diff --git a/installer/nixos-rebuild.sh b/installer/nixos-rebuild.sh
index c173787d585a..80ed41c7d95f 100644
--- a/installer/nixos-rebuild.sh
+++ b/installer/nixos-rebuild.sh
@@ -1,5 +1,10 @@
 #! @shell@ -e
 
+# Allow the location of NixOS sources and the system configuration
+# file to be overridden.
+NIXOS=${NIXOS:-@defaultNIXOS@}
+NIXPKGS=${NIXPKGS:-@defaultNIXPKGS@}
+NIXOS_CONFIG=${NIXOS_CONFIG:-/etc/nixos/configuration.nix}
 
 showSyntax() {
     # !!! more or less cut&paste from
@@ -13,6 +18,13 @@ test:    activate the configuration, but don't make it the boot default
 build:   build the configuration, but don't make it the default or
          activate it
 dry-run: just show what store paths would be built/downloaded
+
+by env overridable settings:
+NIXOS=${NIXOS}
+NIXPKGS=${NIXPKGS}
+NIXOS_CONFIG=${NIXOS_CONFIG}
+"
+
 EOF
     exit 1
 }
@@ -40,13 +52,6 @@ if test "$action" = dry-run; then
 fi
 
 
-# Allow the location of NixOS sources and the system configuration
-# file to be overridden.
-NIXOS=${NIXOS:-@defaultNIXOS@}
-NIXPKGS=${NIXPKGS:-@defaultNIXPKGS@}
-NIXOS_CONFIG=${NIXOS_CONFIG:-/etc/nixos/configuration.nix}
-
-
 # If the Nix daemon is running, then use it.  This allows us to use
 # the latest Nix from Nixpkgs (below) for expression evaluation, while
 # still using the old Nix (via the daemon) for actual store access.