7.3: Multi-file management: Use env_file to separate configurations

As your projects grow more complex, a single file.envcan quickly become cumbersome.DockerCompose offers a practical solution: env_file, which allows you to load additional environment variable files for a specific service.

How does it work?

• .env is always loaded automatically byDockerCompose, even if you don’t specify it.
• env_file allows you to add one or more service-specific files.
• restartThese files can only contain environment variables (no `, `,imageor other YAML directives).

Example project structure:

project/
├─ docker-compose.yml
├─ .env
├─ db.env
├─ app.env

Filedb.env:

DB_PASSWORD=supersecret
DB_NAME=monprojet

Fileapp.env:

APP_ENV=production
APP_DEBUG=false

In thedocker-compose.yml :

services:
  db:
    image: mariadb:10.11
    env_file:
      - db.env
    environment:
      MYSQL_USER: ${DB_USER}  # interpolation depuis .env
  php:
    image: php:8.2-apache
    env_file:
      - app.env

Key points to remember

1. .env vs env_file

Element	Loaded automatically?	What can it contain?	Scope
.env	Yes	Any type of variable (env, image, tag, restart, ports via interpolation)	Global to the entire Docker Compose
env_file	No, must be declared	Only environment variables	Per service

2. Variable priority order
   • Variables in docker-compose.yml (via environment:) → highest priority. Variables in**env_file** → intermediate priority. Variables in**.env** → lowest priority 🔹 Example: ifDB_PASSWORD is defined in.env and in db.env,DockerCompose will use the value from db.env.
3. Mixed usage
   • .env can always be used to interpolate parameters such asrestart: ${RESTART_POLICY} or image: myimage:${TAG}.
   • env_file is especially useful for separating secrets or service-specific configurations.

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