CLI Development

Domain Constraints

Critical Rules

• Errors to stderr, data to stdout — enables piping and scriptability.
• CLI args > env vars > config file > defaults — standard override chain.
• Return non-zero on any error — script integration depends on exit codes.

Key Crates

CLI Structure Pattern

use clap::{Parser, Subcommand};

#[derive(Parser)]
#[command(name = "myapp", about = "My CLI tool")]
struct Cli {
    /// Enable verbose output
    #[arg(short, long)]
    verbose: bool,

    #[command(subcommand)]
    command: Commands,
}

#[derive(Subcommand)]
enum Commands {
    /// Initialize a new project
    Init { name: String },
    /// Run the application
    Run {
        #[arg(short, long)]
        port: Option<u16>,
    },
}

fn main() -> anyhow::Result<()> {
    let cli = Cli::parse();
    match cli.command {
        Commands::Init { name } => init_project(&name)?,
        Commands::Run { port } => run_server(port.unwrap_or(8080))?,
    }
    Ok(())
}

Common Mistakes