How to Use While Loop in Bash

Introduction

Loops are programming constructs that continuously execute specific commands unless interrupted. For instance, the while loop in Bash is a control structure that runs commands if a condition evaluates to true. You can use while loops to handle user input, read files, and execute a series of commands continuously.

This article explains how to use the while loop in Bash for various tasks such as reading files, handling user input, and automating processes.

The `while` Loop Command Syntax

The while loop in Bash follows this syntax:

while [CONDITION]
do
   # Commands
done

In the above command:

while: Starts the while loop.
condition: Sets the condition that you want to evaluate.
do: Marks the beginning of the loop's body including the commands to execute when the condition evaluates to true.
done: Marks the end of the loop and stops execution when the condition evaluates to false.

Create `while` Loops in Bash

Follow these steps to create a while loop in Bash using a shell environment or script.

Create a new inline while loop to continuously increment a numeric variable's value up to a certain number, such as 10.
console
```
$ x=0; while [ $x -le 10 ]; do echo "The value of X is: $x"; ((x++)); done
```
Output:
```
The value of X is: 0
The value of X is: 1
The value of X is: 2
The value of X is: 3
The value of X is: 4
The value of X is: 5
The value of X is: 6
The value of X is: 7
The value of X is: 8
The value of X is: 9
The value of X is: 10
```
In the above inline while loop, the $x -le 10 condition evaluates to true if the variable x is less than or equal to 10 and outputs the current item's value while incrementing it using the ((x++)) command. Inline while loops allow you to execute conditions in Bash without creating a script which is important when evaluating files or capturing real-time user input.
Create a new while.sh script that tests if a variable is less than a number, such as 10, prints the current value, and increments the variable until the condition evaluates to false.
console
```
$ nano while.sh
```
Add the following contents to the while.sh file.
bash
```
#!/bin/bash
var=0

while [ $var -lt 10 ]
do
  echo The current value: $var is less than 10
  ((var++))
done
```
Save and close the file.

In this example, the while loop in Bash checks if the variable is less than 10, and outputs the current value.

Run the script using Bash.

                            console
                            
$ bash while.sh

Output:

The current value: 0 is less than 10
The current value: 1 is less than 10
The current value: 2 is less than 10
The current value: 3 is less than 10
The current value: 4 is less than 10
The current value: 5 is less than 10
The current value: 6 is less than 10
The current value: 7 is less than 10
The current value: 8 is less than 10
The current value: 9 is less than 10

Create a new while-script.sh script.
console
```
$ nano while-script.sh
```
Add the following contents to the file to create a while loop that uses input redirection < instead of a condition to read the file contents line by line.
bash
```
#!/bin/bash

while IFS= read -r line; do
    echo "$line"
done < "/etc/passwd"
```
Save and close the file.

In the above script, the while loop uses the input redirection command < to input the system user information file /etc/htpasswd to the read command and iterate over all lines in the file. The IFS= option keeps all whitespaces in the file.

Run the script using Bash.

                            console
                            
$ bash while-script.sh

Output:

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin

Create Infinite `while` Loops

Infinite While loops run indefinitely each time a condition evaluates to true unless terminated or interrupted. The : command creates an infinite loop that continuously executes a series of commands. Infinite loops are useful in logging and monitoring tasks that output continuous data. Follow the steps below to create an infinite while loop that continuously runs unless you stop it using Ctrl + C.

Create a new infinite.sh script.
console
```
$ nano infinite.sh
```
Add the following contents to the infinite.sh file.
bash
```
#!/bin/bash

while :
do
  echo "This is an Infinite loop message, press <CTRL + C> to stop"
  sleep 1
done
```
Save and close the file.

In the above script, the while loop uses the : option to create an infinite loop that outputs the This is an Infinite loop message, press <CTRL + C> to stop message. The sleep 1 declaration instructs the loop to run every second.

Run the script using Bash.

console

$ bash infinite.sh

Output:

This is an Infinite loop message, press <CTRL + C> to stop
This is an Infinite loop message, press <CTRL + C> to stop
This is an Infinite loop message, press <CTRL + C> to stop
This is an Infinite loop message, press <CTRL + C> to stop
This is an Infinite loop message, press <CTRL + C> to stop

Create Nested `while` Loops

Nested While loops execute multiple commands at different levels to perform complex iterative tasks. They allow you to use a loop inside another loop to handle complex tasks. Follow the steps below to create nested while loops in Bash.

Create a new nested.sh script.
console
```
$ nano nested.sh
```
Add the following contents to the nested.sh file.
bash
```
#!/bin/bash

app_name=("Apache" "Mysql" "Nginx" "Mysql" "PHP")

var=0
while [ $var -lt ${#app_name[@]} ]; do
    j=0
    while [ $j -lt ${#app_name[@]} ]; do
        if [ $var -ne $j ]; then
            echo "Pair: ${app_name[$var]} and ${app_name[$j]}"
        fi
        ((j++))
    done
    ((var++))
done
```
Save and close the file.

In the above script, the nested while loop performs data pairing tasks by iterating over each element in the app_name array variable to pair non-identical package names. The outer while loop iterates through all elements in the array, while the inner loop pairs each service with others using the j variable value in the outer loop to compare the position of each element in the array.

Run the script using Bash.

console

$ bash nested.sh

Output:

Pair: Apache and Mysql
Pair: Apache and Nginx
Pair: Apache and Mysql
Pair: Apache and PHP
Pair: Mysql and Apache
Pair: Mysql and Nginx
Pair: Mysql and Mysql
Pair: Mysql and PHP
Pair: Nginx and Apache
Pair: Nginx and Mysql
Pair: Nginx and Mysql
Pair: Nginx and PHP
Pair: Mysql and Apache
Pair: Mysql and Mysql
Pair: Mysql and Nginx
Pair: Mysql and PHP
Pair: PHP and Apache
Pair: PHP and Mysql
Pair: PHP and Nginx
Pair: PHP and Mysql

To control the flow of a while loop, conditional statements like if-else are essential. Visit the if-else in Bash guide for more information.

Use the `break` Statement in `while` Loops

The break statement terminates and exits a loop when a condition evaluates to true. In the following steps, use the break statement to control the flow of a while loop.

Create a new break.sh script.
console
```
$ nano break.sh
```

Add the following contents to the break.sh file.

                            bash
                            
                        
#!/bin/bash

var=0

while [ $var -lt 10 ]
do
  echo "Updated Value is: $var"
  ((var++))
  if [[ "$var" == '10' ]]; then
    echo "The value is equal to 10, ending..."
    break
  fi
done

echo 'The loop test is complete!'

Save and close the file.

In the above script, the break statement terminates the loop when the var variable reaches 10 and outputs the The loop test is complete! message when the loop stops.

Run the script using Bash.

                            console
                            
$ bash break.sh

Output:

Updated Value is: 0
Updated Value is: 1
Updated Value is: 2
Updated Value is: 3
Updated Value is: 4
Updated Value is: 5
Updated Value is: 6
Updated Value is: 7
Updated Value is: 8
Updated Value is: 9
The value is equal to 10, ending...
The loop test is complete!

Use the `continue` Statement in `while` Loops

The continue statement stops the current iteration in a while loop and starts the next iteration when a specific condition evaluates to true. You can combine the continue and if statements to test for a specific result in a loop to start the next iteration without stopping the loop. Follow the steps below to use the continue statement in a while loop.

Create a new continue.sh script.
console
```
$ nano continue.sh
```
Add the following contents to the continue.sh file.
bash
```
#!/bin/bash

var=0

while [ $var -lt 10 ]
do
  echo "Updated Value is: $var"
  ((var++))
  if [[ "$var" == '5' ]]; then
    echo "The value is equal to 5, skipping..."
    continue
  fi
done
```
Save and close the file.

In the above script, the while loop uses an if conditional statement to compare if the current value is equal to 5. The continue statement starts the next iteration and ignores other commands in the loop.

Run the script using Bash.

                            console
                            
$ bash continue.sh

Output:

Updated Value is: 0
Updated Value is: 1
Updated Value is: 2
Updated Value is: 3
Updated Value is: 4
The value is equal to 5, skipping...
Updated Value is: 5
Updated Value is: 6
Updated Value is: 7
Updated Value is: 8
Updated Value is: 9

Use `while` Loops with Conditional Statements

Conditional statements (if, case, or if-else) control a loop's flow by using the results of a loop's iteration to execute specific commands. You can also combine conditional statements with break and continue commands. In the following steps, use the if statement to control a while loop.

Create a new if-while.sh script.
console
```
$ nano if-while.sh
```

Add the following contents to the if-while.sh file.

                            bash
                            
                        
#!/bin/bash

var=0

while [ $var -le 10 ]; do
    if (( var % 2 == 0 )); then
        echo "The value of $var is EVEN."
    else
        echo "The value of $var is ODD."
    fi

    ((var++))
done

Save and close the file.

The above script uses the if-then conditional statement in a while loop to check if the current var variable value is divisible by 2 to evaluate the result as odd or even.

Run the script using Bash.

                            console
                            
$ bash if-while.sh

Output:

The value of 0 is EVEN.
The value of 1 is ODD.
The value of 2 is EVEN.
The value of 3 is ODD.
The value of 4 is EVEN.
The value of 5 is ODD.
The value of 6 is EVEN.
The value of 7 is ODD.
The value of 8 is EVEN.
The value of 9 is ODD.
The value of 10 is EVEN.

Use `while` Loops with Functions

You can use while loops with functions to create reusable commands and perform different tasks for each condition. Functions work with conditional statements within a loop to execute reusable code or commands at different intervals. Follow the steps below to use a while loop with multiple functions to perform different actions whenever a condition evaluates to true.

Create a new functions-while.sh script.
console
```
$ nano functions-while.sh
```

Add the following contents to the functions-while.sh file.

                            bash
                            
                        
#!/bin/bash

read -p "Enter your application names (apache,nginx,mysql,php or help|-h): " -a app_name

help_page() {
    echo "Usage: ./functions-while.sh [options]"
    echo
    echo "Options:"
    echo "apache"
    echo "nginx"
    echo "mysql"
    echo "php"
    echo "  -h, --help       Display this help page"
}

var=0

while [ $var -lt ${#app_name[@]} ]; do
    service="${app_name[$var]}"

    if [[ $service == "help" || $service == "-h" ]]; then
        help_page
    else
        case $service in
            "apache")
                echo "$service: works as the Web server"
                ;;
            "mysql")
                echo "$service: works as a Database server"
                ;;
            "nginx")
                echo "$service: works as a Reverse proxy or Web server"
                ;;
            "php")
                echo "$service: works as a Hypertext processor"
                ;;                
            *)
                echo "$service: Unknown service"
                ;;
        esac
    fi

    ((var++))
done

Save and close the file.

In the above script, the app_name array stores elements from the user's input, and the while loop iterates over each element in the indexed array that is less than the var variable's value. The loop uses an if-then statement to evaluate if the current list item is help or execute the matching pattern using a case conditional statement. The ((var++)) command increments the current var variable value to iterate over all array elements.

Run the script using Bash.

console

$ bash functions-while.sh

Enter one or more application names, such as apache or mysql when prompted.

Enter your application names (apache,nginx,mysql,php or help|-h):

Output:

Enter your application names (apache,nginx,mysql,php or help|-h): apache mysql
apache: works as the Web server
mysql: works as a Database server

Conclusion

You have used the while loop in Bash to evaluate specific conditions and continuously execute tasks or commands. The while loops are important when evaluating user inputs, variables, or multiple conditions to achieve a specific result. You can use while loops with other Bash features, such as for loops, conditional statements, arrays, and variables to run continuous tasks.

Tags:

Bash

How to Use While Loop in Bash

Introduction

The `while` Loop Command Syntax

Create `while` Loops in Bash

Create Infinite `while` Loops

Create Nested `while` Loops

Use the `break` Statement in `while` Loops

Use the `continue` Statement in `while` Loops

Use `while` Loops with Conditional Statements

Use `while` Loops with Functions

Conclusion

Comments

Products

Features

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Marketplace

Resources

Company

Tech Talks

Vultr Blogs

How to Use While Loop in Bash

Introduction

The while Loop Command Syntax

Create while Loops in Bash

Create Infinite while Loops

Create Nested while Loops

Use the break Statement in while Loops

Use the continue Statement in while Loops

Use while Loops with Conditional Statements

Use while Loops with Functions

Conclusion

Comments

Tech Talks

Vultr Blogs

The `while` Loop Command Syntax

Create `while` Loops in Bash

Create Infinite `while` Loops

Create Nested `while` Loops

Use the `break` Statement in `while` Loops

Use the `continue` Statement in `while` Loops

Use `while` Loops with Conditional Statements

Use `while` Loops with Functions