Cleaning Up Containers

Overview

Teaching: 10 min
Exercises: 0 min

Questions

• How do I interact with a Docker container on my computer?

Objectives

• Explain how to list running and completed containers.

Removing images

The images and their corresponding containers can start to take up a lot of disk space if you don’t clean them up occasionally, so it’s a good idea to periodically remove container images that you won’t be using anymore.

In order to remove a specific image, you need to find out details about the image, specifically, the “image ID”. For example say my laptop contained the following image.

$ docker image ls

REPOSITORY       TAG         IMAGE ID       CREATED          SIZE
hello-world      latest      fce289e99eb9   15 months ago    1.84kB

You can remove the image with a docker image rm command that includes the image ID, such as:

$ docker image rm fce289e99eb9

or use the image name, like so:

$ docker image rm hello-world

However, you may see this output:

Error response from daemon: conflict: unable to remove repository reference "hello-world" (must force) - container e7d3b76b00f4 is using its referenced image fce289e99eb9

This happens when Docker hasn’t cleaned up some of the times when a container has been actually run. So before removing the container image, we need to be able to see what containers are currently running, or have been run recently, and how to remove these.

What containers are running?

Working with containers, we are going to shift to a new docker command: docker container. Similar to docker image, we can list running containers by typing:

$ docker container ls

CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

Notice that this command didn’t return any containers because our containers all exited and thus stopped running after they completed their work.

docker ps

The command docker ps serves the same purpose as docker container ls, and comes from the Unix shell command ps which describes running processes.

What containers have run recently?

There is also a way to list running containers, and those that have completed recently, which is to add the --all/-a flag to the docker container ls command as shown below.

$ docker container ls --all

CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS                     PORTS               NAMES
9c698655416a        hello-world         "/hello"            2 minutes ago       Exited (0) 2 minutes ago                       zen_dubinsky
6dd822cf6ca9        hello-world         "/hello"            3 minutes ago       Exited (0) 3 minutes ago                       eager_engelbart

Keeping it clean

You might be surprised at the number of containers Docker is still keeping track of. One way to prevent this from happening is to add the --rm flag to docker run. This will completely wipe out the record of the run container when it exits. If you need a reference to the running container for any reason, don’t use this flag.

How do I remove an exited container?

To delete an exited container you can run the following command, inserting the CONTAINER ID for the container you wish to remove. It will repeat the CONTAINER ID back to you, if successful.

$ docker container rm 9c698655416a

9c698655416a

If you want to remove all exited containers at once you can use the docker containers prune command. Be careful with this command. If you have containers you may want to reconnect to, you should not use this command. It will ask you if to confirm you want to remove these containers, see output below. If successfull it will print the full CONTAINER ID back to you.

$ docker container prune

WARNING! This will remove all stopped containers.
Are you sure you want to continue? [y/N] y
Deleted Containers:
9c698655416a848278d16bb1352b97e72b7ea85884bff8f106877afe0210acfc
6dd822cf6ca92f3040eaecbd26ad2af63595f30bb7e7a20eacf4554f6ccc9b2b

Removing images, for real this time

Now that we’ve removed any potentially running or stopped containers, we can try again to delete the hello-world image.

$ docker image rm hello-world

Untagged: hello-world:latest
Untagged: hello-world@sha256:5f179596a7335398b805f036f7e8561b6f0e32cd30a32f5e19d17a3cda6cc33d
Deleted: sha256:fce289e99eb9bca977dae136fbe2a82b6b7d4c372474c9235adc1741675f587e
Deleted: sha256:af0b15c8625bb1938f1d7b17081031f649fd14e6b233688eea3c5483994a66a3

The reason that there are a few lines of output, is that a given image may have been formed by merging multiple underlying layers. Any layers that are used by multiple Docker images will only be stored once. Now the result of docker image ls should no longer include the hello-world image.

Key Points

• The docker container command lists containers that have been created.