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 $MY_VARIABLE
You can also use shell parameter expansion to achieve this.
Let's say your mandatory build argument is called MANDATORY_BUILD_ARGUMENT, and you want it to be set and non-empty, your Dockerfile could look like this:
FROM debian:stretch-slim
MAINTAINER Evel Knievel <evel@kniev.el>
ARG MANDATORY_BUILD_ARGUMENT
RUN \
# Check for mandatory build arguments
: "${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 :)
You could do something like this...
FROM ubuntu:14.04
ONBUILD ARG MY_VARIABLE
ONBUILD RUN if [ -z "$MY_VARIABLE" ]; then echo "NOT SET - ERROR"; exit 1; else : ; fi
Then docker build -t my_variable_base .
Then build your images based on this...
FROM my_variable_base
...
It's not super clean, but at least it abstracts the 'bleh' stuff away to the base image.
Another simple way:
RUN test -n "$MY_VARIABLE" || (echo "MY_VARIABLE not set" && false)
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.
