How do I run a command on an already existing Docker container?

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In October 2014 the Docker team introduced docker exec command: https://docs.docker.com/engine/reference/commandline/exec/

So now you can run any command in a running container just knowing its ID (or name):

docker exec -it <container_id_or_name> echo "Hello from container!"

Note that exec command works only on already running container. If the container is currently stopped, you need to first run it with the following command:

docker run -it -d shykes/pybuilder /bin/bash

The most important thing here is the -d option, which stands for detached. It means that the command you initially provided to the container (/bin/bash) will be run in the background and the container will not stop immediately.

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