Bash - Arrays

Arrays in shells are variable to hold more than one value.

Suppose, You have a list of numbers 1 2 3.. 10 and want to store these numbers in Shell Script

Without arrays, You have to declare as follows

let number1=1
let number1=1
...
...
let number10=10

Iteration is difficult and if we want to store 100 numbers, It is very difficult.

So, You can use an array referring to a single variable and store it.

How to declare and create an array?

There are two types of arrays we can create

• indexed arrays: array elements are stored with the index starting from zero

• associated arrays: array is stored with key-value pairs

• Declare an array

To create an array, We need to declare an array.

declares -a array; # indexed array
declare -A array; # associative array

an array is declared with the keyword declare with option -a or A

indexed array example In this, Array values are stored with index=0 onwards. these are created with declare and -a option

declare -a array
array=(one two three)

This array is a store with index=0, incremented by 1 as follows.

array[0]=one
array[1]=two
array[2]=three

associative array example In this, Array values are stored with keys. these are created with declare and -A option

declare -A array
array=(one two three)

In this array is a store with index=0, incremented by 1 as follows

array[key1]=one
array[key2]=two
array[key3]=three

Let’s assign the values.

array=(1,2,3,4)

• Assign the values without declaring an array

  arrayvariable[index]=value
  

  This means, that arrayvariable is declared and assigned an array index with value.

Arrays are zero-indexed based on zero to the length of an array -1 index=0 - returns the first element index=-1 returns the last element

Arrays can contain numbers, strings, and a mix of it Let’s create an array of examples.

Access the array values

An array contains an index to get elements. Array elements can be accessed using the below syntax.

${array_name[index]}

Declare an Array of numbers and loop through

Arrays can contain numbers This example contains an array of numbers and for loop to print

nums=(1 3 12)
for i in "${nums[@]}"
do
  echo "$i"
done

Output:

1
3
12

Declare an Array of strings and loop through

Arrays can contain numbers This example contains an array of numbers and for loop to print

numbers=("element1" "element2" "element3")
for i in "${numbers[@]}"
do
  echo "$i"
done

Output:

element1
element2
element3

Access the first elements of an array

In Array elements, the First element index is zero, and array[0] returns the first element

numbers=("element1" "element2" "element3")

echo ${numbers[0]}
echo ${numbers}

Output:

element1
element1

Get the last element of an array

In a bash script, you can use index=-1 to get the last array element.

numbers=("element1" "element2" "element3")
echo ${numbers[-1]}

With the recent bash 4.0 version, you can use the below syntax to read the last element.

echo ${numbers[${#numbers[@]}-1]}

Iterate or loop array elements

For loop is used to iterate elements.

Here is an example loop array example to print all elements

numbers=("element1" "element2" "element3")

for i in "${numbers[@]}"
do
  echo "$i"
done

Output:

element1
element2
element3

Another way to print the index and elements of an array using for loop.

numbers=("element1" "element2" "element3")

for i in "${!numbers[@]}"
do
  echo "$i" "${numbers[$i]}"
done

Output:

0 element1
1 element2
2 element3

Print all array elements

Use [@] or [*] to print all elements of an array.

arr=("element1" "element2" "element3") //
echo ${arr[@]} #element1 element2 element3
echo ${arr[*]}  #element1 element2 element3

Remove an element from an array

You can remove an element from an array using unset for a given index.

numbers=("element1" "element2" "element3")
echo ${numbers[*]}
unset numbers[-1]
echo ${numbers[*]}

Adding an element to an array

You can add an element at any index position using the below syntax.

array[index]=value

An example of adding elements starting and end as well as middle

numbers=("element1" "element2" "element3")
echo ${numbers[*]}

 numbers[0]="element0"
echo ${numbers[*]}
 numbers[5]="element5"
echo ${numbers[*]}
 numbers[6]="element6"
echo ${numbers[*]}

Output:

element1 element2 element3
element0 element2 element3
element0 element2 element3 element5
element0 element2 element3 element5 element6

Length of an array

In this, Find the count of all elements in an array.

Shell script provides #

arr=("element1" "element2" "element3")
echo ${#arr[@]} # returns 3
echo ${#arr[*]} # returns 3

Array cheat sheet examples