Docker ENTRYPOINT vs CMD

Both ENTRYPOINT and CMD allow you to specify the startup command for an image, but there are subtle differences between them. There are many times where you’ll want to choose one or the other, but they can also be used together.

Override

The ENTRYPOINT or CMD that you specify in your Dockerfile identify the default executable for your image. However, the user has the option to override either of these values at run time.

The recommendation is use CMD in your Dockerfile when you want the user of your image to have the flexibility to run whichever executable they choose when starting the container.

In contrast, ENTRYPOINT should be used in scenarios where you want the container to behave exclusively as if it were the executable it’s wrapping.

Shell vs. Exec

Both the ENTRYPOINT and CMD instructions support two different forms: the shell form and the exec form.

• Shell form: CMD executable param1 param2
• Exec form: CMD ["executable","param1","param2"]

Shell form always start a shell to execute the command.

Whether you’re using ENTRYPOINT or CMD (or both) the recommendation is to always use the exec forms so that’s it’s obvious which command is running as PID 1 inside your container.

ENTRYPOINT and CMD

Combining ENTRYPOINT and CMD allows you to specify the default executable for your image while also providing default arguments to that executable which may be overridden by the user. CMD will be treated as parameters of ENTRYPOINT

FROM ubuntu:trusty
ENTRYPOINT ["/bin/ping","-c","3"]
CMD ["localhost"]

docker build -t ping .

You can run docker run ping google.ca to override cmd, then the command in docker will be /bin/ping -c 3 google.ca