What is the purpose of the CMD directive in a Dockerfile?

The CMD directive in a Dockerfile is specifically designed to set the default command that gets executed when a container is run without a command specified. This means that when you start a container from the built image, if you do not provide an alternate command or arguments in your docker run command, the command defined in the CMD directive will be executed by default.

It's important to note that CMD can be overridden by providing a command in the docker run command, which gives it flexibility in container usage while still providing sensible defaults that aid in the proper execution of the containerized application.

While defining environment variables or specifying an entrypoint are valid aspects of Dockerfile functionality, these actions are handled by other directives such as ENV for setting environment variables and ENTRYPOINT for specifying the initial command that will execute when the container runs. The creation of the initial filesystem is handled through the base image and additional commands like RUN, COPY, or ADD, which modify the filesystem in the image layer. Understanding the role of CMD helps in efficiently managing the behavior of Docker containers in a predictable manner.