Re-using environment variables in docker-compose.yml - docker

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:
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'
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
.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