Pass arg to docker build

Question

I want to pass a variable during build time and start the script with this argument on run. How do I do it?

Dockerfile

FROM alpine
ARG var
# ENV var=${var} # doesn't work
CMD ["echo", "${var}"]
# ENTRYPOINT ["echo", "$var"] # doesn't work
# ENTRYPOINT "echo" "$var" # doesn't work

Running:

docker run -t $(docker build  --build-arg  var=hello -q .)

Produces:

$var

curious because it works fine on my machine... There are 2 differences between our Dockerfiles: ENV var=$var and CMD echo $var — Stefano
– Stefano, Commented Aug 26, 2019 at 16:24

Stefano · Accepted Answer · 2019-08-26 18:33:26Z

4

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.

In other words a correct Dockerfile would be:

FROM alpine
ARG var
ENV var $var
CMD echo $var

In order to build it correctly, you should run:

docker run -t $( docker build --build-arg=var=hello -q . )

src: https://docs.docker.com/engine/reference/builder/#cmd

edited Aug 26, 2019 at 18:33

answered Aug 26, 2019 at 16:38

Stefano

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