How to Use Until Loops to Automate Your Workflow

Introduction

If you’re a developer or sysadmin working with Bash scripts, you’re probably familiar with the for and while loops. But have you ever used the until loop? This lesser-known looping construct can be just as powerful and useful as its more popular counterparts. In this blog, we’ll explore the basics of until loops in Bash and give you some tips and examples to help you incorporate them into your workflow.

Requirements and Dependencies

To use until loops in Bash, you’ll need a Unix-like operating system with the Bash shell installed. This could be a Linux distribution, macOS, or even Windows with a Bash emulator like Cygwin or WSL. You’ll also need some basic knowledge of Bash scripting and looping constructs.

The until Loop in Action

The until loop is similar to the while loop, but with one key difference: the loop continues to execute until the specified condition is met. In other words, the loop will run as long as the condition is false. Here’s the basic syntax:

until [ condition ]
do
    # loop body
done

Let’s take a look at an example. Suppose we want to create a script that checks the status of the website itvraag.nl every 5 seconds until it returns a 200 OK status code. We can use the curl command to check the status, and the sleep command to pause for 5 seconds between checks. Here’s what the script might look like:

#!/bin/bash

until curl -I itvraag.nl | grep "200 OK" > /dev/null
do
    echo "Site is down, trying again in 5 seconds..."
    sleep 5
done

echo "Site is up!"

This script will keep checking the status of the website until it returns a 200 OK code, at which point it will print “Site is up!” and exit the loop.

Surprising Fact

Did you know that you can also use until loops as infinite loops? All you have to do is specify a condition that is always false, like this:

until false
do
    # loop body
done

This loop will run indefinitely, until you manually break out of it using the break command or by killing the script.

Key Take-aways

• until loops are similar to while loops, but they continue to execute until the specified condition is met.
• You can use until loops to repeat a task until a certain condition is met, or as infinite loops.

Examples and Tips

Here are a few examples and tips to help you get started with until loops in Bash:

1. Use until loops to check the status of a server or service, and alert you when it comes back online.
2. Use until loops to retry a task that might fail due to temporary issues, like network outages or server downtime.
3. Use until loops to wait for a file to be created or modified, and then perform some action on it.
4. Use until loops to repeat a task at regular intervals, like checking the status of a website every 5 minutes.
5. Use until loops to create interactive menus or prompts for your Bash scripts.

Challenge

Here’s a challenge for you: try creating a script that uses an until loop to check the status of a server every 5 minutes, and sends an email notification if the server is down for more than 20 minutes. Good luck!