How can I use environment variables in docker- compose ?

Using environment variables in Docker Compose is a common practice to ensure flexible configuration of containerized applications across different environments. Here are some primary methods for introducing and using environment variables in the docker-compose file:

1. .env File

The simplest approach is to create a .env file in the same directory as the docker-compose.yml file. Docker Compose automatically reads the variables from the .env file.

Example:

Create the .env file:

shell
DB_USER=root
DB_PASS=example

Use these variables in the docker-compose.yml file:

yaml
version: '3.8'
services:
  db:
    image: mysql
    environment:
      MYSQL_ROOT_PASSWORD: ${DB_PASS}
      MYSQL_USER: ${DB_USER}
    ports:
      - "3306:3306"

This way, the environment variables MYSQL_ROOT_PASSWORD and MYSQL_USER will utilize the values defined in the .env file.

2. Using the environment Key

You can directly set environment variables within the service definition in the docker-compose.yml file using the environment key.

Example:

yaml
version: '3.8'
services:
  app:
    image: myapp
    environment:
      - DEBUG=1
      - NODE_ENV=production

3. Using the env_file Directive

If you have multiple environment variables, you can place them in separate files and reference them in the docker-compose.yml file using the env_file directive.

Example: Create the app.env file:

shell
DEBUG=1
NODE_ENV=production

Reference it in the docker-compose.yml file:

yaml
version: '3.8'
services:
  app:
    image: myapp
    env_file:
      - app.env

4. Command-Line Variables

You can also pass environment variables via the command line when running the docker-compose command.

Example:

bash
$ DB_USER=root DB_PASS=example docker-compose up

These variables can also be used in the docker-compose.yml file.

Summary

Using environment variables effectively separates configuration from application code, enhancing portability and security. Choosing the appropriate method based on specific requirements allows for more flexible and manageable configuration.