diff --git a/pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh b/pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh
index 795bedb99d4d..95004f447941 100755
--- a/pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh
+++ b/pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh
@@ -49,6 +49,8 @@ while [ "$#" -gt 0 ]; do
         ;;
       switch|boot|test|build|edit|dry-build|dry-run|dry-activate|build-vm|build-vm-with-bootloader)
         if [ "$i" = dry-run ]; then i=dry-build; fi
+        # exactly one action mandatory, bail out if multiple are given
+        if [ -n "$action" ]; then showSyntax; fi
         action="$i"
         ;;
       --install-grub)