Debugging a Running Container

It can be difficult to get everything right in your Dockerfile. It’s helpful to be able to get a running shell inside of the container that will let you explore what’s wrong and try some fixes. In this lab you’ll learn how to use docker exec for debugging.

Run an Existing Container

Use docker run to start the myapp container you built in the Build a Dockerfile lab. This time we’ll run the container in detached mode so the prompt will return.

$ docker run -d -p 8000:8000 myapp 

Verify the container is running:

$ docker ps
CONTAINER ID   IMAGE     COMMAND                  CREATED         STATUS        PORTS                                       NAMES
6191357d529c   myapp     "/bin/sh -c 'python …"   2 seconds ago   Up 1 second   0.0.0.0:8080->8000/tcp, :::8080->8000/tcp   romantic_elion

Note

Make a note of your container’s unique name! You will need to substitute it for romantic_elion in future commands.

Connect to the Container

We can create a new process in the container with docker exec. The most useful is bash or sh depending on the image. The -it switches are mandatory when you want to run a shell, they enable the connection of keyboard signals (e.g. Ctrl-c) to the process in the container.

$ docker exec -it romantic_elion /bin/bash
root@9dbc31635eda:/mysite# 

Notice that the prompt changed!

Get Set for Debugging

The Python container is based on a Debian image and has apt. Let’s use apt to install vim.

root@9dbc31635eda:/mysite# apt update
root@9dbc31635eda:/mysite# apt install vim 

Edit urls.py

Now that you have vim, use it to edit mysite/urls.py:

root@9dbc31635eda:/mysite# vim mysite/urls.py

There’s no need to make a change. Just note that any changes you make are only to this running container. No future containers made with docker run will have this change unless you also edit your source code or Dockerfile.

Clean Up

To stop and remove your container use docker rm:

$ docker rm -f romantic_elion