the --book-dir option is mandatory

cmd/liber/liber.go

@@ -193,9 +193,9 @@ func b2i(b bool) int {
 }
 
 func expandTilde(path string) string {
-	if path[:2] == "~/" {
+	if strings.HasPrefix(path, "~/") {
 		curUser, _ := user.Current()
-		return filepath.Join(curUser.HomeDir, path[2:len(path)])
+		return filepath.Join(curUser.HomeDir, strings.TrimPrefix(path, "~/"))
 	}
 	return path
 }
@@ -207,8 +207,8 @@ func main() {
 	if nset != 1 {
 		log.Fatal("Must specify one of --update, --sync, --search or --http-server")
 	}
-	if *update && *bookDir == "" {
-		log.Fatal("Must specify --book-dir with --update")
+	if *bookDir == "" {
+		log.Fatal("Must specify --book-dir")
 	}
 
 	dbdir := expandTilde(*databaseDir)