There's a Docker file for SpringBoot application with this entry point:
# some lines omitted
ENTRYPOINT java -jar app.jar --spring.config.additional-location=$(ls /config/*.properties | tr '\n' ',')
On container start a host directory is mounted to /config/ directory:
docker run -p 9999:8080 -v C:/path/to/configuration/:/config my_image_name
And it works as expected, capturing all *.properties from the host directory and applying them to the app.
For readability I would like to use the format with array of strings in ENTRYPOINT like this:
ENTRYPOINT ["java", "-jar", "app.jar", "--spring.config.additional-location=$(ls /config/*.properties | tr '\n' ',')"]
Unfortunately, the expression inside $(...) is not evaluated at the container start and the application throws an exception that clearly shows the problem:
ERROR org.springframework.boot.SpringApplication - Application run failed
java.lang.IllegalStateException: Unable to load config data from '')'
How do I express the ENTRYPOINT arguments so the bash expression in $() could be evaluated as in the first case?
To use bash in entrypoint you need to run bash, and not java:
ENTRYPOINT ["/bin/bash", "-c", "java -jar app.jar --spring.config.additional-location=$(ls /config/*.properties | tr '\n' ',')"]
The first element of an entrypoint is a binary or a script (i.e. what) to be executed. The rest (including CMD) goes as arguments to it. bash -c "some string" runs a sequence of commands passed in a string and it possible to use bash expressions in it.