Dockerfile - Optional runtime parameter passed to command

Question

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

Answer 1

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