Environment Variables with Docker-compose

Currently, we have included our database credentials in the code itself. But in production systems and an ideal development workflow, it is not advised to add such critical information in the code.

Clone the project using git clone https://github.com/venky8283/Docker.git

Type git checkout 6e24a37faa2e1bd3401c77b2951f995f410b3d35 to get to the code used in the lesson.

Environment variables are one of the options that are used in production systems to manage credentials. So, let’s see how to work with the environment variables using Docker.

Accessing environment variables in Docker#

Using the .env file#

A file with only the .env extension is used in the production systems to store all the variables. Since it is a hidden file, it is not normally pushed to the code.

We will create a .env file and move the variables to the .env file. To access the variables, we will use ＄{} syntax in docker-compose.

The .env file should be in the same folder where the docker-compose.yml file is located.

You can try running docker-compose up once these changes are done.

Using the env_file#

Another method is to use the env_file keyword instead of the environment keyword in docker-compose.yml. In this method, it is not necessary that the .env file should be located in the same directory as the docker-compose file.

You will provide the location of the .env file like so:

env_file:
      - ./.env

This will fetch all the mentioned environment variables from the .env file.

Priority of .env variables#

There are multiple ways we can use the environment variables in Docker, which includes,

• ENV in Dockerfile
• environment keyword in the docker-compose.yml file
• -e option from the command line

So, docker-compose has precedence levels to overcome the clash of variables. Let’s see those.

When you set the same environment variable in multiple files, here’s the priority used by Compose to choose which value to use:

• Compose file
• Shell environment variables
• Environment file
• Dockerfile
• Variable is not defined

So, if we define a variable in the Compose file in the - environment section and also in a .env file, Compose will consider the variable declared in the Compose file from -environment section.

Sometimes, this might be the solution to any unexpected behavior of the application when multiple environment variables are used.

With this, we have completed the docker-compose section. Try out the exercise and quiz as well.

From the next lesson, we will move towards advanced usage of Docker and look at an introduction to orchestration tools in Docker.