Dockerfile - Optional runtime parameter passed to command-CodePudding

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

CodePudding user response：

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

CodePudding user response：

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.