Nested for loop statement

Nested for loops means loop within loop. They are useful for when you want to repeat something serveral times for several things. For example, create a shell script called nestedfor.sh:

#!/bin/bash
# A shell script to print each number five times.
for (( i = 1; i <= 5; i++ ))      ### Outer for loop ###
do
 
    for (( j = 1 ; j <= 5; j++ )) ### Inner for loop ###
    do
          echo -n "$i "
    done
 
  echo "" #### print the new line ###
done

Save and close the file. Run it as follows:

chmod +x nestedfor.sh
./nestedfor.sh

Sample outputs:

1 1 1 1 1 
2 2 2 2 2 
3 3 3 3 3 
4 4 4 4 4 
5 5 5 5 5 

For each value of i the inner loop is cycled through 5 times, with the variable j taking values from 1 to 5. The inner for loop terminates when the value of j exceeds 5, and the outer loop terminates when the value of i exceeds 5.

Chessboard Example

A chessboard is the type of checkerboard used in the game of chess, and consists of 64 squares - eight rows and eight columns arranged in two alternating colors. The colors are called "black" and "white". Let us write a shell script called chessboard.sh to display a chessboard on screen:

#!/bin/bash
for (( i = 1; i <= 8; i++ )) ### Outer for loop ###
do
   for (( j = 1 ; j <= 8; j++ )) ### Inner for loop ###
   do
        total=$(( $i + $j))   # total 
        tmp=$(( $total % 2))  # modulus
        # Find out odd and even number and change the color 
        # alternating colors using odd and even number logic  
        if [ $tmp -eq 0 ]; 
        then
            echo -e -n "\033[47m  "
        else
            echo -e -n "\033[40m  "
        fi
  done
 echo "" #### print the new line ###
done

Save and close the file. Run it as follows:

chmod +x chessboard.sh
./chessboard.sh

Sample outputs: