Images versus containers

This explains the difference between a Docker image and a container. These terms are sometimes used interchangeably however they are not the same and have two different meanings.

Previously you downloaded the busybox image and showed that it was present on your computer. Run these two commands if you haven't done so already.

docker pull busybox
docker images

When you run a Docker image using docker run you are creating a container from that instance. This can be illustrated by creating a container that sleep for 60 seconds. The flag --detach means the Docker process will run in the background and you may still continue to type in the terminal.

docker run --detach busybox sleep 60

After you have started this command, you can list the running containers on your system using the docker ps command. You should see your busybox container and the time it was created.

docker ps

Remember this running container was started from the busybox image. Images are like blueprints for a 'box' with all the software and data inside. When you start an image using docker run you are creating a container from that image blueprint. You can illustrate this further by creating two busybox containers running at the same time.

docker run --detach busybox sleep 60
docker run --detach busybox sleep 60
docker ps

In the output here you can see the created containers will have an ID like 'cd2e3b4c60b4' and a name like 'determined_mayer'. There are used to manage multiple containers created from the same image.

Data in containers

A further point to clarify in the relationship between images and containers is that filesystem changes made in a container do not affect the image. This can be initially confusing because if you create a file in a container, then create a new container the files are not there. This is because the 'blueprint' image has not been changed, and the created container has not changed.

We can illustrate this with an example. The touch command is used to create files. We will use touch to create a busybox container then list the container contents.

docker run busybox sh -c "touch i_made_a_file && ls -l"

The && symbols are used to combined two commands together into a single line. This is part of bash and can be done on your own computer, this is not specific to Docker. You should see the created file i_made_a_file in the container file system. Now repeat the listing command again.

docker run busybox ls -l

The file i_made_a_file does not appear. This is because everytime you use docker run you create a new, fresh container from the image blueprint. If you are ever confused about why data is missing in a container, this may be the cause.

Exercises

• Create a sleeping busybox container using --detach, then stop it early using docker kill. You'll need to get the container ID or name from using docker ps.

• Do the same thing with docker pause and docker unpause. This illustrates how you can manage running containers on your system.

• Change the internal state of a container again using touch. Then use docker diff to show which file have changed inside the container. This exercise may be tricky. You will have to work out how to identify your container after is no longer running. Try docker help ps for help.