Can I set/get variables in docker-compose.yml files?

Question

I want to have a variable in a docker-compose.yml for my project that represents a port and instead of putting 8080 everywhere in the file, I'd rather use it as a variable.

Is it possible to set variable, say in the beginning of the file, then reuse it in other places of the file?

Answer 1

You can use an external .env file and include it in the docker-compose file.

$ cat .env
port=8080

$ cat docker-compose.yml
version: '3'
services:
  web:
    port: "${port}"

docker-compose.yml and .env should be in the same directory.

Answer 2

Starting from version 3.4, you can re-use configuration parts using extension fields. Valid extension fields have two requirements:

1. Should be located at the root of your Compose file.
2. Their name starts with the "x-" character sequence.

Here's an example:

version: '3.4'
x-logging:
  &default-logging
  options:
    max-size: '12m'
    max-file: '5'
  driver: json-file

services:
  web:
    image: myapp/web:latest
    logging: *default-logging
  db:
    image: mysql:latest
    logging: *default-logging