How can I use a variable inside a Dockerfile CMD?

Question

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

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"

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 $(eval echo "$@")

For the Java developers, following my solution below gonna work:

if you tried to run your container with a Dockerfile like below

ENTRYPOINT ["/docker-entrypoint.sh"]
# does not matter your parameter $JAVA_OPTS wrapped as ${JAVA_OPTS}
CMD ["java", "$JAVA_OPTS", "-javaagent:/opt/newrelic/newrelic.jar", "-server", "-jar", "app.jar"]

with an ENTRYPOINT shell script below:

#!/bin/bash
set -e
source /work-dir/env.sh
exec "$@"

it will build the image correctly but print the error below during the run of container:

Error: Could not find or load main class $JAVA_OPTS
Caused by: java.lang.ClassNotFoundException: $JAVA_OPTS

instead, Java can read the command line parameters either through the command line or by _JAVA_OPTIONS environment variable. so, it means we can pass the desired command line parameters through _JAVA_OPTIONS without changing anything on Dockerfile as well as to allow it to be able to start as parent process of container for the valid docker signalization via exec "$@".

The below one is my final version of the Dockerfile and docker-entrypoint.sh files:

...
ENTRYPOINT ["/docker-entrypoint.sh"]
CMD ["java", "-server", "-jar", "app.jar"]

#!/bin/bash
set -e
source /work-dir/env.sh
export _JAVA_OPTIONS="-XX:+PrintFlagsFinal"
exec "$@"

and after you build your docker image and tried to run it, you will see the logs below that means it worked well:

Picked up _JAVA_OPTIONS: -XX:+PrintFlagsFinal
[Global flags]
      int ActiveProcessorCount                     = -1                                        {product} {default}