What is the difference between CMD and ENTRYPOINT in a Dockerfile?

CMD sets the default command that will be executed when the container starts if the user does not specify another command. It can be overridden at runtime. ENTRYPOINT also sets a command to run, but it is not as easily overridden; it more likely defines a base command to which arguments passed at runtime are appended. CMD is often used to provide default arguments for ENTRYPOINT.

Examples:

Dockerfile with CMD:

FROM ubuntu
CMD ["echo", "Hello from CMD"]

Run:

docker run <image>
# Output: Hello from CMD

docker run <image> echo "Hello from overridden CMD"
# Output: Hello from overridden CMD

Dockerfile with ENTRYPOINT:

FROM ubuntu
ENTRYPOINT ["echo"]

Run:

docker run <image>
# Output:

docker run <image> "Hello from ENTRYPOINT"
# Output: Hello from ENTRYPOINT

Dockerfile with ENTRYPOINT and CMD:

FROM ubuntu
ENTRYPOINT ["echo"]
CMD ["Default message"]

Run:

docker run <image>
# Output: Default message

docker run <image> "Custom message"
# Output: Custom message

Summary table:

Feature	CMD	ENTRYPOINT
Purpose	Default command or arguments	Base command to execute
Override	Easily overridden at runtime	Not easily overridden, more often supplemented with arguments
Usage with ENTRYPOINT	Provides default arguments	Defines the command to which arguments from CMD or runtime are added
Formats	Executable form, Shell form	Executable form, Shell form