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.

Answer 1

Another simple way:

RUN test -n "$MY_VARIABLE" || (echo "MY_VARIABLE  not set" && false)

Answer 2

Long time ago I had a need to introduce a required (mandatory) ARG, and for better UX include the check at the beginning:

FROM ubuntu:bionic
ARG MY_ARG
RUN [ -z "$MY_ARG" ] && echo "MY_ARG is required" && exit 1 || true

...

RUN ./use-my-arg.sh

But this busts the build cache for every single layer after the initial MY_ARG, because MY_ARG=VALUE is prepended to every RUN command afterwards.

Whenever I changed MY_ARG it would end up rebuilding the whole image, instead of rerunning the last RUN command only.

To bring caching back, I have changed my build to a multi-staged one:

• The first stage uses MY_ARG and checks it's presence.
• The second stage proceeds as usual and declares ARG MY_ARG right at the end.

FROM alpine:3.11.5
ARG MY_ARG
RUN [ -z "$MY_ARG" ] && echo "MY_ARG is required" && exit 1 || true

FROM ubuntu:bionic
...
ARG MY_ARG
RUN ./use-my-arg.sh

Since ARG MY_ARG in the second stage is declared right before it's used, all the previous steps in that stage are unaffected, thus cache properly.

Answer 3

I cannot comment yet because I do not have 50 reputation, but I would like to add onto @Jan Nash's solution because I had a little difficulty getting it to work with my image.

If you copy/paste @Jan Nash's solution, it will work and spit out the error message that the build argument is not specified.

What I want to add

When I tried getting it to work on a CentOS 7 image (centos:7), Docker ran the RUN command without erroring out.

Solution

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.}"]

I hope that helps for future incoming people. Otherwise, I believe @Jan Nash's solution is just brilliant.

Answer 4

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 5

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 <evel@kniev.el>

    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 :)

Answer 6

You could do something like this...

 FROM ubuntu:14.04
 ONBUILD ARG MY_VARIABLE
 ONBUILD RUN if [ -z "$MY_VARIABLE" ]; then echo "NOT SET - ERROR"; exit 1; else : ; fi

Then docker build -t my_variable_base .

Then build your images based on this...

FROM my_variable_base
...

It's not super clean, but at least it abstracts the 'bleh' stuff away to the base image.