Re-using environment variables in docker-compose.yml

Question

Is it possible to re-use environment variables that are shared among multiple containers?

The idea is to avoid duplication, as illustrated in this example:

version: '2'

services:

  db:
    image: example/db
    ports:
      - "8443:8443" 
    container_name: db
    hostname: db
    environment:
      - USER_NAME = admin 
      - USER_PASSWORD = admin 

svc:
  image: example/svc
  depends_on:
    - db
  ports:
    - "9443:9443"
  container_name: svc
  hostname: svc
  environment:
    - DB_URL = https://db:8443
    - DB_USER_NAME = admin
    - DB_USER_PASSWORD = admin 

Answer 1

The extends option can be nice but it's not supported in 3.x compose files. Other ways to go are:

1. Extension fields (compose file 3.4+)

   If you can use 3.4+ compose files, extension fields are probably the best option:

   docker-compose.yml

   version: '3.4'
   
   x-common-variables: &common-variables
     VARIABLE: some_value
     ANOTHER_VARIABLE: another_value
   
   services:
     some_service:
       image: someimage
       environment: *common-variables
   
     another_service:
       image: anotherimage
       environment:
         <<: *common-variables
         NON_COMMON_VARIABLE: 'non_common_value'
   

2. env_file directive

   docker-compose.yml

   version: '3.2'
   
   services:
     some_service:
       image: someimage
       env_file:
         - 'variables.env'
   
     another_service:
       image: anotherimage
       env_file:
         - 'variables.env'
   

   variables.env

   VARIABLE=some_value
   ANOTHER_VARIABLE=another_value
   

3. .env file in project root (or variables at actual compose environment)

   Variables from .env file can be referenced in service configuration:

   docker-compose.yml

   version: '3.2'
   
   services:
     some_service:
       image: someimage
       environment:
         - VARIABLE
   
     another_service:
       image: anotherimage
       environment:
         - VARIABLE
         - ANOTHER_VARIABLE
   

   .env

   VARIABLE=some_value
   ANOTHER_VARIABLE=another_value