Question 81471 in Docker Question posted in
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Dockerfile – Optional runtime parameter passed to command

I need to pass optional, runtime parameter to a command in Docker.

The idea is that if PARAM env variable is set when docker is being run – it should be passed to java command as --key VALUE , and when runtime parameter is not set – it shoulddn’t pass anything – in particular it shouldn’t pass --key parameter name.

I.e. it should run following command if PARAM env variable is set:
/bin/java -jar artifact.jar --key $PARAM

And following if it’s not:
/bin/java -jar artifact.jar

I wanted to use :+ syntax, but it’s resolved during build time, which means it won’t be affected by runtime env variable.

docker build -t test-abc . && docker run -e "PARAM=oooo" test-abc

FROM openjdk:17

ENV PARAM=${PARAM:+"--key $PARAM"}


ENTRYPOINT /bin/java -jar artifact.jar $PARAM
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2

Answers


  1. 0

    Prepare a script that will correctly handle arguments:

    #!/bin/bash
args=()
if [[ -v PARAM ]]; then
   args=(--key "$PARAM")
fi
/bin/java -jar artifact.jar "${args[@]}" "$@"

    And add it:

    ADD entrypoint.sh /entrypoint.sh
CMD chmod +x /entrypoint.sh
ENTRYPOINT /entrypoint.sh
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  2. 0

    If you are using ENTRYPOINT here, and you rewrite it in JSON-array syntax, then you can pass additional arguments to your program after the docker run image-name. (Technically they replace the CMD.)

    # Dockerfile: do use JSON-array syntax; do not attempt to handle options;
# must use ENTRYPOINT for "container-as-command" layout
ENTRYPOINT ["java", "-jar", "artifact.jar"]

    docker run test-abc --key oooo

    You can also write the parameter expansion directly inside the command string. This requires you to use shell-syntax ENTRYPOINT or CMD, and so will essentially forbid you from providing extra options you don’t set up this way. The ${VARIABLE:+value} syntax is supported in the POSIX specification so this should work with all Linux distribution base images, even if they don’t include bash.

    ENTRYPOINT java -jar artifact.jar ${PARAM:=--key "$PARAM"}

    BashFAQ/050 has some more syntax suggestions, some of which are bash-specific, but the more detailed ones do require a good understanding of shell splitting and expansions.

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