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

You can use the extends directive (available in compose 1.x and 2.x) to have multiple containers inherit the environment configuration from an underlying service description. For example, put the following in a file named base.yml:

version: '2'

services:
  base:
    environment:
      DB_URL: https://db:8443
      DB_USER_NAME: admin
      DB_USER_PASSWORD: admin 

Then in your docker-compose.yml:

version: '2'

services:
  container1:
    image: alpine
    command: sh -c "env; sleep 900"
    extends:
      file: base.yml
      service: base

  container2:
    image: alpine
    command: sh -c "env; sleep 900"
    extends:
      file: base.yml
      service: base
    environment:
      ANOTHERVAR: this is a test

Then inside of container1, you will see:

DB_URL=https://db:8443
DB_USER_NAME=admin
DB_USER_PASSWORD=admin

And inside of container2 you will see:

DB_URL=https://db:8443
DB_USER_NAME=admin
DB_USER_PASSWORD=admin
ANOTHERVAR=this is a test

You can obviously use extends for things other than the environment directive; it's a great way to avoid duplication when using docker-compose.

Answer 2

You can reference local environment variables from within a docker-compose file. Assuming what you're wanting to do is make USER_NAME the same as DB_USER_NAME:

docker-compose.yml

version: '2'

services:
  db:
    image: example/db
    ports:
      - "8443:8443" 
    container_name: db
    hostname: db
    environment:
      - USER_NAME = ${USERNAME}
      - USER_PASSWORD = ${PASSWORD}

svc:
  image: example/svc
  depends_on:
    - db
  ports:
    - "9443:9443"
  container_name: svc
  hostname: svc
  environment:
    - DB_URL = https://db:8443
    - DB_USER_NAME = ${USERNAME}
    - DB_USER_PASSWORD = ${PASSWORD}

Then, run docker-compose like:

$ USERNAME="admin" PASSWORD="admin" docker-compose up

Alternately, for something more permanent, and easier to type on a recurring basis:

$ printf '%s\n%s\n' 'export USERNAME="admin"' 'export PASSWORD="admin"' >> ~/.bash_profile
$ source ~/.bash_profile
$ docker-compose up

Answer 3

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