How to Use Bash Until Loop

Introduction

Loops are fundamental constructs in Bash that run a series of commands multiple times until a specific condition is met. A Bash Until Loop continuously executes a block of commands while a specific condition evaluates to false and stops when the condition evaluates to true. The until loops are the opposite of while loops. The Bash while loop runs when a condition is true and terminates when a condition returns false.

This article explains how to use the Bash until Loop in scripts.

The Bash Until Loop Syntax

The following is a basic until loop syntax in Bash.

until [CONDITION]
do
  # Commands
done

In the above Bash until loop syntax:

• until: Creates a new until loop.
• condition: Specifies the condition to evaluate and run the commands in the loop's body if the condition is false.
• do: Specifies the start of the loop's body and runs the commands while the condition evaluates to false.
• done: Ends the loop.

Create until Loops

The until loops work in Bash shell environment and scripts. You can use inline until loops to evaluate conditions and perform tasks in a shell or create until loops to run multiple commands in a script. Follow the steps below.

1. Create a new inline until loop to evaluate if 0 is greater than 3 and print the result after incrementing the value.

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   $ count=0; until [ $count -ge 3 ]; do echo "Counter: $count"; ((count++)); done

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   Output:

   Counter: 0
   Counter: 1
   Counter: 2

2. Create a new until.sh script using a text editor like vim.

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   $ vim until.sh

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3. Add the following content to the until.sh file.

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   #!/bin/bash
   
   count=1
   
   until [ "$count" -gt 10 ]; do
       echo "The current count is: $count"
       ((count++))
   done

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   Save and close the file.

   In the above script, the until loop continuously displays the result of the count variable when the condition evaluates to false until the value reaches 10. The count++ command increments the variable in each loop cycle.

4. Run the script using Bash.

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   $ bash until.sh 

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   Output:

   The current count is: 1
   The current count is: 2
   The current count is: 3
   The current count is: 4
   The current count is: 5
   The current count is: 6
   The current count is: 7
   The current count is: 8
   The current count is: 9
   The current count is: 10

Infinite until Loops

An Infinite until loop continuously runs a specific block of commands unless a user stops the action or if the loop encounters a break statement. You can create infinite until loops using constant conditions. Infinite loops are useful when viewing real-time file updates such as logs. Follow the steps below to create an infinite until loop.

1. Create a new infinite.sh script.

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   $ vim infinite.sh

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2. Add the following contents to the infinite.sh file.

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   #!/bin/bash
   
   until false; do
       tail -n 10 /var/log/syslog 
       echo "Press Ctrl + C to stop the loop"
       sleep 3                   
   done

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   Save and close the file.

   In the above script, the until loop continuously runs and displays the last lines in the /var/log/syslog file using the tail command and waits 3 seconds between the updates. The infinite loop monitors the log file and stops if the user presses Ctrl + C.

3. Run the script using Bash.

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   $ bash infinite.sh

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   Output:

   2024-10-16T00:00:17.106598+00:00 acs-server systemd[1]: Starting sysstat-collect.service - system activity accounting tool...
   2024-10-16T00:00:17.124697+00:00 acs-server systemd[1]: Starting logrotate.service - Rotate log files...
   2024-10-16T00:00:17.139593+00:00 acs-server systemd[1]: sysstat-collect.service: Deactivated successfully.
   2024-10-16T00:00:17.142083+00:00 acs-server systemd[1]: Finished sysstat-collect.service - system activity accounting tool.
   2024-10-16T00:00:17.187803+00:00 acs-server systemd[1]: dpkg-db-backup.service: Deactivated successfully.
   2024-10-16T00:00:17.189907+00:00 acs-server systemd[1]: Finished dpkg-db-backup.service - Daily dpkg database backup service.
   2024-10-16T00:00:17.218479+00:00 acs-server systemd[1]: logrotate.service: Deactivated successfully.
   2024-10-16T00:00:17.219024+00:00 acs-server systemd[1]: Finished logrotate.service - Rotate log files.
   Press Ctrl + C to stop the loop

Create Nested until Loops

Nested until loops execute commands using multiple levels of conditions to perform a specific task. Such loops use the following syntax to run multiple commands at different levels.

until [CONDITION]
do
   # Commands

   until [CONDITION]
   do
      # Commands
   done

   # Commands
done

Follow the steps below to create nested until loops to form a visual countdown of numbers.

1. Create a new nested-until.sh script.

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   $ vim nested-until.sh

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2. Add the following contents to the nested-until.sh file.

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   #!/bin/bash
   
   count=15
   until [ $count -le 0 ]
   do
       asterisks=0
       until [ $asterisks -ge $count ]
       do
           echo -n "*"
           ((asterisks++))
       done
   
       echo " ($count)"
       
       ((count--))
   done

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   Save and close the file.

   In the above Bash script, the outer Bash until loop checks if the count condition is less than 1 to assign the inner until loop an asterisk variable to display asterisks that match the count variable value. The outer loop displays the current count variable value and decrements it using count-- to repeat the loop.

3. Run the script using Bash.

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   $ bash nested-until.sh 

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   Output:

   *************** (15)
   ************** (14)
   ************* (13)
   ************ (12)
   *********** (11)
   ********** (10)
   ********* (9)
   ******** (8)
   ******* (7)
   ****** (6)
   ***** (5)
   **** (4)
   *** (3)
   ** (2)
   * (1)

Use until Loop break and continue Statements

In Bash, break and continue statements affect how the until loop executes. The break statement stops the loop and the continue statement instructs the loop to skip specific commands. Follow the steps below to use the break and continue statements with until loops.

1. Create a new break.sh file.

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   $ vim break.sh

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2. Add the following contents to the break.sh file.

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   #!/bin/bash
   
   count=10 
   
   until [ "$count" -lt 1 ]; do
       echo "Count: $count"
       
       if [ "$count" -eq 5 ]; then
           echo "Count is 5, terminating the loop."
           break
       fi
       
       ((count--))
   done

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   Save and close the file.

   In the above script, the until loop uses a conditional statement to check the current count variable value and terminate the loop using the break statement when the count variable evaluates to 5.

3. Run the script using Bash.

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   $ bash break.sh 

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   Output:

   Count: 10
   Count: 9
   Count: 8
   Count: 7
   Count: 6
   Count: 5
   Count is 5, terminating the loop.

4. Create a new continue.sh script.

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   $ vim continue.sh

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5. Add the following contents to the continue.sh file.

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   #!/bin/bash
   
   count=10
   
   until [ "$count" -lt 1 ]; do
      if [ "$count" -eq 5 ]; then
          echo "Count is 5, skipping the iteration."
          ((count--))  
          continue 
      fi
      
      echo "Count: $count"
      ((count--)) 
   done

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   Save and close the file.

   In the above Bash script, the until loop uses a conditional if statement to test the count variable's value. The loop skips the current iteration when the count value evaluates to 5 using the continue statement.

6. Run the script.

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   $ bash continue.sh 

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   Output:

   Count: 10
   Count: 9
   Count: 8
   Count: 7
   Count: 6
   Count is 5, skipping the iteration.
   Count: 4
   Count: 3
   Count: 2
   Count: 1

Use until Loops with Conditional Statements

You can use the until loops with advanced conditional statements to execute a specific set of commands. These Bash conditional statements include if,, if-else,case, and elif. Follow the steps below to create an until loop and use an if-else conditional statement to validate the user input.

1. Create a new until-if.sh script.

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   $ vim until-if.sh

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2. Add the following contents to the until-if.sh file.

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   #!/bin/bash
   
   user_password="admin123"
   
   until [ "$user_input" = "$user_password" ]; do
       read -p "Enter password: " user_input
       if [ "$user_input" = "$user_password" ]; then
           echo "Access granted!"
       else
           echo "Incorrect password, try again with the correct password."
       fi
   done

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   Save and close the file.

   In the above script, the until loop uses the read command to prompt for user input and continuously runs the if-else statement until the condition evaluates to true and stops the loop.

3. Run the script using Bash.

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   $ bash until-if.sh 

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   Enter a wrong password like 11 to test the if-else statement.

   Output:

   Enter password: 11
   Incorrect password, try again with the correct password.
   Enter password: 

Combine until Loops with While Loops

The until loops continuously run until a condition evaluates to true but while loops continuously run until a condition evaluates to false. Combining Bash until and while loops allows you to validate, control, and execute specific commands based on certain conditions. Follow the steps below to use Bash until loop with while loops to validate user input.

1. Create a new until-while.sh file.

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   $ vim until-while.sh

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2. Add the following contents to the until-while.sh file.

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   #!/bin/bash
   
   user_password="admin123"
   user_input=""
   attempts=1
   maximum_attempts=5
   
   until [ "$attempts" -gt "$maximum_attempts" ]; do
       echo "You have $attempts of $maximum_attempts attempts"
       
       while [ "$user_input" != "$user_password" ]; do
           read -p "Enter the password: " user_input
           
           if [ "$user_input" == "$user_password" ]; then
               echo "Access granted!"
               exit 0 
           else
               echo "Incorrect password, try again with the correct password.."
               break 
           fi
       done
   
       ((attempts++))
   done
   
   if [ "$attempts" -gt "$maximum_attempts" ]; then
       echo "Maximum password attempts reached. Access denied!"
   fi

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   Save and close the file.

   In the above script, the outer until loop displays user password attempts and runs the inner while loop to capture the user input using the read command. The if-else conditional statement validates the user input and returns a result. The script increments the attempts variable to run the until loop again if the inner while loop evaluates to false. The loop stops when the attempts and maximum_attempts variables are equal.

3. Run the script using Bash.

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   $ bash until-while.sh 

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   Enter wrong passwords when prompted to test the loop functionalities.

   Output:

   You have 1 of 5 attempts
   Enter the password: 1
   Incorrect password, try again with the correct password..
   You have 2 of 5 attempts
   Enter the password: 12
   Incorrect password, try again with the correct password..
   You have 3 of 5 attempts
   Enter the password: e
   Incorrect password, try again with the correct password..
   You have 4 of 5 attempts
   Enter the password: rr
   Incorrect password, try again with the correct password..
   You have 5 of 5 attempts
   Enter the password: 55
   Incorrect password, try again with the correct password..
   Maximum password attempts reached. Access denied!

   Run the script again and enter the correct admin123 user password .

   You have 1 of 5 attempts
   Enter the password: admin123
   Access granted!

Conclusion

You have created and used the Bash until loop to continuously run commands if a condition evaluates to false. The until loops run commands when a condition is false and stops when the condition evaluates to true. Combining until loops with conditional statements such as if-else, while, and for loops allows advanced iteration control to run complex commands or automate specific tasks in Bash.