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:

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