How to Append to an Array in Bash? [4 Easy Methods]

You can use the following 4 methods to append to an array in Bash:

1. Using the shorthand operator (+=): array_name+=('element to append').
2. Defining the last index: array_name[${#array_name[@]}]='element to append'.
3. Using bracket (): array_name=(${array_name[@]} 'element to append').
4. Via array concatenation: array1=(${array1[@]} ${array2[@]}).

Append to an array in Bash refers to adding elements to a Bash array. It is a fundamental yet essential operation for the seamless addition of data within the array data structure. This article discusses 4 easy methods on how to append to an array in Bash. It discusses the appending of single elements to an array as well as appending multiple elements to a Bash array. It also mentions the methods to append an entire array to another array. Additionally, it mentions 2 common mistakes that may occur during the append operation in Bash arrays with solutions and tips.

1. Using the Shorthand Operator “+=”

Use the += operator to append elements to a Bash array. The full syntax is like the following:

array+=(elements_to_append)

Let’s see a simple example of appending to an array in bash using the += operator:

#!/bin/bash

#The main array
digit=(one two three)

#prints array with 3 existing elements
echo "array before appending: ${digit[@]}"

#new value to append
digit+=(four)

#Prints array with newly added element four
echo "array after appending: ${digit[@]}"

As you can see the above script appends a single entity four to the array digit.

Append Multiple Elements

Apart from appending a single element to an array, it might become necessary to append multiple elements to a Bash array. Fortunately, Bash arrays provide the following syntax to append more than one element:

array_name+=(element1 element2 elementN)

Here’s an example of appending multiple elements to a Bash array:

#!/bin/bash

#The main array
digit=(one two three)

#prints array with 3 existing elements
echo "array before appending: ${digit[@]}"

#new value to append
digit+=(four five six seven)

#Prints array with newly added element four
echo "array after appending: ${digit[*]}"

See from the above image that the script appends 4 new elements to the array digit.

2. Defining the Last Index

Another simple way to append an element to an array is to add data explicitly to the last index. Now, to append an element using the last index, use the expression ${#array_name[@]} with the syntax: array_name[${#array_name[@]}]="Data to append". This will append the new Data to append to the end of the array by defining the last index. Here’s an example:

#!/bin/bash

#Main Array to work with
Linux=("Ubuntu" "Debian" "openSUSE")

#print main array
echo "Before append: ${Linux[*]}"

#Value RHEL to append 
Linux[${#Linux[@]}]="RHEL"

#Printing the values of an array
echo "After append: ${Linux[*]}"

As you can see, defining the last index in the above script appends an element successfully to an array.

3. Append an Array Using Parenthesis

An array element can be appended by using the array variable name and the new element value enclosed within the first bracket (). The full syntax is: array_name=(${array_name[@]} "element to append").

Below is an example script to append a value to a Bash array using first brackets ():

#!/bin/bash

# Declare a string array
fruit=("orange" "watermelon" "apple")

#print the array
echo "before append: ${fruit[@]}"

# Add new element at the end of the array using parenthesis () 
fruit=(${fruit[@]} "grape")

#print the array elements in a single line 
echo "After append: ${fruit[@]}"

Terminal output to append to an array using first brackets ().

4. Using Array Concatenation

Array concatenation is the process of combining two arrays to form a single array. This is very easy to achieve using the syntax array1=(${array1[@]} ${array2[@]}) which will expand and concatenate array1 and array2 into array1. Let’s see array concatenation into action to append elements to a Bash array:

#!/bin/bash

#main array
digit=(1 2 3)

#print array
echo "array1: ${digit[@]}"

#array elements to append
alsoDigit=(5 6 7)

#print array2
echo "array2: ${alsoDigit[@]}"

#array appending via array concatenation
digit=(${digit[@]} ${alsoDigit[@]})

#print the new array 
echo "the new array: ${digit[@]}"

Upon execution, the script prints both individual & concatenated arrays.

How to Append Array to Another Array in Bash?

Apart from appending individual or multiple data elements, the smart approach of using the shorthand operator += is capable of appending an array to another array. The complete syntax is array1+=(${array2[@]}) where the += operator appends the array2 to the existing array1.

Here’s how to append an array to another array in Bash:

#!/bin/bash

# Main array
main_array=("Apple" "Orange" "Banana")
echo "main array: ${main_array[@]}"

# Array to append
to_append=("Grapes" "Mango")
echo "to_append: ${to_append[@]}"

# append array to array using compound assignment +=
main_array+=("${to_append[@]}")

# Print the updated array
echo "updated array: ${main_array[@]}"

In the above image, the array to_append has been appended to the main_array to form an array of 5 elements from 3 existing elements.

2 Ways to Avoid Common Mistakes When Appending to Bash Arrays

The append operation in Bash array is very simple which you already have learned. However, you might encounter 2 common pitfalls during appending to Bash arrays and employ solutions like the following:

1. Handling Spaces in Bash Array Elements

One common issue arises when you have to handle array elements containing spaces. Look at the below example:

tennis=("nadal" "roddick" "roger federer")
tennis+=("sampras" "suffin")
echo ${tennis[*]}
echo "length: ${#tennis[*]}"

In the above example, when the array elements and length are printed, Bash treats ‘roger federer’ as a single element, which is desirable in this case.

However, if the multi-word element is not quoted around, then it will be treated as 2 other elements rather than a single one like the following:

#!/bin/bash
tennis=("nadal" "roddick" roger federer)
tennis+=("sampras" "suffin")
echo ${tennis[*]}
echo "length: ${#tennis[*]}"

In the above image, the array length is now 6 instead of 5 since Bash treats roger and federer as 2 different array elements due to the absence of quotes around.

Solution: Always use quotes around array elements that contain space to ensure Bash treats them as a single data value.

2. Escape Overwriting Existing Arrays

Another common error is mistakenly using the equal sign (=) instead of the += operator in the array append syntax where the syntax becomes array=(element to append) and accidentally overwrites the array instead of appending it. See the script below:

#!/bin/bash
tennis=('nadal' 'roddick' 'federer')
tennis=('sampras' 'suffin')
echo ${tennis[*]}

The above script prints sampras suffin instead of printing all the 5 elements nadal roddick federer sampras suffin since the use of = in the syntactical format tennis=(‘sampras’ ‘suffin’) has overwritten the previous array with 2 elements.

Solution: Double-check to make sure you are using the += operator in appending to the array. Otherwise, you might accidentally end up overwriting the array.

Conclusion

This article discusses the concept of how to append to arrays in Bash with 4 different methods. It mentions appending single, or multiple elements to an array. it sheds light on the advanced concept of appending arrays to another Bash array. In addition, it mentions 2 common pitfalls with effective solutions to ensure smooth appending to Bash arrays. Hope this article aids your journey to mastering Bash arrays.

Frequently Asked Questions

How to add elements to a Bash array at specific positions?

To add an element to a Bash array at a specific position, use the concept of direct index assignment with the syntax array_name[index]=value. For instance, to add an element named apple at the 5th position (index 4) to the array food=(rice meat wheat), use the syntax food[4]=apple.

Can I append an array to another array in Bash?

Yes, to append an array to another array, use the <strong>+=</strong> operator with the syntax new_array+=(${array[@]}) to append array to new_array. Here’s a script for example:

#!/bin/bash

arr1=("apple" "banana" "cherry")
arr2=("mango" "grape")
arr1+=(${arr2[@]})

echo ${arr1[@]}

Output: apple banana cherry mango grape

What’s the difference between array+=element and array+=(element)?

The first syntax array+=element adds a string element to the first element while the latter one appends element as a new individual value at the end of the array. Take the array num=(1 2 3) for example. The syntax num+=4 will add 4 as a string at the end of the element 1. However, the syntax  num+=(4) will add 4 as a new array element to the end of the array. See the below Bash script:

#!/bin/bash
num=( 1 2 3)

num+=4
num+=(4)

echo ${num[@]}

How do I append to an array in Bash?

To append to an array in Bash, use the “+=” operator, with the syntax, your_array+=(element_To_add). This allows you to add an element to the end of an array.

Is it possible to append multiple arrays to another Bash array?

Yes, to append multiple arrays to another array, use += with the syntax array1+=("${array2[@]}" "${array3[@]}" "${array4[@]}") to append the 3 arrays (array2 array3 array4) to the existing array1.

What is meant by Bash array append?

Bash array append means to add elements to an array in Bash. The append operation adds elements at the end of an existing array. To append an element to a Bash array, use the shorthand operator += with the syntax array_name+=(element to append). For example, the syntax cars+=(honda) adds the element honda to the end of the array cars.

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