Docker echo environment variable

Question

I'm trying to write a little docker file that sets a User and just echos the current user as a little example to prove to myself it is working. I've tried a number of variants and couldn't find much help in the documentation.

FROM ubuntu
USER daemon
# ENTRYPOINT ["echo", "$USER"]
# just gives "$USER"
# ENTRYPOINT ["echo", "-e", "${USER}"]
# just gives "$USER"
# ENTRYPOINT echo $USER
# gives empty string
# ENTRYPOINT ["/bin/echo", "$USER"]
# just gives "$USER"

I'm running docker build . on the dockerfile and then running docker run <image-id> and getting the results

Expected result is daemon, or without the USER daemon line, I expect root. Probably a really simple answer.

Answer 1

This is the expected behavior, as weird as it seems!

When ENTRYPOINT is a list (as in ENTRYPOINT ["echo", "$USER"]), it is used as-is, without further parsing or interpretation. So $USER remains $USER, because there is no shell involved in the process to replace it with the value of the USER environment variable.

Now, when ENTRYPOINT is a string (as in ENTRYPOINT echo $USER), what is actually executed is sh -c "echo $USER", and $USER is replaced with the value of the environment variable (as you would expect).

However, the environment variable USER is not set by default. It is set by the login process; and when you just run sh -c ... the login process is not involved.

Compare the environment when running docker run -t -i ubuntu bash and docker run -t -i ubuntu login -f root. In the former case, you will get a very basic environment; in the latter case, you will get the complete environment that you are used to (including USERvariable).