How to Use while Loop in Bash

1. Syntax: while Loop in Bash
2. Example: while Loop in Bash
3. Example: Infinite while Loop in Bash
4. Example: while Loop in Bash With break Statement
5. Example: while Loop in Bash With continue Statement

while loop is one of the most widely used loop structures in almost every programming language. It is used when we don’t know the number of times we need to run a loop. We can specify a condition for the while loop, and the statements in the loop are executed until the condition becomes false.

Syntax: while Loop in Bash

while [condition]
do
   command-1
   command-2
     ...
     ...
   command-n
done

Here, the condition represents the condition that needs to be checked every time before executing commands in the loop. If the condition is true, we execute the statements in the loop. If the condition is false, we exit out of the loop. The statements from command-1 to command-n are statements executed in a loop until the condition becomes false.

Example: while Loop in Bash

#!/bin/bash

num=5

while [ $num -ge 0 ]
do
  echo $num
  ((num--))
done

Output:

5
4
3
2
1
0

Here, initially, num is set to 5. We keep printing num in the terminal and decrementing num by 1 in a loop as long as the num value is greater than or equal to 0.

Example: Infinite while Loop in Bash

#!/bin/bash

while true
do
  echo "This is an infinite while loop. Press CTRL + C to exit out of the loop."
  sleep 0.5
done

Output:

This is an infinite while loop. Press CTRL + C to exit out of the loop.
This is an infinite while loop. Press CTRL + C to exit out of the loop.
This is an infinite while loop. Press CTRL + C to exit out of the loop.
^C

It is an infinite while loop that prints This is an infinite while loop. Press <kbd>Ctrl</kbd>+<kbd>C</kbd> to exit out of the loop. in every 0.5 seconds. To exit out of the loop, we can Press Ctrl+C.

Example: while Loop in Bash With break Statement

#!/bin/bash

num=5

while [ $num -ge 0 ]
do
  echo $num
  ((num--))
  if [[ "$num" == '3' ]]; then
    echo "Exit out of loop due to break"
    break
  fi
done

Output:

5
4
Exit out of loop due to break

In the above program, num is initialized as 5. The loop is executed as long as num is greater than or equal to 0. But as we have a break statement in the loop when num is 3. So, we exit out of the loop as the value of num becomes 3.

Example: while Loop in Bash With continue Statement

#!/bin/bash

num=6

while [ $num -ge 1 ]
do
  ((num--))
  if [[ "$num" == '3' ]]; then
    echo "Ignore a step due to continue"
    continue
  fi
  echo $num
  
done

Output:

5
4
Ignore a step due to continue
2
1
0

In the above program, num is initialized as 6. In the loop, we first decrease num by 1 and then print the num latest value. The loop is executed as long as the num value is greater than or equal to 1. When num becomes 3, the script does not print the value of num as we have the continue statement when num is 3.