How can I use a variable inside a Dockerfile CMD?

Question

Inside my Dockerfile:

ENV PROJECTNAME mytestwebsite
CMD ["django-admin", "startproject", "$PROJECTNAME"]

Error:

CommandError: '$PROJECTNAME' is not a valid project name

What is the quickest workaround here? Does Docker have any plan to "fix" or introduce this functionality in later versions of Docker?

NOTE: If I remove the CMD line from the Docker file and then run the Docker container, I am able to manually run Django-admin startproject $PROJECTNAME from inside the container and it will create the project...

Answer 1

Lets say you want to start a java process inside a container:

Example Dockerfile excerpt:

ENV JAVA_OPTS -XX +UnlockExperimentalVMOptions -XX:+UseCGroupMemoryLimitForHeap -XX:MaxRAMFraction=1 -XshowSettings:vm 
... 
ENTRYPOINT ["/sbin/tini", "--", "entrypoint.sh"] 
CMD ["java", "${JAVA_OPTS}", "-myargument=true"]

Example entrypoint.sh excerpt:

#!/bin/sh 
... 
echo "*** Startup $0 suceeded now starting service using eval to expand CMD variables ***"
exec su-exec mytechuser $(eval echo "$@")

Answer 2

Inspired on above, I did this:

#snapshot by default. 1 is release.
ENV isTagAndRelease=0

CMD     echo is_tag: ${isTagAndRelease} && \
        if [ ${isTagAndRelease} -eq 1 ]; then echo "release build"; mvn -B release:clean release:prepare release:perform; fi && \
        if [ ${isTagAndRelease} -ne 1 ]; then echo "snapshot build"; mvn clean install; fi && \ 
       .....

Answer 3

When you use an execution list, as in...

CMD ["django-admin", "startproject", "$PROJECTNAME"]

...then Docker will execute the given command directly, without involving a shell. Since there is no shell involved, that means:

• No variable expansion
• No wildcard expansion
• No i/o redirection with >, <, |, etc
• No multiple commands via command1; command2
• And so forth.

If you want your CMD to expand variables, you need to arrange for a shell. You can do that like this:

CMD ["sh", "-c", "django-admin startproject $PROJECTNAME"]

Or you can use a simple string instead of an execution list, which gets you a result largely identical to the previous example:

CMD django-admin startproject $PROJECTNAME

Answer 4

If you want to use the value at runtime, set the ENV value in the Dockerfile. If you want to use it at build-time, then you should use ARG.

Example :

ARG value
ENV envValue=$value
CMD ["sh", "-c", "java -jar ${envValue}.jar"]

Pass the value in the build command:

docker build -t tagName --build-arg value="jarName"