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

Answer 2

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 3

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