Introduction
Arrays are one of the most used and fundamental data structures. You can think of an array as a variable that stores multiple variables within it. Arrays offer a convenient way to represent and manipulate a group of related data items as a single entity.
In this article we will cover Bash arrays and explain how to use them in your Bash scripts.
Array Bash
Bash supports one-dimensional associative and numerically indexed array types. Numeric arrays are referenced using integers and associative using strings.
Numerically indexed arrays can be accessed from the end using negative indices; the index of -1 references to the last element. Indexes do not have to be contiguous.
Unlike most programming languages, Bash array elements do not have to be of the same data type. This means you can create an array containing both strings and numbers.
Bash does not support multidimensional arrays, and it is not possible to have array elements that are also arrays.
There is no limit to the maximum number of elements that can be stored in an array.
Creating Bash arrays
In Bash, arrays can be initialized using several methods.
Creating numerically indexed arrays
Bash variables are not typed. Any variable can be used as an indexed array without declaring it.
To explicitly declare an array, use the declare builtin command:
declare -a array_name
One way to create an indexed array is to use the following form:
array_name[index_1]=value_1
array_name[index_2]=value_2
array_name[index_n]=value_n
Where index_* is a positive integer.
Another way to create a numeric array is to specify the list of elements in parentheses, separated by white space:
array_name=( element_1 element_2 element_N )
When the array is created using the previous form, indexing starts from zero, that is, the first element has an index of 0.
Creating associative arrays
Associative arrays are a type of array in which each element is identified by a unique key rather than an index.
Unlike numerically indexed indices, associative arrays must be declared before they can be used.
To declare an associative array, use the declare built-in with the -A (uppercase) option:
declare -A array_name
Associative arrays are created using the following form:
declare -A array_name
array_name[index_foo]=value_foo
array_name[index_bar]=value_bar
array_name[index_xyz]=value_xyz
Where index_* can be any string.
You can also create an associative array using the following form:
declare -A array_name
array_name=(
[index_foo]=value_foo
[index_bar]=value_bar
[index_xyz]=value_xyz
)
Array operations
If you're new to Bash programming, arrays in Bash can be confusing at first. However, after reading this article, you will understand them better.
Reference elements
To reference a single element, you need to know the index of the element.
You can refer to any element using the following syntax:
${array_name[index]}
The syntax for accessing an array element is similar to the syntax of most programming languages. The curly braces ${} are needed to avoid the shell's filename expansion operators.
We print the element with an index of 1:
## declare the array
declare -a my_array=( "Hydrogen" "Helium" "Lithium" "Beryllium" )
## print element
echo ${my_array[1]}
Helium
If you use @ or * as an index, the word expands to all members of the array.
To print all items, you should use:
## declare the array
declare -a my_array=( "Hydrogen" "Helium" "Lithium" "Beryllium" )
## print all elements
echo "${my_array[@]}"
Hydrogen Helium Lithium Beryllium
The only difference between @ and * is when the form ${my_array[x]} is enclosed in double quotes. In this case, * expands to a single word where the array elements are separated by a space. @ expands each element of the array into a separate word. This is especially important when using the illiterate form through array elements.
To print the array keys, add the ! operator before the array name:
${!array_name[index]}
Here is an example:
## declare the array
declare -a my_array=( "Hydrogen" "Helium" "Lithium" "Beryllium" )
## print all elements
echo "${!my_array[@]}"
0 1 2 3
Length of the matrix
If you want to determine the number of elements in an array, you can use the following syntax to get the length of the array:
${#array_name[@]}
The syntax is the same as when referencing all elements by adding the # character before the array name.
## declare the array
declare -a my_array=( "Hydrogen" "Helium" "Lithium" "Beryllium" )
## array Length
echo ${#my_array[@]}
4
Go through the array
When you have a set of items, you often need to perform an operation on each item. One way to achieve this is to iterate over the array using a loop. The for loop allows you to access each element of the array individually and perform the desired operation on it.
declare -a my_array=( "Hydrogen" "Helium" "Lithium" "Beryllium" )
## Array Loop
for i in "${my_array[@]}"
do
echo "$i"
done
The code above will iterate over the array and print each element in a new line:
Hydrogen
Helium
Lithium
Beryllium
Here is an example of how to print all keys and values:
declare -a my_array=( "Hydrogen" "Helium" "Lithium" "Beryllium" )
## Array Loop
for i in "${!my_array[@]}"
do
echo "$i" "${my_array[$i]}"
done
0 Hydrogen
1 Helium
2 Lithium
3 Beryllium
Another way to iterate through an array is to get the length of the array and use the C-style loop:
declare -a my_array=( "Hydrogen" "Helium" "Lithium" "Beryllium" )
# Length of the array
length=${#my_array[@]}
# Array Loop
for (( i=0; i < ${length}; i++ ))
do
echo $i ${my_array[$i]}
done
0 Hydrogen
1 Helium
2 Lithium
3 Beryllium
Adding a new item
To add a new element to a bash array and specify its index, use the following form:
my_array[index_n]="New Element"
Here is an example:
declare -a my_array=( "Hydrogen" "Helium" "Lithium" "Beryllium" )
## add new element
my_array[9]="Aluminum"
## print all elements
echo "${my_array[@]}"
Hydrogen Helium Lithium Beryllium Aluminum
Another way to add a new element to an array without specifying the index is to use the += operator. You can add one or more elements:
declare -a my_array=( "Hydrogen" "Helium" "Lithium" "Beryllium" )
## add new elements
my_array+=( Cobalt Nickel )
## print all elements
echo "${my_array[@]}"
Hydrogen Helium Lithium Beryllium Cobalt Nickel
Delete an item
To delete a single item, you will need to know the index of the item. An element can be removed using the unset command:
unset my_array[index]
Let's see an example:
declare -a my_array=( "Hydrogen" "Helium" "Lithium" "Beryllium" )
## remove element
unset my_array[2]
## print all elements
echo "${my_array[@]}"
Hydrogen Helium Beryllium
Conclusion
We have explained how to create numerically indexed and associative arrays. We also showed how to iterate through arrays, calculate array length, and add and remove elements.
If you have any questions or feedback, feel free to leave a comment.
