Docker: understanding ENTRYPOINT and CMD instructions

Question

I'd like to ask some question about ENTRYPOINT and CMD instructions available for use in a Dockerfile.

1. Providing that I'm mounting local directories as volumes in a container using fig or docker-compose. When exactly are ENTRYPOINT and CMD instructions executed?
   • After the volumes were mounter or before?
2. If I pass a bash script to ENTRYPOINT, will this script be executed each time a container is started?
3. If there is a bash script added as ENTRYPOINT, will all commands executed with docker run or docker exec be passed as arguments to this script ?
4. When exactly are CMD instauctions executed? Once a container was started and volumes mounted ?
5. Why can there only be one CMD in a Dockerfile? What if I want to start a container with several processes / run severa exacutables?

Answer 1

1) ENTRYPOINT and CMD are executed in the order they appear in the Dockerfile, regardless of the volumes mount

2) if you have an ENTRYPOINT launching a verb, you can pass a parameter

3) yes for docker run but some examples might clarify this, and docker exec just gets you inside the container

4) CMD executes when a container is launched

5) you can use several CMD in a Dockerfile, but only the last one will be used, docker is designed to run one process,

if you want to run several, you will need some tools such as supervisor http://docs.docker.com/articles/using_supervisord or runit or s6 or daemontools see http://docs.docker.com/faq

As CMD is easily overriden and not ENTRYPOINT (unless you docker run --entrypoint) usually you have ENTRYPOINT as the last by one line in your Dockerfile and CMD as the last line, being in fact the parameter, that can change