How to Build a Configuration Hierarchy (Defaults, File, Env, CLI)

Build a configuration hierarchy by defining defaults in code, loading overrides from a file, checking environment variables, and finally applying CLI flags in that specific order of precedence.

use std::env;
use std::fs;

fn main() {
    // 1. Defaults
    let mut config = Config { port: 8080, debug: false };

    // 2. File (e.g., config.toml)
    if let Ok(content) = fs::read_to_string("config.toml") {
        // Parse content and update config.port, config.debug
        // config.port = parsed_port;
    }

    // 3. Environment Variables
    if let Ok(port_str) = env::var("APP_PORT") {
        config.port = port_str.parse().unwrap_or(config.port);
    }
    if let Ok(debug_str) = env::var("APP_DEBUG") {
        config.debug = debug_str == "true";
    }

    // 4. CLI Flags (Highest Precedence)
    let args: Vec<String> = env::args().collect();
    for arg in args.iter().skip(1) {
        if arg.starts_with("--port=") {
            config.port = arg.trim_start_matches("--port=").parse().unwrap();
        } else if arg == "--debug" {
            config.debug = true;
        }
    }

    println!("Final Config: port={}, debug={}", config.port, config.debug);
}

struct Config {
    port: u16,
    debug: bool,
}

A configuration hierarchy ensures your application uses sensible defaults but allows users to override settings easily. It works like a set of rules where the most specific instruction wins: a file overrides the code, an environment variable overrides the file, and a command-line argument overrides everything else. This gives you flexibility without breaking the default setup.