Add -device-id command line option (fixes #4387)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4390

cmd/syncthing/main.go

@@ -236,6 +236,7 @@ type RuntimeOptions struct {
 	resetDeltaIdxs bool
 	showVersion    bool
 	showPaths      bool
+	showDeviceId   bool
 	doUpgrade      bool
 	doUpgradeCheck bool
 	upgradeTo      string
@@ -301,6 +302,7 @@ func parseCommandLineOptions() RuntimeOptions {
 	flag.BoolVar(&options.doUpgradeCheck, "upgrade-check", false, "Check for available upgrade")
 	flag.BoolVar(&options.showVersion, "version", false, "Show version")
 	flag.BoolVar(&options.showPaths, "paths", false, "Show configuration paths")
+	flag.BoolVar(&options.showDeviceId, "device-id", false, "Show the device ID")
 	flag.StringVar(&options.upgradeTo, "upgrade-to", options.upgradeTo, "Force upgrade directly from specified URL")
 	flag.BoolVar(&options.auditEnabled, "audit", false, "Write events to audit file")
 	flag.BoolVar(&options.verbose, "verbose", false, "Print verbose log output")
@@ -390,6 +392,17 @@ func main() {
 		return
 	}
 
+	if options.showDeviceId {
+		cert, err := tls.LoadX509KeyPair(locations[locCertFile], locations[locKeyFile])
+		if err != nil {
+			l.Fatalln("Error reading device ID:", err)
+		}
+
+		myID = protocol.NewDeviceID(cert.Certificate[0])
+		fmt.Println(myID)
+		return
+	}
+
 	if options.browserOnly {
 		openGUI()
 		return