How do I use Docker environment variable in ENTRYPOINT array?

Question

You're using the exec form of ENTRYPOINT. Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, ENTRYPOINT [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: ENTRYPOINT [ "sh", "-c", "echo $HOME" ].
When using the exec form and executing a shell directly, as in the case for the shell form, it is the shell that is doing the environment variable expansion, not docker.(from Dockerfile reference)

In your case, I would use shell form

ENTRYPOINT ./greeting --message "Hello, $ADDRESSEE\!"

I tried to resolve with the suggested answer and still ran into some issues...

This was a solution to my problem:

ARG APP_EXE="AppName.exe"
ENV _EXE=${APP_EXE}

# Build a shell script because the ENTRYPOINT command doesn't like using ENV
RUN echo "#!/bin/bash \n mono ${_EXE}" > ./entrypoint.sh
RUN chmod +x ./entrypoint.sh

# Run the generated shell script.
ENTRYPOINT ["./entrypoint.sh"]

Specifically targeting your problem:

RUN echo "#!/bin/bash \n ./greeting --message ${ADDRESSEE}" > ./entrypoint.sh
RUN chmod +x ./entrypoint.sh
ENTRYPOINT ["./entrypoint.sh"]

After much pain, and great assistance from @vitr et al above, i decided to try

• standard bash substitution
• shell form of ENTRYPOINT (great tip from above)

and that worked.

ENV LISTEN_PORT=""

ENTRYPOINT java -cp "app:app/lib/*" hello.Application --server.port=${LISTEN_PORT:-80}

e.g.

docker run --rm -p 8080:8080 -d --env LISTEN_PORT=8080 my-image

and

docker run --rm -p 8080:80 -d my-image

both set the port correctly in my container

Refs

see https://www.cyberciti.biz/tips/bash-shell-parameter-substitution-2.html

For me, I wanted to store the name of the script in a variable and still use the exec form.

Note: Make sure, the variable you are trying to use is declared an environment variable either from the commandline or via the ENV directive.

Initially I did something like:

ENTRYPOINT [ "${BASE_FOLDER}/scripts/entrypoint.sh" ]

But obviously this didn't work because we are using the shell form and the first program listed needs to be an executable on the PATH. So to fix this, this is what I ended up doing:

ENTRYPOINT [ "/bin/bash", "-c", "exec ${BASE_FOLDER}/scripts/entrypoint.sh \"${@}\"", "--" ]

Note the double are required

What this does is to allow us to take whatever extra args were passed to /bin/bash, and supply those same arguments to our script after the name has been resolved by bash.

---

man 7 bash

--
A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as filenames and arguments. An argument of - is equivalent to --.

In my casw worked this way: (for Spring boot app in docker)

ENTRYPOINT java -DidMachine=${IDMACHINE} -jar my-app-name

and passing the params on docker run

docker run --env IDMACHINE=Idmachine -p 8383:8383 my-app-name