ENTRYPOINT vs CMD

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.

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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.

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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.

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Key Differences

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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.