print ARG value in docker build

Question

Is there a way to print ARGs values, that are passed via the --build-arg flags to a docker build command?

Just using RUN echo $ARG_NAME isn't enough since I want it to be printed before the FROM section, where it's not allowed.

The point is to see these values immediately so I can stop the build quickly, preventing downloading the wrong base images.

Already searched docker docs and google. Maybe someone here can shed some light.

score 11 · Answer 1 · answered Jun 07 '20 at 20:58

11

You could use a multi-stage build where the first stage is just for diagnostics and otherwise gets completely ignored.

FROM busybox
ARG ARG_NAME
RUN echo $ARG_NAME

FROM python:3.8
ARG ARG_NAME
...
CMD ["my_app"]

Note that Docker layer caching can cause RUN steps to be totally skipped so even this isn't 100% reliable.

answered Jun 07 '20 at 20:58

David Maze

Good idea! The skipping problem may be solved with the flag `docker build --no-cache .` – RoBeaToZ Jun 09 '20 at 06:02

RoBeaToZ · Answer 2 · 2020-06-07T20:24:59.723

0

This is not possible

A Dockerfile must begin with a FROM instruction

The execution of the RUN command will be performed in an intermediate container that has to be created of an image.

edited Jun 07 '20 at 20:24

answered Jun 07 '20 at 20:21

RoBeaToZ

Thanks for the answer :). I'm aware of that. That's why I mentioned I can't use `RUN echo....`. I was wondering whether there's some other way / workaround, maybe some verbosity level... – yair Jun 07 '20 at 20:23
2

Sorry but a Dockerfile may not start with FROM instruction since it is possible to pass the base image as a build variable. In this case, Dockerfile will start with ARG VARNAME, then FROM VARNAME. https://docs.docker.com/engine/reference/builder/#understand-how-arg-and-from-interact – breakthewall Jul 08 '20 at 13:44

2 Answers2