How to add/remove an element to/from the array in bash?

Question

I have an array containing some elements, but I want to push new items to the beginning of the array; How do I do that?

Answer 1

To add an element to the beginning of an array use.

arr=("new_element" "${arr[@]}") 

Generally, you would do.

arr=("new_element1" "new_element2" "..." "new_elementN" "${arr[@]}") 

To add an element to the end of an array use.

arr=( "${arr[@]}" "new_element" ) 

Or instead

arr+=( "new_element" ) 

Generally, you would do.

arr=( "${arr[@]}" "new_element1" "new_element2" "..." "new_elementN") #Or arr+=( "new_element1" "new_element2" "..." "new_elementN" ) 

To add an element to specific index of an array use.

Let's say we want to add an element to the position of Index2 arr[2], we would actually do merge on below sub-arrays:

1. Get all elements before Index position2 arr[0] and arr[1];
2. Add an element to the array;

3. Get all elements with Index position2 to the last arr[2], arr[3], ....

   arr=( "${arr[@]:0:2}" "new_element" "${arr[@]:2}" ) 

Removing an element from the array

In addition to removing an element from an array (let's say element #3), we need to concatenate two sub-arrays. The first sub-array will hold the elements before element #3 and the second sub-array will contain the elements after element #3.

arr=( "${arr[@]:0:2}" "${arr[@]:3}" ) 

• ${arr[@]:0:2} will get two elements arr[0] and arr[1] starts from the beginning of the array.

• ${arr[@]:3} will get all elements from index3 arr[3] to the last.

  one possible handy way to re-build the arr excluding element#3 (arr[2]) from that:

  del_element=3; arr=( "${arr[@]:0:$((del_element-1))}" "${arr[@]:$del_element}" ) 

  specify which element you want to exclude in del_element=.

Another possibility to remove an element is

1. Using unset (actually assign 'null' value to the element)

   unset -v 'arr[2]' 

2. Use replace pattern if you know the value of your array elements to truncate their value (replace with empty string).

   arr=( "${arr[@]/PATTERN/}" ) 

Print the array

printf '%s\n' "${arr[@]}" 

Answer 2

Note that arrays in bash (copied from ksh) are rather associative arrays (with keys limited to positive integers also called sparse arrays).

a=(newvalue "$a[@]") 

would make a new $a array with newvalue as ${a[0]} and the elements of the original array appended in the numerical order of their key with keys 1, 2...

For instance, if you had:

bash-4.4$ typeset -p a declare -a a=([0]="foo" [12]="bar") bash-4.4$ a=(newvalue "${a[@]}") bash-4.4$ typeset -p a declare -a a=([0]="newvalue" [1]="foo" [2]="bar") 

That explains why there's no builtin operator for that.

If you wanted to insert the newvalue as ${a[0]} and shift all the other keys by one, you'd need a temporary array:

b=newvalue for k in "${!a[@]}"; do b[k+1]=${a[k]} done unset a for k in "${!b[@]}"; do a[k]=${b[k]} done unset b 

Shells like zsh or yash that have normal arrays have operators for that:

• zsh:

   a[1,0]=newvalue 

(also works for prepending strings to scalar variables)

• yash:

   array -i a 0 newvalue 

Answer 3

# array declaration arr=() #Function to read data from file a and add into array fun_add_in_array() { input=$1 while IFS=',' read -r f1 f2 do echo "Element1 : $f1" echo "Element2 : $f2" arr+=( "$f1" ) done < "$input" } #Function to print a array fun_read_array() { arr=("$@") for i in "${arr[@]}" do echo $i done } 

Answer 4

Create an Indexed Array:

$ declare -a A $ declare -p A 

declare -a A

Add some elements to the array:

$ A+=(foo) $ A+=(bar) $ A+=("baz quux") $ declare -p A 

declare -a A=([0]="foo" [1]="bar" [2]="baz quux")

Remove the middle element, making it a Sparse Indexed array:

$ unset A[1] $ declare -p A 

declare -a A=([0]="foo" [2]="baz quux")

Remove the last element from the Sparse Indexed Array:

$ unset A[-1] $ declare -p A 

declare -a A=([0]="foo")