How to clean up unused Docker images and containers automatically from AWS EC2 instance

If you have a large number of containers that update often, your associated EBS volume may become overloaded with old images and containers. This can result in a lot of wasted space and slows down your instance. If this occurs, you will need to SSH into the instance and remove any unnecessary Docker images and containers to free up space on your EC2 instance. However, if there is insufficient storage space on the root volume, the instance may become unresponsive and inaccessible via SSH until you restart it.
In this blog article, I will demonstrate how to automatically clear up unused Docker images and containers using a simple shell script and cronjob.

Prune Docker images and containers

You can use the following command:

docker system prune -a

This command will remove any stopped containers, networks that are not in use by at least one container, dangling images, and build cache. However, prior to removal, it will request confirmation. If you want to bypass the confirmation, you can use the following command:

docker system prune -af

If you simply want to remove the dangling images and not other resources such as networks, stopped containers or build cache, you can use the following command:

docker image prune -a

These commands will reclaim the storage space that was being used by the stopped containers and unused images.

Automate the process

Now, let's create a simple shell script to automate the process and then we will schedule it to run at a specific time using cronjob.

cd /etc/cron.daily
sudo nano docker-cleanup

Once you are in the file, add the following content:

#!/bin/bash
docker system prune -af  --filter "until=$((7*24))h"

Save the file and exit from it. This command will remove all unused images, networks, stopped containers and build cache that are older than 7 days. You can change the time value as per your requirement and containers update frequency you got. Now, let's make this file executable so OS can run this command inside it on daily basis.

sudo chmod +x /etc/cron.daily/docker-cleanup

That's it. Docker will now execute this command daily. You can change the time interval as per your requirement. There are cron.hourly/cron.daily/cron.weekly/cron.monthly directories in /etc directory. You can put your script in the appropriate directory to run it at the desired interval.

To insure the command will run while daily cronjob, you can test the following command manually to check if it's working fine or not:

run-parts /etc/cron.daily

This will run all the scripts in the /etc/cron.daily directory. We added one shell script "docker-cleanup" above in this directory to clean up the unused docker content.

Conclusion

In this blog post, we learned how to clean up unused Docker images and containers automatically using a simple shell script and cronjob. This will help you to keep your EC2 instance clean and free up space on your attached volume.