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skills/clap-patterns/templates/env-variables.rs

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+/// Environment Variable Integration Template
+///
+/// This template demonstrates:
+/// - Reading from environment variables
+/// - Fallback to CLI arguments
+/// - Default values
+/// - Sensitive data handling (API keys, tokens)
+
+use clap::Parser;
+use std::path::PathBuf;
+
+#[derive(Parser)]
+#[command(name = "envapp")]
+#[command(about = "CLI with environment variable support")]
+struct Cli {
+    /// API key (or set API_KEY env var)
+    ///
+    /// Sensitive data like API keys should preferably be set via environment
+    /// variables to avoid exposing them in shell history or process lists.
+    #[arg(long, env = "API_KEY", hide_env_values = true)]
+    api_key: String,
+
+    /// Database URL (or set DATABASE_URL env var)
+    #[arg(long, env = "DATABASE_URL")]
+    database_url: String,
+
+    /// Log level: debug, info, warn, error
+    ///
+    /// Defaults to "info" if not provided via CLI or LOG_LEVEL env var.
+    #[arg(long, env = "LOG_LEVEL", default_value = "info")]
+    log_level: String,
+
+    /// Configuration file path
+    ///
+    /// Reads from CONFIG_FILE env var, or uses default if not specified.
+    #[arg(long, env = "CONFIG_FILE", default_value = "config.toml")]
+    config: PathBuf,
+
+    /// Number of workers (default from env or 4)
+    #[arg(long, env = "WORKER_COUNT", default_value_t = 4)]
+    workers: usize,
+
+    /// Enable debug mode
+    ///
+    /// Can be set via DEBUG=1 or --debug flag
+    #[arg(long, env = "DEBUG", value_parser = clap::value_parser!(bool))]
+    debug: bool,
+
+    /// Host to bind to
+    #[arg(long, env = "HOST", default_value = "127.0.0.1")]
+    host: String,
+
+    /// Port to listen on
+    #[arg(short, long, env = "PORT", default_value_t = 8080)]
+    port: u16,
+}
+
+fn main() {
+    let cli = Cli::parse();
+
+    println!("Configuration loaded:");
+    println!("  Database URL: {}", cli.database_url);
+    println!("  API Key: {}...", &cli.api_key[..4.min(cli.api_key.len())]);
+    println!("  Log level: {}", cli.log_level);
+    println!("  Config file: {}", cli.config.display());
+    println!("  Workers: {}", cli.workers);
+    println!("  Debug mode: {}", cli.debug);
+    println!("  Host: {}", cli.host);
+    println!("  Port: {}", cli.port);
+
+    // Initialize logging based on log_level
+    match cli.log_level.to_lowercase().as_str() {
+        "debug" => println!("Log level set to DEBUG"),
+        "info" => println!("Log level set to INFO"),
+        "warn" => println!("Log level set to WARN"),
+        "error" => println!("Log level set to ERROR"),
+        _ => println!("Unknown log level: {}", cli.log_level),
+    }
+
+    // Your application logic here
+    println!("\nStarting application...");
+    println!("Listening on {}:{}", cli.host, cli.port);
+}
+
+// Example usage:
+//
+// 1. Set environment variables:
+//    export API_KEY="sk-1234567890abcdef"
+//    export DATABASE_URL="postgres://localhost/mydb"
+//    export LOG_LEVEL="debug"
+//    export WORKER_COUNT="8"
+//    cargo run
+//
+// 2. Override with CLI arguments:
+//    cargo run -- --api-key "other-key" --workers 16
+//
+// 3. Mix environment and CLI:
+//    export DATABASE_URL="postgres://localhost/mydb"
+//    cargo run -- --api-key "sk-1234" --debug