Bash Script to Merge Two Arrays Easily

In Bash, you can merge two arrays by combining them like this: merged_array=("${array1[@]}" "${array2[@]}") which creates a new array containing all elements from both arrays.

Examples

Inputarray1=(1 2 3), array2=(4 5 6)

Outputmerged_array=(1 2 3 4 5 6)

Inputarray1=(apple banana), array2=(cherry date)

Outputmerged_array=(apple banana cherry date)

Inputarray1=(), array2=(x y z)

Outputmerged_array=(x y z)

How to Think About It

To merge two arrays in Bash, think of creating a new array that first takes all elements from the first array, then adds all elements from the second array. This is done by expanding both arrays with "${array[@]}" syntax and putting them together inside parentheses.

Algorithm

1

Define the first array with its elements.

2

Define the second array with its elements.

3

Create a new array by combining all elements from the first and second arrays using array expansion.

4

Print or use the merged array as needed.

Code

bash

#!/bin/bash

array1=(1 2 3)
array2=(4 5 6)

merged_array=("${array1[@]}" "${array2[@]}")

echo "Merged array elements: ${merged_array[@]}"

Output

Merged array elements: 1 2 3 4 5 6

Dry Run

Let's trace merging array1=(1 2 3) and array2=(4 5 6) through the code

1

Define array1

array1 contains: 1 2 3

2

Define array2

array2 contains: 4 5 6

3

Merge arrays

merged_array becomes: 1 2 3 4 5 6

Step	merged_array contents
After step 3	1 2 3 4 5 6

Why This Works

Step 1: Array Expansion

Using "${array[@]}" expands all elements of an array preserving each element as a separate word.

Step 2: Combining Arrays

Putting both expanded arrays inside parentheses () creates a new array with all elements combined.

Step 3: Resulting Array

The new array contains all elements from the first array followed by all elements from the second array in order.

Alternative Approaches

Appending elements in a loop

bash

#!/bin/bash

array1=(a b c)
array2=(d e f)
merged_array=()
for element in "${array1[@]}" "${array2[@]}"; do
  merged_array+=("$element")
done

echo "${merged_array[@]}"

This method uses a loop to append elements one by one, which is more flexible but longer and slower for large arrays.

Using declare -a and indirect expansion

bash

#!/bin/bash

declare -a array1=(1 2)
declare -a array2=(3 4)
merged_array=()
merged_array=("${array1[@]}" "${array2[@]}")
echo "${merged_array[@]}"

This is similar to the main method but explicitly declares arrays, useful for clarity and complex scripts.

Complexity: O(n) time, O(n) space

Time Complexity

Merging arrays requires visiting each element once, so time grows linearly with total elements.

Space Complexity

A new array is created to hold all elements, so space also grows linearly with input size.

Which Approach is Fastest?

Direct array expansion is fastest and simplest; looping is slower but more flexible for processing elements.

Approach	Time	Space	Best For
Direct array expansion	O(n)	O(n)	Simple merging with no processing
Appending in loop	O(n)	O(n)	Merging with element processing
Explicit declare -a	O(n)	O(n)	Clarity and complex scripts