Dockerfile CMD not accepting variables for substitution

Question

Unable to use variables on the CMD line:

#CMD ["/opt/jdk/bin/java", "-jar", "${ARTIFACTID}-${VERSION}.${PACKAGING}"]
CMD ["/opt/jdk/bin/java", "-jar", "ssltools-domain-LATEST.jar"]

Latest line works, not the first.

Any help or workaround very appreciated

Answer 1

When you write the arguments to CMD (or ENTRYPOINT) as a JSON string, as in...

CMD ["/opt/jdk/bin/java", "-jar", "ssltools-domain-LATEST.jar"]

...the command is executed directly with the exec system call and is not processed by a shell. That means no i/o redirection, no wildcard processing...and no variable expansion. You can fix this in a number of ways:

• You can just write it as a plain string instead, as in:

    CMD /opt/jdk/bin/java -jar ${ARTIFACTID}-${VERSION}.${PACKAGING}
  

  When the argument is not a JSON construct, it gets passed to sh -c.

• You can do as suggested by Philip, and pass the arguments to sh -c yourself:

    CMD ["sh", "-c", "/opt/jdk/bin/java -jar ${ARTIFACTID}-${VERSION}.${PACKAGING}"]
  

  Those two options are basically equivalent.

• A third option is to put everything into a shell script, and then run that:

    COPY start.sh /start.sh
    CMD ["sh", "/start.sh"]
  

  This is especially useful if you need to perform more than a simple command line.