When you start a container with docker run it is given an unique id that you can use for interacting with the container. Let’s try to run a container from the image we just created:

docker run my_docker_conda

If everything worked run_qc.sh is executed and will first download and then analyse the three samples. Once it’s finished you can list all containers, including those that have exited.

docker container ls --all

This should show information about the container that we just ran. Similar to:

CONTAINER ID        IMAGE                    COMMAND                  CREATED             STATUS                      PORTS               NAMES
39548f30ce45        my_docker_conda     "/bin/bash -c 'bas..."    3 minutes ago       Exited (0) 3 minutes ago                             el

If we run docker run without any flags, your local terminal is attached to the container. This enables you to see the output of run_qc.sh, but also disables you from doing anything else in the meantime. We can start a container in detached mode with the -d flag. Try this out and run docker container ls to validate that the container is running.

By default, Docker keeps containers after they have exited. This can be convenient for debugging or if you want to look at logs, but it also consumes huge amounts of disk space. It’s therefore a good idea to always run with --rm, which will remove the container once it has exited.

If we want to enter a running container, there are two related commands we can use, docker attach and docker exec. docker attach will attach local standard input, output, and error streams to a running container. This can be useful if your terminal closed down for some reason or if you started a terminal in detached mode and changed your mind. docker exec can be used to execute any command in a running container. It’s typically used to peak in at what is happening by opening up a new shell. Here we start the container in detached mode and then start a new interactive shell so that we can see what happens. If you use ls inside the container you can see how the script generates file in the data, intermediate and results directories. Note that you will be thrown out when the container exits, so you have to be quick.

docker run -d --rm --name my_container my_docker_conda
docker exec -it my_container /bin/bash

Bind mounts

There are obviously some advantages to isolating and running your data analysis in containers, but at some point you need to be able to interact with the host system to actually deliver the results. This is done via bind mounts. When you use a bind mount, a file or directory on the host machine is mounted into a container. That way, when the container generates a file in such a directory it will appear in the mounted directory on your host system.

Tip
Docker also has a more advanced way of data storage called volumes. Volumes provide added flexibility and are independent of the host machine’s filesystem having a specific directory structure available. They are particularly useful when you want to share data between containers.

Say that we are interested in getting the resulting html reports from FastQC in our container. We can do this by mounting a directory called, say, fastqc_results in your current directory to the /course/results/fastqc directory in the container. Try this out by running:

docker run --rm -v $(pwd)/fastqc_results:/course/results/fastqc my_docker_conda

Here the -v flag to docker run specifies the bind mount in the form of directory/on/your/computer:/directory/inside/container. $(pwd) simply evaluates to the working directory on your computer.

Once the container finishes validate that it worked by opening one of the html reports under fastqc_results/.

We can also use bind mounts for getting files into the container rather than out. We’ve mainly been discussing Docker in the context of packaging an analysis pipeline to allow someone else to reproduce its outcome. Another application is as a kind of very powerful environment manager, similarly to how we’ve used Conda before. If you’ve organized your work into projects, then you can mount the whole project directory in a container and use the container as the terminal for running stuff while still using your normal OS for editing files and so on. Let’s try this out by mounting our current directory and start an interactive terminal. Note that this will override the CMD command, so we won’t start the analysis automatically when we start the container.

docker run -it --rm -v $(pwd):/course/ my_docker_conda /bin/bash

If you run ls you will see that all the files in the docker directory are there. Now edit run_qc.sh on your host system to download, say, 12000 reads instead of 15000. Then rerun the analysis with bash run_qc.sh. Tada! Validate that the resulting html reports look fine and then exit the container with exit.

Quick recap
In this section we’ve learned:

How to use docker run for starting a container and how the flags -d and --rm work.

How to use docker container ls for displaying information about the containers.

How to use docker attach and docker exec to interact with running containers.

How to use bind mounts to share data between the container and the host system.