Shell Programming - While Loop

In this tutorial we will learn about While loop in Shell Programming.

The while loop is similar to a for loop and help in executing a block of code multiple times as long as some given condition is satisfied.

while syntax

while [ condition ]
do
  # body of while loop
done

Where condition is some condition which if satisfied results in the execution of the body of the loop.

To come out of the while loop we make the condition fail.

To quit the script we can use the exit command.

Example #1: Write a Shell Script to print from 1 to 5 using while loop

#!/bin/sh

# initialise i
i=1

while [ $i -le 5 ]
do
  # echo i
  echo $i

  # update i
  i=`expr $i + 1`
done

Output:

$ sh example01.sh 
1
2
3
4
5

We can achieve the same result by write the following code.

#!/bin/sh

# initialise i
i=1

while [ $i -le 5 ]
do
  # echo i
  echo $i

  # update i
  i=$(( $i + 1 ))
done

Nested while loop

We can nest while loop by placing a while loop in the body of another while loop.

while [ condition_outer ]
do

  # body of the outer while loop

  while [ condition_inner ]
  do
    # body of the inner while loop
  done

done

Example #2: Write a Shell Script to print the following pattern

1
1 3
1 3 5
1 3 5 7

For this we will use r variable to count the rows and c variable to count the columns. And we will use counter variable to print the number.

#!/bin/sh

# for the rows
r=1

while [ $r -le 4 ]
do

  # for the output
  count=1
  
  # for the column
  c=1

  while [ $c -le $r ]
  do
  	# print the value
  	printf "$count "

    # update count
    count=$(( $count + 2 ))

    # update c
    c=$(( $c + 1 ))
  done

  # go to new line
  printf "\n"

  # update r
  r=$(( $r + 1 ))
  
done

Output:

$ sh example02.sh 
1 
1 3 
1 3 5 
1 3 5 7