How can environment variables be used within an application running in a container?

Environment variables can be used within an application running in a container to convey configuration information. Here's how you can set and access them:

Setting Environment Variables

When you run a container, you can set environment variables using the -e flag in the docker run command. For example:

docker run -d --name my-app -e APP_ENV=production -e APP_PORT=8080 my-image

In this example:

• APP_ENV is set to production.
• APP_PORT is set to 8080.

Accessing Environment Variables in Code

In your application code, you can access these environment variables using the appropriate method for your programming language. For example, in Java, you can use:

String appEnv = System.getenv("APP_ENV");
String appPort = System.getenv("APP_PORT");

Verifying Environment Variables

You can verify that the environment variables are set correctly by executing a command inside the running container:

docker exec my-app env | grep APP_

This command will list all environment variables that start with APP_, allowing you to confirm their values.

Summary

Environment variables provide a flexible way to configure applications running in containers without hardcoding values, making it easier to manage different environments (development, testing, production) and configurations.