What is the difference between CMD and RUN commands in a Dockerfile, and what is each of them used for?

In the Dockerfile:

• CMD sets the default command that will be executed when the container starts. It can be overridden when running the container with docker run.

• RUN executes a command during the image build. It is used for installing packages, configuring the environment, etc. Each RUN command creates a new layer in the image.

Example:

FROM ubuntu:20.04
RUN apt-get update && apt-get install -y curl  # executed during build
CMD ["/bin/bash"]  # default command when starting the container

Thus, RUN is for preparing the image, CMD is for setting the container's behavior upon startup.