add default config paths

src/main.rs

@@ -3,23 +3,70 @@ mod config;
 use clap::Parser;
 use std::path::PathBuf;
 
+const CONFIG_FILENAME: &str = "bot.yml";
+const APP_NAME: &str = "deltachat-remotecontrol-bot";
+const APP_DIR: &str = APP_NAME;
+
 /// Delta Chat bot for remote control of local network machines.
 #[derive(Parser)]
 #[command(version, about)]
 struct Args {
     /// Path to the YAML configuration file.
+    /// If omitted, the following paths are tried in order:
+    /// ./bot.yml, ~/.config/deltachat-remotecontrol-bot/bot.yml,
+    /// /etc/deltachat-remotecontrol-bot/bot.yml
     #[arg(short = 'c', long = "config")]
-    config: PathBuf,
+    config: Option<PathBuf>,
+}
+
+/// Returns the ordered list of default config paths to try when `-c` is not given.
+/// The home-directory entry is omitted if $HOME is not set.
+fn default_config_paths() -> Vec<PathBuf> {
+    let mut paths = vec![PathBuf::from(CONFIG_FILENAME)];
+
+    if let Ok(home) = std::env::var("HOME") {
+        paths.push(
+            PathBuf::from(home)
+                .join(".config")
+                .join(APP_DIR)
+                .join(CONFIG_FILENAME),
+        );
+    }
+
+    paths.push(PathBuf::from("/etc").join(APP_DIR).join(CONFIG_FILENAME));
+
+    paths
 }
 
 fn main() {
     let args = Args::parse();
 
-    let _ = match config::read_config(&args.config) {
+    let config_path: PathBuf = match args.config {
+        Some(path) => path,
+        None => {
+            let defaults = default_config_paths();
+            match defaults.iter().find(|p| p.exists()) {
+                Some(path) => path.clone(),
+                None => {
+                    eprintln!("Error: no configuration file found. Tried:");
+                    for path in &defaults {
+                        eprintln!("  {}", path.display());
+                    }
+                    eprintln!("Use -c <path> to specify a config file explicitly.");
+                    std::process::exit(1);
+                }
+            }
+        }
+    };
+
+    let config = match config::read_config(&config_path) {
         Ok(c) => c,
         Err(e) => {
             eprintln!("Error: {e}");
             std::process::exit(1);
         }
     };
+
+    // TODO: start the bot using `config`.
+    let _ = config;
 }