Docker Compose: Set Environment Variables

In a Docker Compose, you can set environment variables in a service’s containers or overwrite variables that are defined in the Dockerfile of the images you’re running.

To pass the environment variables to the containers, you can use the environment key in a docker-compose.yml file, that works just like a docker run -e VAR=VALUE ... command.

Alternatively, you can pass multiple environment variables from an external file with the env_file option, just like with a docker run --env-file=FILE ... command.

In this short note i will show the examples of how to set the environment variables in the Docker Compose using the environment and env_file options.

Cool Tip: How to specify a path to the Dockerfile in a Docker Compose! Read more →

Set Environment Variables in Docker Compose

To set the environment variables in the docker-compose.yml file, use the environment option as in the example below:

version: '3'
services:
  db:
    image: mysql:latest
    environment:
      - MYSQL_DATABASE: 'db'
      - MYSQL_USER: 'user'
      - MYSQL_PASSWORD: 'password'
      - MYSQL_ROOT_PASSWORD: 'password'

To pass multiple environment variables from an external file, create the .env file with the contents as follows:

# .env
MYSQL_DATABASE: 'db'
MYSQL_USER: 'user'
MYSQL_PASSWORD: 'password'
MYSQL_ROOT_PASSWORD: 'password'

… and use the env_file option in the docker-compose.yml file to pass these variables:

version: '3'
services:
  db:
    image: mysql:latest
    env_file: .env

You can also set environment variables from multiple files as follows:

version: '3'
services:
  db:
    image: mysql:latest
    env_file:
      - ./common.env
      - ./apps/db.env

Environment Files Order: The environment files in the Docker Compose are processed from the top down. For example, for the same variable specified in the file ./common.env and assigned a different value in the file ./apps/db.env, the value from ./apps/db.env stands as this file is listed below (after) the ./common.env file.