How can I use a variable inside a Dockerfile CMD?

146
votes

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...

4
dockerdockerfile
How and when are you defining $PROJECTNAME? – Piotr Wittchen
At the beginning of my Dockerfile using ENV. Also I forgot to mention that if I remove the CMD line from the Dockerfile and then run the container, from inside the container I can run this command and it will create the project (meaning the ENV variable is valid). – david
What type of variable do you mean: dockerfile variable or environmental variable (as in your system runtime)? – emix

4 Answers

220
votes

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
31
votes

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"
12
votes

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 $(eval echo "$@")
0
votes

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 && \ 
       .....