diff --git a/book/makefile b/book/makefile
index c6a4423..05a6ff8 100644
--- a/book/makefile
+++ b/book/makefile
@@ -6,6 +6,11 @@ CLEAN := latexmk -cd -lualatex -c -use-make
 CHECK_1 := lacheck
 CHECK_2 := chktex
 
+# we want bash as shell
+SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; \
+	 else if [ -x /bin/bash ]; then echo /bin/bash; \
+	 else echo sh; fi; fi)
+
 ifdef DEBUG
 EBOOK += -a debug
 WEBSITE += -a debug