Docker: how to copy env var from 1 service to another

Question

here is the docker-compose.yml I would like to achieve:

version: "3.4"

services:

  service1:
    image: dorowu/ubuntu-desktop-lxde-vnc

  service2:
    build:
      context: .
    environment:
      - DISPLAY=[use env variable DISPLAY of service1]

Is there a way in docker compose to tell service2 that the value of env variable DISPLAY must be the same as the env variable DISPLAY defined in service1 ?

Edit

The DISPLAY env var is set in dorowu/ubuntu-desktop-lxde-vnc directly in the Dockerfile.

It's not a duplicate of Re-using environment variables in docker-compose.yml because the question asked how to write a common env variables for differents services.

In my case I don't want to explicitly write a common variables (with a .env file for example) but I want to set the env variable DISPLAY of service2 to the same value as DISPLAY in service 1 without having to rewrite it in a .env file.

For example if I wanted my service2 to have the same DISPLAY as my host I could write:

version: "3.4"

services:

  service1:
    image: dorowu/ubuntu-desktop-lxde-vnc

  service2:
    build:
      context: .
    environment:
      - DISPLAY=$DISPLAY

Now in this case I'm looking for something like:

version: "3.4"

services:

  service1:
    image: dorowu/ubuntu-desktop-lxde-vnc

  service2:
    build:
      context: .
    environment:
      - DISPLAY=service1:$DISPLAY

Does this answer your question? [Re-using environment variables in docker-compose.yml](https://stackoverflow.com/questions/36283908/re-using-environment-variables-in-docker-compose-yml) — Anton, Mar 14 '23 at 06:24
If `$DISPLAY` is X Window System configuration, and `service1` is running an X server and it's listening over TCP, you probably need to set this up like other cross-container communication: set `DISPLAY=service1:0.0`, which will most likely be a different value from the default in `service1` (probably `:0`). — David Maze, Mar 14 '23 at 10:41
@Anton, in fact I did not ask the same question. I edited the question to add more clarity. — Lenny4, Mar 14 '23 at 12:31
@DavidMaze even though your solution might work, it's not what I'm looking for. I took this example but I could also take another env var as `SERVER_PORT` (or anything else). In fact I could make it work by setting `DISPLAY=:1.0` (in service2) and binding the volume `/tmp/.X11-unix` between the 2 services. Please read the edit for more clarity. — Lenny4, Mar 14 '23 at 12:39

score 2 · Accepted Answer · answered Mar 14 '23 at 14:42

Compose has no way of extracting information from an image and republishing it to another container. You clarify that the environment variable is set in the first image's Dockerfile, and you want its value in the second container; there is no way to do that in Compose.

Of note, there's also no way to do this in plain Docker. The closest you could come is launching a temporary container to get the environment variable out, but that's awkward

docker run \
  -e DISPLAY=$(docker run --rm dorowu/ubuntu-desktop-lxde-vnc sh -c 'echo $DISPLAY) \
  ...

A similarly awkward docker inspect invocation could retrieve the value.

But fundamentally, this capability isn't a standard part of the Docker tool set, and you can't do it at all in Compose (or other container environments like Kubernetes).

If you're setting the environment-variable value in Compose then there are tricks like YAML anchors and env_file: that can set the same value in multiple places (but not "share them from one container to another" per se). You mention the X Window System $DISPLAY variable as an example, and for things that refer to other containers it's likely the server won't know its own name, so you'd need to set the environment variable directly in the client container in any case.

score 0 · Answer 2 · answered Mar 14 '23 at 08:34

0

ENV_FILE is what you need. You can share everything between apps by setting up variables in a common file.

# docker-compose.yml
build:
env_file: .env # here

.env

DISPLAY=env variable

answered Mar 14 '23 at 08:34

Andy Su

131
7

Hello, yes I know but it's not what I'm looking for, the variable DISPLAY is already defined in `service1` by ther Dockerfile. I want to use it without having to write a `.env` file – Lenny4 Mar 14 '23 at 12:17

Docker: how to copy env var from 1 service to another

Edit

2 Answers2

.env