ENTRYPOINT vs CMD in Dockerfile

CMD (Command)

• Purpose: Provides default arguments for the container.


• Usage: Use CMD when you want to provide default behavior but allow it to be overridden by arguments supplied when running the container (docker run).

Example 1: Default command with override

FROM ubuntu:latest

CMD ["echo", "Hello, World!"]

• If you run the container as docker run <image>, it will print Hello, World!.


• If you provide a different command, like docker run <image> echo "Goodbye", it will override the CMD.

ENTRYPOINT

• Purpose: Sets the main executable for the container that is not easily overridden.


• Usage: Use ENTRYPOINT when you want to enforce a specific command or script as the container's entry point. Additional arguments provided with docker run are passed as arguments to the ENTRYPOINT.

Example 2: Enforce entry point

FROM ubuntu:latest

ENTRYPOINT ["echo"]

• Running docker run <image> Hello, World! will always use echo, so the result will be Hello, World!.


• You cannot replace echo unless you use the --entrypoint flag.

Combining ENTRYPOINT and CMD

• Combine ENTRYPOINT and CMD for flexible and reusable images.


• ENTRYPOINT specifies the main command, and CMD provides default arguments that can be overridden.

Example 3: Flexible combination

FROM ubuntu:latest

ENTRYPOINT ["echo"]

CMD ["Hello, World!"]

• Running docker run <image> will print Hello, World!.


• Running docker run <image> Goodbye will print Goodbye, overriding CMD but not ENTRYPOINT.

Key Differences

When to Use Which?

• Use CMD when you want to provide a default, flexible behavior that users can easily override.


• Use ENTRYPOINT when you want to enforce a specific script or command while still allowing extra arguments.