Add -check flag for dry-run config validation

Author: brvier

CHANGELOG.md

@@ -3,6 +3,13 @@
 All notable changes to Gjallar are documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
+## [0.6.0] - 2026-07-06
+
+### Added
+
+- `-check` flag: validate the configuration (monitors, rules, alert URLs)
+  and exit without running any check or sending any alert.
+
 ## [0.5.0] - 2026-07-05
 
 ### Added

main.go

@@ -28,11 +28,24 @@ var version = "dev"
 func main() {
 	configPath := flag.String("config", "gjallar.yaml", "path to YAML configuration")
 	showVersion := flag.Bool("version", false, "print version and exit")
+	checkOnly := flag.Bool("check", false, "validate the configuration (including checkers and alert URLs) and exit")
 	flag.Parse()
 	if *showVersion {
 		fmt.Println("gjallar", version)
 		return
 	}
+	if *checkOnly {
+		cfg, err := config.Load(*configPath)
+		if err == nil {
+			_, err = prepare(cfg)
+		}
+		if err != nil {
+			fmt.Fprintln(os.Stderr, "gjallar:", err)
+			os.Exit(1)
+		}
+		fmt.Printf("config OK: %d monitors, %d alert channels\n", len(cfg.Monitors), len(cfg.Alerts))
+		return
+	}
 	slog.SetDefault(slog.New(slog.NewTextHandler(os.Stderr, nil)))
 	slog.Info("starting gjallar", "version", version)