Nested for loop in Linux Shell Script

By: Dorris in Linux Tutorials on 2011-01-15

As you see the if statement can be nested, similarly loop statement can be nested. You can nest the for loop. To understand the nesting of for loop see the following shell script.

$ vi nestedfor.sh
        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

Run the above script as follows:

$ chmod +x nestedfor.sh
$ ./nestefor.sh
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

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

Following script is quite intresting, it prints the chess board on screen.

$ vi chessboard

        for (( i = 1; i <= 9; i++ )) ### Outer for loop ###
        do
           for (( j = 1 ; j <= 9; j++ )) ### Inner for loop
        ###
           do
                tot=`expr $i + $j`
                tmp=`expr $tot % 2`
                if [ $tmp -eq 0 ]; then
                    echo -e -n "\033[47m "
                else
                    echo -e -n "\033[40m "
                fi
          done
         echo -e -n "\033[40m" #### set back background colour to
        black
         echo "" #### print the new line ###
        done

Run the above script as follows:

$ chmod +x chessboard
$ ./chessboard

Above shell script cab be explained as follows:

Command(s)/Statements	Explanation
for (( i = 1; i <= 9; i++ )) do	Begin the outer loop which runs 9 times., and the outer loop terminets when the value of i exceeds 9
for (( j = 1 ; j <= 9; j++ )) do	Begins the inner loop, for each value of i the inner loop is cycled through 9 times, with the varible j taking values from 1 to 9. The inner for loop terminates when the value of j exceeds 9.
tot=`expr $i + $j` tmp=`expr $tot % 2`	See for even and odd number positions using these statements.
if [ $tmp -eq 0 ]; then echo -e -n "\033[47m " else echo -e -n "\033[40m " fi	If even number posiotion print the white colour block (using echo -e -n "\033[47m " statement); otherwise for odd postion print the black colour box (using echo -e -n "\033[40m " statement). This statements are responsible to print entier chess board on screen with alternet colours.
done	End of inner loop
echo -e -n "\033[40m"	Make sure its black background as we always have on our terminals.
echo ""	Print the blank line
done	End of outer loop and shell scripts get terminted by printing the chess board.