How to make a build arg mandatory during Docker build?

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.

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

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.}"]

Another simple way:

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

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.

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.

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.

In case anybody is looking for a the solution but with docker compose build, I used mandatory variables.

version: "3.9"
services:
  my-service:
    build:
      context: .
      args:
        - ENVVAR=${ENVVAR:?See build instructions}

After running docker compose build:

• Before exporting ENVVAR: Invalid template: "required variable ENVVAR is missing a value: See build instructions"
• After exporting ENVVAR: build proceeds

Support for Required Environment variables Compose Environment Variables