How do you pass environment variables to services in Docker Compose

Environment variables are essential for configuring services in Docker Compose. They allow you to customize the behavior of your applications without hardcoding values into your code. This guide will explain how to pass environment variables to services in Docker Compose using two primary methods: the environment attribute and the env_file attribute.

1. Using the `environment` Attribute

The environment attribute allows you to define environment variables directly in your docker-compose.yml file. You can specify variables using either a list or a mapping syntax.

Example of Using `environment`

version: '3.8'
services:
  webapp:
    image: my-web-app
    environment:
      - DEBUG=true
      - PORT=8080

In this example, the webapp service will have the DEBUG variable set to true and the PORT variable set to 8080.

Using Shell Variables

You can also pass environment variables from your shell by not providing a value:

version: '3.8'
services:
  webapp:
    image: my-web-app
    environment:
      - DEBUG

In this case, the value of DEBUG will be taken from the shell environment where Docker Compose is executed.

2. Using the `env_file` Attribute

The env_file attribute allows you to specify a file containing environment variables. This method is useful for managing multiple variables and keeping your docker-compose.yml file clean.

Example of Using `env_file`

version: '3.8'
services:
  webapp:
    image: my-web-app
    env_file:
      - webapp.env

In this example, the webapp service will load environment variables from the webapp.env file.

Contents of `webapp.env`

DEBUG=true
PORT=8080
DATABASE_URL=mysql://user:password@db:3306/mydatabase

This file contains the environment variables that will be available to the webapp service.

3. Key Considerations

Security: Avoid using environment variables for sensitive information like passwords. Instead, consider using Docker secrets.
Variable Precedence: Environment variables defined in the docker-compose.yml file will override those in the env_file if they share the same name.
Multiple env_file Support: You can specify multiple env_file entries, and they will be evaluated in order.

4. Conclusion

Passing environment variables to services in Docker Compose is straightforward and can be done using the environment or env_file attributes. By utilizing these methods, you can effectively manage your application's configuration and ensure a flexible deployment process.