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

Question

In Dockerfiles there are two commands that look similar to me: CMD and ENTRYPOINT. But I guess that there is a (subtle?) difference between them -

Answer 1

Yes, that is a good question. I don't understand it fully yet, but:

I understand that ENTRYPOINT is the binary that is being executed. You can overide entrypoint by --entrypoint="".

docker run -t -i --entrypoint="/bin/bash" ubuntu

CMD is the default argument to container. Without entrypoint, default argument is command that is executed. With entrypoint, cmd is passed to entrypoint as argument. You can emulate a command with entrypoint.

# no entrypoint
docker run ubuntu /bin/cat /etc/passwd

# with entry point, emulating cat command
docker run --entrypoint="/bin/cat" ubuntu /etc/passwd

So, main advantage is that with entrypoint you can pass arguments (cmd) to your container. To accomplish this, you need to use both:

# Dockerfile
FROM ubuntu
ENTRYPOINT ["/bin/cat"]

and

docker build -t=cat .

then you can use:

docker run cat /etc/passwd
#              ^^^^^^^^^^^
#                   CMD
#          ^^^      
#          image (tag)- using the default ENTRYPOINT