr+=("val")`	`arr[key]="val"`
Delete	`unset arr[0]`	`unset arr[key]`

Best practices:

Always quote: "${arr[@]}"
Use declare -A for associative arrays
Check existence with [[ -v arr[key] ]]
Use mapfile -t to read files
Associative arrays require Bash 4.0+