Execute a process in running container

You can either execute programs in the image from the command line (see above) or execute a container interactively, i.e. “enter” the container.

docker run -it ubuntu:18.04 /bin/bash

In this case, you will see the shell of the container where you can execute commands.

docker run --name myubuntu ubuntu:18.04 tail -f /dev/null
docker exec myubuntu uname -a

The first command starts an the ubuntu image and runs it as a daemon.
On this image, you can interactively execute commands.

Alternatively, you can enter a running container interactively to e.g. inspect what is executed in the container.

docker exec -it myubuntu /bin/bash