How to share environment variables in Docker Compose services

I knew how to set environment variables in a Docker Compose service but I didn’t know how to share. So I’ve written notes for my output.

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Background

I had an issue in that I had to set many environment variables in Docker Compose YAML and most of them were shared with some of the services. So I thought there should be a solution to it.

What I did

I used thex- prefix to reuse variables.

Extension

It can be used not only for sharing environment variables but also for sharing functions like volume and depends_on

Then my docker-compose.yml changed like below.

Before

version: '3.8'
services:
  service1:
    image: path/to/image1
    environment:
      - COMMON_ONE
      - COMMON_TWO
      ...other common variables...
  service2:
    image: path/to/image2
    environment:
      - COMMON_ONE
      - COMMON_TWO
      ...other common variables...
      - NOT_COMMON_ONE
      - NOT_COMMON_TWO

After

version: '3.8'
x-common-environment: &common-env
  - COMMON_ONE
  - COMMON_TWO
  ...other common variables...
services:
  service1:
    image: path/to/image
    environment:
      <<: *common-env
  service2:
    image: path/to/image2
    environment:
      <<: *common-env
      - NOT_COMMON_ONE
      - NOT_COMMON_TWO

Small note. If you have variables with the same name as variables in shell or .env , you don’t have to set the value ( — COMMON_ONE=${COMMON_ONE}).

Ways to set environment variables in Compose

That’s it!