Pass environment variable from external env file to container using docker compose

Question

I have a docker-compose file which has a service that runs a postgres database image. The environment variables for the database (user, password, db_name) are in a env file called db.env:

DB_USERNAME=some_user
DB_PASSWORD=thepassword
DB_NAME=database

As I want to have these variables only in one file I wanted to use this backand/db.env file. The problem is, that the environemnt variable names differ from the ones which are used by postgres (DB_USERNAME <=> POSTGRES_USER).

This is the docker-compose.yml file:

services:
  db:
    image: postgres
    ports:
      - 5432:5432
    env_file: 
      - backend/db.env
    environment:
      - POSTGRES_PASSWORD=${DB_PASSWORD}
      - POSTGRES_USER=${DB_TEST}
      - POSTGRES_DB=${DB_DATABASE}

I tried to use the same approach as when using the standard .env file: Environment variables in Compose.

Answer 1

env_file will be available in the container .env in compose file itself so you need to use .env here

Workaround:

for i in $(cat < db.env); do export $i;done && docker-compose up -d

db.env:

DB_USERNAME=some_user
DB_PASSWORD=thepassword
DB_NAME=database