Cleaning Unused Docker Data Without External Tools

Warning: read the full article before typing commands you may regret.

If you are a docker user (may it be in production or on your development machine), you may have accumulated quite a bit of now useless data. There are some existing tools to clean your daemon of unused images and containers (like docker-gc), but since the docker API 1.25 docker has an easy to use prune command that could me good enough for you.

Basic usage is pretty simple:

$ sudo docker system prune
WARNING! This will remove:
        - all stopped containers
        - all networks not used by at least one container
        - all dangling images
        - all build cache
Are you sure you want to continue? [y/N] y

Deleted Containers:
deleted: sha256:ea43728b2d10e7b0fe24036f9531caac96bd02f779b95a6620110f00ccd3b002
deleted: sha256:022db612b3070971ce7d51778806a1f995a9c3aa1a741a6c0be0bca603787387
... approximately 2 gajillion hashes later...

Total reclaimed space: 5.64GB

That’s nice, but it does not clean everything unused. For that you should add the --all option to remove every image and not only the dangling ones:

$ sudo docker system prune --all
WARNING! This will remove:
        - all stopped containers
        - all networks not used by at least one container
        - all images without at least one container associated to them
        - all build cache
Are you sure you want to continue? [y/N] y
... moar hashes...

Total reclaimed space: 26.78GB

Of course that may not be ideal. Having caches is pretty useful after all. And there is just the option for that: --filter (only available starting at API 1.28). At the time of my writing there are only two filters: untiland label. In my case I only use the until one which allows you to clean everything older than a specified timestamp.

Of course I don’t put a timestamp in the command, I use date to generate a timestamp based on a human readable date:

$ sudo docker system prune --all --filter until=$(date -d "1 month ago" +%s)

This command uses a substitution via $( command ), this allows you to get the result of a command and inject it in another one as a string. The date command itself is decomposed as: date -d <time description> <time format>, where+%s is the format to get a timestamp. You can check man date for more info.

And voilà 😃 I rarely need the cache to be older than a month on my laptop (and one month is already pretty conservative, but that’s because I rarely have a good internet so I cache as much as possible).

Finally, if you feel adventurous enough you can add the --force option so that the command will not ask for confirmation. I use the following in acronjob on my continuous integration server:

$ sudo docker system prune --force --all --filter until=$(date -d "1 week ago" +%s)

Happy cleaning ♻️

A note about sudo: on my dev machine (which is a GNU/Linux system) I only use docker via sudo because not doing so mean that your user has the same power as root all the time. This is due to the fact that the docker daemon is a privileged process running as root. See the official Docker security page.

This post was originally published on mayeu.me.