Docker - The 'docker' command
The ‘docker’ command
Having a Dockerfile is great but it does nothing by itself - it’s just a plain text file. To use it you will need to invoke ‘docker’ and ask it to build the image you want to use.
This starts a new container using an image. A very simple example of this is:
docker run -it ubuntu /bin/bash
This tells docker to:
run a container from an image
the ‘-it’ instructs it to be interactive and use the TTY; this is so we can type commands and get responses.
use image ‘ubuntu’
start the command /bin/bash
The image will be downloaded from hub.docker.com if you don’t already have one labelled ‘ubuntu’
As you might expect, this builds an image from a Dockerfile. It doesn’t upload it anywhere, it just assembles it for use as containers.
docker build -t antonyh/sprocketserver ./sprocketserver
This will build an image as follows:
‘build’ is the command to Docker
the -t sets the tag name; we need this to create the container
look for a Dockerfile in directory ./sprocketserver or in other words, use ./sprocketserver/Dockerfile as the source to build the image.
This loads up the Dockerfile, processes it, and records the image as ‘antonyh/sprocketserver’. We can start this in the background with a -d flag:
docker run -d antonyh/sprocketserver
Note that we don’t have to tell it what command to run; this is set in the Dockerfile, although we could still override it here if we wanted to. The ‘-d’ tells Docker to ‘detach’, so it runs in the background and the output from Docker is the container ID instead of log output.
You can list the running dockers with ‘docker ps’ or all containers including terminated with ‘docker ps -a’. You’ll use this command a lot to get container IDs.
If you started Docker as a detached process, this will give you the log (stdout / stderr) output so you can find out what the container is doing. There’s an option to ‘follow’ the logs just like the ‘tail’ shell command:
docker logs -f <containerid>
This is a useful command for obvious reasons. You need to stop containers before you can remove them.
Stopped containers can be removed with ‘docker rm’. Note that data volumes will be dropped if this is the last container that references them, so be careful not to lose data when removing unwanted containers.
Using ‘docker images’ will give a list of all the local images available together with some information such as size.
Removing images is possible with ‘docker rmi’.
The help provided with Docker is your friend, and is the equivalent to ‘man’ pages. Use it liberally, there’s a lot of information available and will be current to your version of Docker. On the downside, I personally find it a little dry.