CODEWITHSUNDEEP
dockerenvironment-variables
Marvin Richter
Marvin Richter

Reputation: 621

Docker environment substitution with environment variable replacement

I'm trying to use environment variable substitution inside a Dockerfile paired with environment variable replacement from docker but it looks like the variable replacement takes place after the substitution.

The following Dockefile:

FROM alpine:3.7

ENV name="World"
ENV message="Hello, ${name}"

ENTRYPOINT ["env"]

With the Docker run command:

$ docker run -it --rm -e "name=Marvin" envtest/helloworld

Prints the following environment variables:

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=49d702faf257
TERM=xterm
name=Marvin
message=Hello, World
HOME=/root

You can see that even though I replaced the name variable, the message variable is still substituted with the original value from name.

Does anyone know how to do environment variable substitution with dockers environment variable replacement?

EDIT: I found a comment in the Docker forum stating that environment variables are interpreted at build time but can be replaced one by one at runtime. So the documentation is a little misleading.

Upvotes: 3

Views: 6788

Answers (1)

Rafaf Tahsin
Rafaf Tahsin

Reputation: 8626

When you build the Dockerfile, you get an image. And the image has no knowledege about what was written in the Dockerfile.

That means, the docker image has no knowledge weather ENV message="Hello, ${name}" or ENV message="Hello, world" was written in the Dockerfile.

It just has it's environment variable as it is, i.e., name="World", message="Hello, world"

So, when you start your image using $ docker run -it --rm -e "name=Marvin" envtest/helloworld, it overrides the variable name, i.e. now name is Marvin.

But message remains "Hello, world".

Because, inside the image, message is "Hello, world" not "Hello, ${name}"

Upvotes: 6

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