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.