Summary

The time example introduced you to the difference between ENTRYPOINT and CMD, and how you can use both together.

In summary, use CMD when you might want to run other processes in the container. Some containers are specifically designed to make multiple tool commands available, so using CMD is convenient.

Use ENTRYPOINT when you want a container to behave like a specific tool and you want to easily supply arguments without inadvertently overriding the default command.

Use ENTRYPOINT and CMD together when you want the behavior of using ENTRYPOINT, but want to specify potentially more appropriate default arguments using CMD that will override the containerized program’s defaults.

🤯

The nice thing about this combination is that you can still override the CMD defaults (which override the program’s defaults) when you launch a container!

For options that should not be overridden, they should be specified as part of the ENTRYPOINT instruction. Any options set using CMD or when running a container will be appended to the ENTRYPOINT options.

Overriding the ENTRYPOINT when running a container

Finally, as with CMD, you can even override the ENTRYPOINT when you run a container. It just takes requires a little more effort on the command line.

You use the --entrypoint option to specify the command (ex, echo), and then supply an argument after the image name (ex, hello) as usual.

docker run -it --rm --name timectr --entrypoint echo get-time hello

hello