Dockerfile with both

What if we want to override the container to behave the way it does with an ENTRYPOINT, but we want to override the default frequency of 5 seconds?

You use both ENTRYPOINT and CMD.

• Dockerfile.using_both

FROM debian
COPY get-time.sh /usr/local/bin
RUN chmod +x /usr/local/bin/get-time.sh
ENTRYPOINT [ "/usr/local/bin/get-time.sh" ]
CMD [ "2" ]

Rebuild the Docker image.

docker build -f Dockerfile.using_entrypoint -t get-time .

Run a container.

docker run -it --rm --name timectr get-time

• Output

Fetch UTC(NIST) time every 2 seconds...
21-09-02 19:30:53
^C

What happens here is that the value for CMD acts as a default argument that will be passed to the command specified by ENTRYPOINT. You can still pass an argument when you create the container and it will just override CMD’s default value.

docker run -it --rm --name timectr get-time 3

• Output

Fetch UTC(NIST) time every 3 seconds...