350-901 Question #7: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. Which additional line results in the output of Test 1 upon execution of the docker run -rm devnet 1 command in a Dockerfile with this content? FROM alpine:3.7 RUN apk add --no-cache bash

Options

• ACMD ["/bin/echo", "Test"]
• BRUN ["/bin/echo", "Test"]
• CENTRYPOINT ["/bin/echo", "Test"]
• DCMD ["/bin/echo Test"]

Explanation

To achieve the output "Test 1" when running docker run -rm devnet 1, the Dockerfile must use ENTRYPOINT ["/bin/echo", "Test"] so that the 1 passed during docker run is appended as an argument to the entrypoint command.

Common mistakes.

• A. If CMD ["/bin/echo", "Test"] is used without an ENTRYPOINT, the arguments provided to docker run (in this case, 1) completely override the CMD instruction, leading to an attempt to execute 1 which would fail.
• B. The RUN instruction executes commands during the Docker image build process, not when the container is run, so it would not define the runtime command for the container.
• D. CMD ["/bin/echo Test"] uses the shell form, which would typically be overridden by docker run arguments. Even if not overridden, it would output "Test" only and not accept "1" as a separate argument to echo.

Concept tested. Dockerfile ENTRYPOINT vs CMD

Reference. https://docs.docker.com/engine/reference/builder/#entrypoint

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