What is a dangling image and what is an unused image?

Question

An unused image means that it has not been assigned or used in a container. For example, when running docker ps -a - it will list all of your exited and currently running containers. Any images shown being used inside any of containers are a "used image".

On the other hand, a dangling image just means that you've created the new build of the image, but it wasn't given a new name. So the old images you have becomes the "dangling image". Those old image are the ones that are untagged and displays "<none>" on its name when you run docker images.

When running docker system prune -a, it will remove both unused and dangling images. Therefore any images being used in a container, whether they have been exited or currently running, will NOT be affected.

Safest and Easiest way to cleanup Dangling Images

docker images --quiet --filter=dangling=true | xargs --no-run-if-empty docker rmi

Docker images consist of multiple layers. Dangling images, are layers that have no relationship to any tagged images. They no longer serve a purpose and consume disk space.

Note: I recommend not to use prune in production, because docker system prune -a will remove all the images which are not referenced by the container, by which we can't roll back to the previous release.

To list dangling images by adding the filter flag, -f with a value of dangling=true to the docker images.

List Dangling images

docker images -f dangling=true

Remove Dangling Images

docker rmi $(docker images -f dangling=true -q)

OR

docker images --quiet --filter=dangling=true | xargs --no-run-if-empty docker rmi

When we run any cron jobs to delete tha dangling stuff use the above to make sure the job runs successfully. Like in Jenkins if we run a free style job with beloow commad job will never fail even if no dangling stuff exists in the machine.

This is the safest and easiest way to cleanup dangling images and get back our disk space back for use.

Images in docker are referenced by a sha256 digest, often referred to as the image id. That digest is all you need for the image to exist on the docker host. Typically, you will have tags that point to these digests, e.g. the tag busybox:latest current points to image id c30178c523... on my system. Multiple tags can point to the same image, and any tag can be changed to point to a different id, e.g. when you pull a new copy of busybox:latest or build a new version of your application image.

Dangling images are images which do not have a tag, and do not have a child image (e.g. an old image that used a different version of FROM busybox:latest), pointing to them. They may have had a tag pointing to them before and that tag later changed. Or they may have never had a tag (e.g. the output of a docker build without including the tag option). These are typically safe to remove as long as no containers are still running that reference the old image id. The main reason to keep them around is for build caching purposes.

In addition, you may have downloaded images that you are not currently used by containers (including stopped containers). These are entirely different from dangling images and may be safe to remove as long as you don't plan to use them in the future or don't mind downloading another copy when you do need them.

Dangling images are layers that have no relationship to any tagged images. They no longer serve a purpose and consume disk space.

An unused image is an image that has not been assigned or used in a container.

List Dangling images

docker images -f dangling=true

dangling images are untagged images. Following command gives list of dangling images.

docker images --filter "dangling=true"

docker image prune deletes all dangling images.

Unused images are images that have tags but currently not being used as a container. You may or may not need it in future.

docker image prune -a delete all dangling as well as unused images.

You generally don't want to remove all unused images until some time. Hence it is better to remove with a filter.

docker image prune -f --filter "until=6h"

I saw useful commands (aliases) for removing dangling images, courtesy of andyneff here: https://forums.docker.com/t/how-to-delete-cache/5753:

alias docker_clean_images='docker rmi $(docker images -a --filter=dangling=true -q)' 
alias docker_clean_ps='docker rm $(docker ps --filter=status=exited --filter=status=created -q)' 

The first one cleans all dangling images. This is useful for removing intermediate images left over from multiple builds. The second one is for removing stopped containers. These are aliases I use for routine maintenance

If you want to remove ALL of your cache, you first have to make sure all containers are stopped and removed, since you cannot remove an image in use by a container. So something similar

docker kill $(docker ps -q) docker_clean_ps docker rmi $(docker images
-a -q)

This would kill and remove all images in your cache.