How to Use env_file in Docker Compose for Environment Variables

Use the env_file option in your docker-compose.yml to specify a file containing environment variables. Docker Compose loads these variables into the container, making configuration simple and clean.

Syntax

The env_file option in docker-compose.yml points to one or more files containing environment variables. Each file should have lines in the format KEY=VALUE. This option is placed under the service definition.

Example parts:

• env_file: key to specify environment files
• - ./myenv.env path to the env file

yaml

services:
  app:
    image: myapp:latest
    env_file:
      - ./myenv.env

Example

This example shows a Docker Compose file using env_file to load environment variables from app.env. The container prints the variable GREETING to demonstrate it works.

yaml

version: '3.8'
services:
  hello:
    image: alpine
    env_file:
      - app.env
    command: sh -c "echo \"$GREETING\""

Output

Hello from env_file!

Common Pitfalls

Common mistakes when using env_file include:

• File path is incorrect or file does not exist.
• Environment file has spaces around = which are not allowed.
• Using environment and env_file with conflicting variables; environment overrides env_file.
• Not restarting containers after changing the env file.

Example of wrong and right usage:

yaml

services:
  app:
    image: myapp
    env_file:
      - ./wrong path.env  # Wrong: path with space or file missing

services:
  app:
    image: myapp
    env_file:
      - ./correct_path.env  # Right: correct path and file exists

Quick Reference

Option	Description	Notes
env_file	Specifies file(s) with environment variables	Supports multiple files as a list
environment	Defines environment variables inline	Overrides variables from env_file if duplicated
File format	KEY=VALUE per line	No spaces around '='; comments start with '#'
Variable precedence	environment > env_file > Dockerfile ENV	Last wins if duplicated

Key Takeaways

Use env_file in docker-compose.yml to load environment variables from a file.

Ensure the env file path is correct and the file format is KEY=VALUE without spaces around '='.

Variables defined in environment override those in env_file.

Restart containers after changing env files to apply updates.

You can specify multiple env files as a list under env_file.