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I have a Dockerfile where an ARG is used in the CMD instruction:
ARG MASTER_NAME
CMD spark-submit --deploy-mode client --master ${MASTER_URL}
The arg is passed via docker-compose:
spark:
build:
context: spark
args:
- MASTER_URL=spark://master:7077
However, the ARG does not seem to get expanded for CMD. After I docker-compose up.
Here's what inspect shows:
docker inspect -f "{{.Name}} {{.Config.Cmd}}" $(docker ps -a -q)
/spark {[/bin/sh -c spark-submit --deploy-mode client --master ${MASTER_URL}]}
644
ARG instruction defines a variable that can be passed at build time. Once it is defined in the Dockerfile you can pass with this flag --build-arg while building the image. We can have multiple ARG instruction in the Dockerfile. ARG is the only instruction that can precede the FROM instruction in the Dockerfile.
When building a Dockerfile, the CMD instruction specifies the default program that will execute once the container runs. A quick point to note: CMD commands will only be utilized when command-line arguments are missing.
Example of using CMD and ENTRYPOINT togetherIf both ENTRYPOINT and CMD are present, what is written in CMD is executed as an option of the command written in ENTRYPOINT. If an argument is added at the time of docker run , the contents of CMD will be overwritten and the ENTRYPOINT command will be executed.
The thing is that args only can be used at build time, and the CMD is executing at run time. I guess that the only approach now to achieve what you want is setting an environment variable in the Dockerfile with the MASTER_NAME value.
ARG MASTER_NAME
ENV MASTER_NAME ${MASTER_NAME}
CMD spark-submit --deploy-mode client --master ${MASTER_NAME}
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