How to make a build arg mandatory during Docker build?

Question

Is there any way to make a build argument mandatory during docker build? The expected behaviour would be for the build to fail if the argument is missing.

For example, for the following Dockerfile:

FROM ubuntu  ARG MY_VARIABLE ENV MY_VARIABLE $MY_VARIABLE  RUN ... 

I would like the build to fail at ARG MY_VARIABLE when built with docker build -t my-tag . and pass when built with docker build -t my-tag --build-arg MY_VARIABLE=my_value ..

Is there any way to achieve that behaviour? Setting a default value doesn't really do the trick in my case.

(I'm running Docker 1.11.1 on darwin/amd64.)

EDIT: One way of doing that I can think of is to run a command that fails when MY_VARIABLE is empty, e.g.:

FROM ubuntu  ARG MY_VARIABLE RUN test -n "$MY_VARIABLE" ENV MY_VARIABLE $MY_VARIABLE  RUN ... 

but it doesn't seem to be a very idiomatic solution to the problem at hand.

Answer 1

I tested with RUN test -n <ARGvariablename> what @konradstrack mentioned in the original (edit) post... that seems do the job of mandating the variable to be passed as the build time argument for the docker build command:

FROM ubuntu  ARG MY_VARIABLE RUN test -n "$MY_VARIABLE" ENV MY_VARIABLE $MY_VARIABLE 

Answer 2

You can also use shell parameter expansion to achieve this.

Let's say your mandatory build argument is called MANDATORY_BUILD_ARGUMENT, and you want it to be set and non-empty, your Dockerfile could look like this:

    FROM debian:stretch-slim     MAINTAINER Evel Knievel <[email protected]>          ARG MANDATORY_BUILD_ARGUMENT          RUN \     # Check for mandatory build arguments         : "${MANDATORY_BUILD_ARGUMENT:?Build argument needs to be set and non-empty.}" \          # Install libraries     &&  apt-get update \     &&  apt-get install -y \             cowsay \             fortune \      # Cleanup     &&  apt-get clean \     &&  rm -rf \             /var/lib/apt/lists/* \             /var/tmp/* \             /tmp/* \      CMD ["/bin/bash", "-c", "/usr/games/fortune | /usr/games/cowsay"] 

Of course, you would also want to use the build-argument for something, unlike I did, but still, I recommend building this Dockerfile and taking it for a test-run :)

EDIT

As mentioned in @Jeffrey Wen's answer, to make sure that this errors out on a centos:7 image (and possibly others, I admittedly haven't tested this on other images than stretch-slim):

Ensure that you're executing the RUN command with the bash shell.

RUN ["/bin/bash", "-c", ": ${MYUID:?Build argument needs to be set and not null.}"]