CLI Configuration Skill

Patterns and best practices for managing configuration in command-line applications.

Configuration Precedence

The standard precedence order (lowest to highest priority):

1. Compiled defaults - Hard-coded sensible defaults
2. System config - /etc/myapp/config.toml
3. User config - ~/.config/myapp/config.toml
4. Project config - ./myapp.toml or ./.myapp.toml
5. Environment variables - MYAPP_KEY=value
6. CLI arguments - --key value (highest priority)

use config::{Config as ConfigBuilder, Environment, File};

pub fn load_config(cli: &Cli) -> Result<Config> {
    let mut builder = ConfigBuilder::builder()
        // 1. Defaults
        .set_default("port", 8080)?
        .set_default("host", "localhost")?
        .set_default("log_level", "info")?;

    // 2. System config (if exists)
    builder = builder
        .add_source(File::with_name("/etc/myapp/config").required(false));

    // 3. User config (if exists)
    if let Some(config_dir) = dirs::config_dir() {
        builder = builder.add_source(
            File::from(config_dir.join("myapp/config.toml")).required(false)
        );
    }

    // 4. Project config (if exists)
    builder = builder
        .add_source(File::with_name("myapp").required(false))
        .add_source(File::with_name(".myapp").required(false));

    // 5. CLI-specified config (if provided)
    if let Some(config_path) = &cli.config {
        builder = builder.add_source(File::from(config_path.as_ref()));
    }

    // 6. Environment variables
    builder = builder.add_source(
        Environment::with_prefix("MYAPP")
            .separator("_")
            .try_parsing(true)
    );

    // 7. CLI arguments (highest priority)
    if let Some(port) = cli.port {
        builder = builder.set_override("port", port)?;
    }

    Ok(builder.build()?.try_deserialize()?)
}

Config File Formats

TOML (Recommended)

Clear, human-readable, good error messages.

# con...