How do applications use them?

Applications use environment variables in several ways:

1. Configuration Settings: Applications can read environment variables to configure settings without hardcoding values. For example, a web application might read a database connection string from an environment variable.

2. Environment-Specific Behavior: Applications can behave differently based on the environment (development, testing, production) by checking specific environment variables. For instance, an application might enable debugging features if a DEBUG variable is set to true.

3. Security: Sensitive information, such as API keys or passwords, can be stored in environment variables instead of being included in the source code. This helps keep sensitive data secure.

4. Path Management: Applications can use environment variables to determine where to find executables or libraries. For example, the PATH variable specifies directories to search for executable files.

5. Custom User Preferences: Applications can allow users to set preferences through environment variables, enabling customization of the application's behavior.

To access environment variables in a programming language, you typically use built-in functions or libraries. For example, in Python, you can use os.environ to access them:

import os

# Get the value of an environment variable
db_host = os.environ.get('DB_HOST')

This allows the application to adapt its behavior based on the values of the environment variables.