diff -Napru bmake.orig/mk/sys.mk bmake/mk/sys.mk
--- bmake.orig/mk/sys.mk	2020-08-19 19:52:06.000000000 +0200
+++ bmake/mk/sys.mk	2020-12-22 20:24:45.701882157 +0100
@@ -119,7 +119,7 @@ unix ?= We run ${_HOST_OSNAME}.
 
 # We need a Bourne/POSIX shell
 MAKE_SHELL ?= sh
-SHELL ?= ${MAKE_SHELL}
+SHELL := ${.SHELL}
 
 # A race condition in mkdir, means that it can bail if another
 # process made a dir that mkdir expected to.