How to Read Environment Variables from an Environment File in Docker?

To read environment variables from an environment file in Docker, we use the --env-file option with the docker run command. This helps us to choose a file that has key-value pairs of environment variables. This makes it simple to manage and set up our Docker containers. By using an environment file, we can make our container setups easier and keep our code cleaner. This helps both in development and in production.

In this article, we will look at different ways to read environment variables in Docker. We will focus on using environment files. We will talk about understanding environment files, using Docker Compose, reading variables from a Dockerfile, passing variables directly in the docker run command, and the best ways to manage environment variables in our Docker environment.

  • How to Read Environment Variables from an Environment File in Docker
  • Understanding Environment Files in Docker
  • Using Docker Compose to Read Environment Files
  • Reading Environment Variables from Dockerfile
  • Passing Environment Variables Directly in Docker Run
  • Best Practices for Managing Environment Variables in Docker
  • Frequently Asked Questions

Understanding Environment Files in Docker

Environment files in Docker are simple text files. They define environment variables for our Docker containers. We can use these variables to set up applications and services inside the containers. This way, we do not have to hardcode values in the Dockerfile or Docker Compose files.

Structure of an Environment File

An environment file usually has key-value pairs. The format looks like this:

VARIABLE_NAME=value
ANOTHER_VARIABLE_NAME=another_value

Usage

  1. Creating an Environment File: First, we need to create a file. We can name it .env or choose another name like app.env:

    touch app.env

  2. Populating the File: Next, we add the environment variables to the file:

    DATABASE_URL=postgres://user:password@db:5432/mydatabase
SECRET_KEY=mysecretkey

Loading Environment Variables in Docker

To use our environment file when we run a Docker container, we use the --env-file flag with the docker run command:

docker run --env-file app.env my-docker-image

Using Environment Files with Docker Compose

In a docker-compose.yml file, we can mention the environment file under the services section:

version: '3'
services:
  web:
    image: my-web-app
    env_file:
      - app.env

Best Practices

  • Keep Environment Files Secure: We should not put sensitive information directly in our environment files or version control.
  • Use Different Files for Different Environments: It is better to create separate environment files for development, testing, and production.
  • Document Variables: We can add comments in our environment files to explain the purpose of each variable.

For more information on managing environment variables in Docker, we can check how to effectively use environment variables in Docker Compose.

Using Docker Compose to Read Environment Files

Docker Compose makes it easier to manage applications with many containers. It helps us define environment variables using an environment file. This way, we keep things consistent and portable in different settings.

To read environment variables from an environment file in Docker Compose, we can follow these steps:

  1. Create an Environment File: First, we need to create a file called .env or any name we like (for example, myenv.env). Inside this file, we write our environment variables like this: KEY=VALUE.

    Here is an example of a .env file:

    DATABASE_URL=mysql://user:password@db:3306/mydb
SECRET_KEY=mysecretkey

  2. Reference the Environment File in docker-compose.yml: Next, we tell Docker Compose about our environment file in the docker-compose.yml file. We use the env_file option under the service definition.

    Here is an example of docker-compose.yml:

    version: '3.8'

services:
  app:
    image: myapp:latest
    env_file:
      - myenv.env
    ports:
      - "8000:8000"
  db:
    image: mysql:5.7
    environment:
      MYSQL_ROOT_PASSWORD: example

  3. Using Environment Variables in the Application: Now, our application can use the environment variables from the .env file as if they are set in the container’s environment.

  4. Running Docker Compose: We can start our services and read the environment variables from the file with this command:

    docker-compose up

  5. Verification: To check if the environment variables are set right, we can open a shell in the running container:

    docker-compose exec app env

This method gives us a simple and effective way to manage environment variables. It is really helpful when we deploy applications in different places. It helps keep our settings consistent and easy to handle. For more details on Docker Compose and what it can do, we can look at What is Docker Compose and How Does it Simplify Multi-Container Applications?.

Reading Environment Variables from Dockerfile

We can read environment variables from a Dockerfile by using the ENV instruction. This lets us set environment variables that will be available in the container when it runs. Here’s how we do it:

Setting Environment Variables in Dockerfile

  1. Basic Syntax:

    ENV VARIABLE_NAME=value

  2. Example:

    FROM ubuntu:latest
ENV APP_ENV=production
ENV APP_PORT=8080

  3. Using Environment Variables: We can use these environment variables later in our Dockerfile or in our app running inside the container. For example, to use the APP_PORT variable in a command:

    CMD ["node", "app.js", "--port", "$APP_PORT"]

Accessing Environment Variables

  • Inside the running container, we can access these environment variables in our app code or through the shell:

    echo $APP_ENV

Build-Time Variables

If we want to set environment variables while we build the image, we use the ARG instruction. This lets us pass values during the build:

ARG BUILD_ENV
ENV APP_ENV=$BUILD_ENV

Example with ARG

FROM ubuntu:latest
ARG BUILD_ENV
ENV APP_ENV=$BUILD_ENV
CMD ["echo", "Environment is $APP_ENV"]

To build the Docker image and pass the argument, we run:

docker build --build-arg BUILD_ENV=development -t myapp .

Best Practices

  • We should not hardcode sensitive data in Dockerfiles. Instead, we can use Docker secrets or environment files.
  • We should keep environment variables short and clear for better reading and maintenance.
  • It is good to document environment variables in our project’s README for clear understanding.

For more about using environment variables well in Docker, we can check out how to use environment variables in Docker Compose.

Passing Environment Variables Directly in Docker Run

We can pass environment variables straight to a Docker container by using the -e flag with the docker run command. This way, we can set one or more environment variables when we start the container.

Syntax

docker run -e VAR_NAME=value -e ANOTHER_VAR=value IMAGE_NAME

Example

Let’s say we want to set environment variables for a container that runs a web app:

docker run -e DB_HOST=localhost -e DB_USER=root -e DB_PASS=password my_web_app

Using Environment Variables from a File

If we have many environment variables, we can load them from a file using the --env-file option. The file should have key-value pairs. Each one should be on a new line.

Example Environment File (env.list)

DB_HOST=localhost
DB_USER=root
DB_PASS=password

Running the Container with the Environment File

docker run --env-file ./env.list my_web_app

Best Practices

  • Security: We should not hardcode sensitive information. It is better to use secrets management tools or environment files that only certain people can access.
  • Consistency: Let’s use the same environment variables in different places like development, staging, and production. This helps us keep things steady.
  • Documentation: We need to write down the important environment variables for our app. This makes it easier for others to set things up.

For more information about handling environment variables in Docker, we can look at this article.

Best Practices for Managing Environment Variables in Docker

Managing environment variables in Docker is very important. It helps with security, consistency, and making deployment easier in different environments. Here are some best practices we can think about:

  1. Use Environment Files: Store your environment variables in a .env file. This keeps your setup neat and apart from your code.

    Example .env file:

    DATABASE_URL=mysql://user:password@localhost/dbname
SECRET_KEY=your_secret_key

    We can reference it in our docker-compose.yml:

    version: '3'
services:
  app:
    image: your_app_image
    env_file:
      - .env

  2. Declare Variables in Dockerfile: It is usually better to keep sensitive info out of the Dockerfile. But we can declare default environment variables for our app here.

    Example:

    FROM node:14
ENV NODE_ENV production

  3. Avoid Hardcoding Sensitive Data: We should not hardcode sensitive info like passwords or API keys in Dockerfiles or source code. It is better to use environment variables or Docker secrets for this kind of data.

  4. Use Docker Secrets for Sensitive Data: In production, we can use Docker secrets to handle sensitive info safely. This is very helpful when we use Docker Swarm.

    Example:

    echo "my_secret_password" | docker secret create db_password -

    We can reference it in our service:

    secrets:
  - db_password

  5. Documentation and Consistency: We should write clear documentation about what each environment variable does and how to use it. Also, we need to keep naming consistent across our projects.

  6. Use Default Values in Docker Compose: Let’s provide default values for environment variables in our docker-compose.yml. This makes sure our app has good defaults if certain variables are missing.

    Example:

    environment:
  - NODE_ENV=${NODE_ENV:-development}

  7. Limit Environment Variable Scope: We should keep environment variables limited to the containers that need them. This reduces the risk of accidental exposure and makes things less complex.

  8. Regularly Review and Clean Up: We need to check our environment variables often. This way, we can make sure they are still useful and remove any that we don’t need anymore.

  9. Use Environment Variable Validation: We can add checks in our app to make sure required environment variables are set and have correct values before we start the app.

By following these best practices for managing environment variables in Docker, we can make our containerized applications more secure and easier to maintain. For more insights on Docker environment variables and how to manage them, we can look at this article.

Frequently Asked Questions

1. How do we specify an environment file in Docker Compose?

To specify an environment file in Docker Compose, we use the env_file directive in our docker-compose.yml file. This lets us define environment variables in a separate file and load them into our containers. Here is an example:

version: '3'
services:
  app:
    image: your-image
    env_file:
      - .env

With this setup, the environment variables we define in our .env file are available to the application running in the container.

2. Can we read environment variables from an environment file in a Dockerfile?

In a Dockerfile, we cannot directly read environment variables from an outside environment file. However, we can pass environment variables at build time using the --build-arg flag or define them with the ARG instruction in our Dockerfile. This is important for configuring our Docker images dynamically. For more info, check how to retrieve an environment variable value in a Dockerfile during the Docker build process.

3. What is the difference between using environment variables in Docker run and Docker Compose?

Using environment variables in docker run lets us pass them directly when we start a container with the -e flag. On the other hand, Docker Compose allows us to define environment variables in a more organized way, like in a .env file or directly in the docker-compose.yml file. This organized way helps us manage many services and their settings. For more insights, see how to effectively use environment variables in Docker Compose.

4. How can we access environment variables inside a running Docker container?

We can access environment variables inside a running Docker container by using the printenv or echo command in the container’s shell. For example, we can run:

docker exec -it your_container_name printenv

This command shows all environment variables set in the container. It helps us check configurations and solve problems.

5. What are best practices for managing environment variables in Docker?

Best practices for managing environment variables in Docker include using .env files for sensitive information. We can use Docker secrets for sensitive data too. We should keep things consistent across environments by documenting environment variables. Also, we should avoid hardcoding environment variables in Dockerfiles or images. This improves security and flexibility. For detailed guidance, refer to Docker security best practices.