Docker Command vs Entrypoint - Understanding how to control your container's behavior

 When creating Docker images, we have the power to define how the container will behave when it starts. Two key instructions in a Dockerfile, CMD and ENTRYPOINT, play a crucial role in this behavior.
While both seem similar at first, they serve distinct purposes and understanding their differences is essential for building effective Docker images.

CMD: The Default Command:
Think of CMD as the default command that the container will execute when it starts.
It's like setting a preferred program to launch when you turn on your computer. However, just like opening a different program on our computer, CMD can be overridden by providing a different command when starting the container.

For an example; Consider a Docker file for a simple web server;

FROM nginx:latest
CMD ["nginx", "-g", "daemon off;"]

This Dockerfile uses CMD to specify that the nginx command should run by default when the container starts. However, if you run the container with a different command, like bash, the CMD will be overridden, and the container will start a bash shell instead.

ENTRYPOINT: The Inflexible One:
ENTRYPOINT, on the other hand, defines the main purpose of your container.
It's like hardcoding the computer to always run a specific program, regardless of any other commands you try to give it. ENTRYPOINT cannot be easily overridden when starting the container.

For an exanple; Consider we want to create a container that always runs a spedfic Python script the docker file should be;

FROM python:3.9
ENTRYPOINT ["python", "my_script.py"]

In above use case, even if we try to start the container with a different command, the ENTRYPOINT will ensure that your Python script is always executed.

How to make CMD and ENTRYPOINT work together?
CMD and ENTRYPOINT can also work together. ENTRYPOINT sets the main command, while CMD provides default arguments for that command. This combination offers flexibility while ensuring the container's primary function remains intact.

We could use CMD and ENTRYPOINT together in a Docker file similar as below;

FROM python:3.9
ENTRYPOINT ["python", "my_script.py"]
CMD ["--verbose"]

Here, ENTRYPOINT ensures that my_script.py is always executed, while CMD provides the default argument --verbose. You can still override the CMD arguments when starting the container, but the script will always run.