What is the difference between CMD and ENTRYPOINT in a Dockerfile?

Both CMD and ENTRYPOINT instructions define what command gets executed when running a container. There are few rules that describe their co-operation.

1. Dockerfile should specify at least one of CMD or ENTRYPOINT commands.
2. ENTRYPOINT should be defined when using the container as an executable.
3. CMD should be used as a way of defining default arguments for an ENTRYPOINT command or for executing an ad-hoc command in a container.
4. CMD will be overridden when running the container with alternative argumen

Docker has a default entrypoint which is /bin/sh -c but does not have a default command. When you run docker like this:

docker run -i -t ubuntu bash

The entrypoint is the default /bin/sh -c, the image is ubuntu and the command is bash. The command is run via the entrypoint. i.e., the actual thing that gets executed is /bin/sh -c bash. This allowed Docker to implement RUN quickly by relying on the shell’s parser. Later on, people asked to be able to customize this, so ENTRYPOINT and --entrypoint were introduced.