16 Apr 2014

Some Docker Tips and Tricks

Docker is a great tool, which can be daunting at first. Shells can be annoying to work with and have their gotchas. It took me some time to figure these out and I want to spare you the time. This post has some quick tips, tricks and shell one liners to help you use Docker.

Removing All Containers and Images (Spring Cleaning)

Spring cleaning one liner:

 docker rm -v -f $(docker ps -a -q) ; docker rmi $(docker images -q -a) 

This might give a warning when there are no running containers or containers at all, but it’s a nice one liner when you’re trying things out. If you just want to remove all containers, you can run:

 docker rm -f -v $(docker ps -a -q) 

Remove Containers On Exit

If you only want to quickly run a command in a container and exit and aren’t worried about the end state, add --rm to the docker run command, this will really end up saving you a lot of containers to clean up!

Example: docker run --rm -i -t busybox /bin/bash

Commands Don’t Run in a Shell

If you docker run something that needs shell expansion, for example docker run --rm busybox ls '/var/log/*', it will fail. Why this fails took me a while to figure out. The gotcha here is: you don’t have a shell, the * is shell expansion and so you need a shell to be able to use that. The proper way to do this is:

docker run --rm busybox sh -c 'ls /var/log/*'

Boot2Docker and Laptops on the Move Means DNS Woes

I have about three locations where I use my laptop and they’re all on different ISPs. Boot2docker tends to hold on to DNS servers for a bit too long and because of that, you might get weird errors when trying to build images. If you see

cannot lookup archive.ubuntu.com

on Ubuntu or something similar on CentOS, it might be wise to stop and start your boot2docker just to be sure:

boot2docker-cli down && boot2docker-cli up

Things should work again after that.

Volumes Beat docker logs and docker copy

If you want to monitor log files or use files from one container on another, have a look at volumes. For example, to check if Tomcat has started up:

tomcat_id=$(docker run -d -v /var/log/tomcat6 wouterd/tomcat6)
# Give Tomcat some time to wake up...
sleep 5
while ! docker run --rm --volumes-from ${tomcat_id} busybox /bin/sh -c "grep -i -q 'INFO: Server startup in' /var/log/tomcat6/catalina*.log" ; do
    echo -n "."
    sleep 5

You can also specify the volumes to share inside a Dockerfile, which I did in my previous blog post on continuous integration with docker.

Docker Inspect Plus Go Templates Equals Profit

The command docker inspect allows the use of Go templates to format the output of the inspect command. If you’re good at that, you can get a lot of great information out of docker containers from the commandline in shell scripts. Here’s one to get the IP of a running container:

container_ip=$(docker inspect --format '{{.NetworkSettings.IPAddress}}' ${container_id}) 

Here’s a very silly one to get the host:port mapping of all exposed ports and put them in a java properties file:

template='{{ range $key, $value := .NetworkSettings.Ports }}{{ $key }}='"${BOOT_2_DOCKER_HOST_IP}:"'{{ (index $value 0).HostPort }} {{ end }}'
tomcat_host_port=$(docker inspect --format="${template}" ${container_id})
for line in ${tomcat_host_port} ; do
    echo "${line}" >> ${work_dir}/docker_container_hosts.properties

See also

My post on continuous integration using docker and maven

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