Difference between CMD and ENTRYPOINT in Docker

Every commands, either passed as an argument or specified from CND instruction are passed as argument of binary specified in ENTRYPOINT.

/bin/sh -c is the default entrypoint. So if you specify CMD date without specifying entrypoint, Docker executes it as /bin/sh -c date.

By using entrypoint, you can change the behaviour of your container at run time that makes container operation a bit more flexible.

["/bin/date"]

With the entrypoint above, the container prints out current date with different format.

docker run -i clock_container +"%s"
docker run -i clock_container +"%F"

You can only have one of each of CMD and ENTRYPOINT in your Dockerfile. CMD has a few different forms: it can specify an executable, or just specify parameters for the executable in ENTRYPOINT. ENTRYPOINT always specifies the executable as its first argument.

CMD [“apachectl”, “-D”, “FOREGROUND”]

or

CMD [“-D”, “FOREGROUND”]
ENTRYPOINT [“apachectl”]

As you can see, each token in the full command is specified as a different argument to CMD and ENTRYPOINT and wrapped in double quotes. All arguments are enclosed within brackets.