Docker ENV in CMD - docker

Question

I am trying to do the following:
Pass a build arg to my docker build command
Store that as an env variable in my container
Use it in my CMD to start when my container launches.
Below is my setup:
FROM ubuntu:xenial
ARG EXECUTABLE
ENV EXECUTABLE ${EXECUTABLE}
CMD ["/opt/foo/bin/${EXECUTABLE}", "-bar"]
Here is how i'm building container
docker build --build-arg EXECUTABLE=$EXECUTABLE -t test_image .
Here is how i'm running image
docker run -d test_image
When I run the container it crashes and tells me
docker: Error response from daemon: OCI runtime create failed:
container_linux.go:296: starting container process caused
"exec: \"/opt/foo/bin/${EXECUTABLE}\": stat /opt/foo/bin/${EXECUTABLE}:
no such file or directory": unknown.

Answer 1

To use environment variables, you need to use shell.
https://docs.docker.com/engine/reference/builder/#cmd
Note: Unlike the shell form, the exec form does not invoke a command
shell. This means that normal shell processing does not happen. For
example, CMD [ "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: CMD [ "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.
Based on this, I think you can work fine by the following Dockerfile.
FROM ubuntu:xenial
ARG EXECUTABLE
ENV EXECUTABLE ${EXECUTABLE}
CMD [ "sh", "-c", "/opt/foo/bin/${EXECUTABLE}", "-bar"]

Answer 2

You'll have to write out an executable or shim as ARG / ENV substitution is not supported for CMD.
The list of supported substitutions:
ADD
COPY
ENV
EXPOSE
FROM
LABEL
STOPSIGNAL
USER
VOLUME
WORKDIR
as well as:
ONBUILD (when combined with one of the supported instructions above)
A workaround is to write your executable to a file and execute that:
FROM ubuntu:xenial
ARG EXECUTABLE
RUN : \
&& /bin/echo -e "#!/bin/sh\nexec '/bin/$EXECUTABLE' -bar" > /exe \
&& chmod +x /exe
CMD ["/exe"]
Build:
docker build -t test --build-arg EXECUTABLE=echo .
Run:
$ docker run -ti test
-bar

Answer 3

Another way to access the environment variable passed is by running it as:
docker run -e EXECUTABLE=<some_value> <docker_image>
Then, in the dockerfile
CMD exec /opt/foo/bin/${EXECUTABLE} -bar