How to read environment variables

Use std::env::var to read an environment variable by name, which returns a Result containing the value or an error if it is missing.

use std::env;

fn main() {
    let key = "MY_VAR";
    match env::var(key) {
        Ok(val) => println!("Value: {}", val),
        Err(_) => println!("Variable not found"),
    }
}

Environment variables are settings stored outside your code that your program can read to change its behavior. Think of them like a configuration file that the operating system manages for you. You use them to switch between modes, like development and production, without rewriting your code.