How to ensure that the docker build fails if an ENV has not been set?

Question

This github discussion was found. It seems that it is allowed to build a docker image while a certain ENV does not have been set.

How to ensure that the docker build fails if an ENV has not been set?

Answer 1

An empty of non existent environment variable is the same thing, there's no reason Docker do not allow you to pass an empty variable as it could be a valid use case.

It's up to your responsibility to ensure needed environment variables are there and give an error message back when not.

Something along the line of:

RUN 'if [ -z "$AN_ENV_VAR" ]; then echo 'Environment variable AN_ENV_VAR must be specified. Exiting.'; exit 1; fi'

You can play with the exit code to automate some actions on failure around your docker build command.

Answer 2

To error out the build, just add the following with your env variable in the docker-compose file.

${ENV_VAR?Variable ENV_VAR not set}

example:

image: consul:${TAG?Variable TAG not set}

Answer 3

If the if command construct is very complex for you, you can always simply it like

RUN [ -n "$VAR" ]

This asserts the variable value length is nonzero. If it's empty or not set it'll exit with 1. This is also same as running RUN test -n "$VAR"

I see this error message on my machine

The command '/bin/sh -c [ -n "$VAR" ]' returned a non-zero code: 1