Re-using environment variables in docker-compose.yml

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

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:8443DB_USER_NAME=adminDB_USER_PASSWORD=admin

And inside of container2 you will see:

DB_URL=https://db:8443DB_USER_NAME=adminDB_USER_PASSWORD=adminANOTHERVAR=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.

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