Chris Sainty

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Useful Docker Commands

Today I'll share some quick, helpful, docker commands I use in my workflow. These help keep my development and production machines clean of old and unused images.

I'm no bash expert, so tweet me or make a PR if you have any suggestions.

Remove dangling images

docker rmi $(docker images --filter="dangling=true" -q)

In development you will generally be experimenting building images and you tend to re-use tags while tweaking and perfecting your build.
Each time you re-use a tag though, it does not delete the old image, it just untags it. These are called dangling images and can be seen like this

$ docker images
REPOSITORY                      TAG                 IMAGE ID    
<none>                          <none>              bc53f0c6cce0
mysql                           5.6                 01bbb21c400c
centos                          7                   05188b417f30

Luckily these are easy to filter on with docker images --filter="dangling=true", and once you can list something, you can easily use an expansion to do something useful with it, like rmi to remove the images.

$ docker rmi $(docker images --filter="dangling=true" -q)
Deleted: sha256:bc53f0c6cce0
Deleted: sha256:2ac48b1345ab

Remove unused images

docker rmi $(grep -xvf <(docker ps -a --format '{{.Image}}' | sed 's/:latest//g') <(docker images | tail -n +2 | grep -v '<none>' | awk '{ print $1":"$2 }' | sed 's/:latest//g'))

In production, however, the problem is different. Assuming you use versioned tags, then you are going to end up with a lot of old images around when new versions have been deployed.

Let's break the command down going from right to left

First, find all the images on the system

$ docker images | tail -n +2 | grep -v '<none>' | awk '{ print $1":"$2 }' | sed 's/:latest//g'

Next, find all the images which have containers based off them

$ docker ps -a --format '{{.Image}}' | sed 's/:latest//g'

Then, feed these two lists in to grep to find the mismatch

$ grep -xvf <(docker ps -a --format '{{.Image}}' | sed 's/:latest//g') <(docker images | tail -n +2 | grep -v '<none>' | awk '{ print $1":"$2 }' | sed 's/:latest//g')

Finally this list is fed in to the rmi call.