diff --git a/book/preamble.tex b/book/preamble.tex
index d795828..7c48a0c 100644
--- a/book/preamble.tex
+++ b/book/preamble.tex
@@ -182,8 +182,8 @@
 	{\lstset{language=sh, %
 		numbers=left,numberstyle=\tiny,stepnumber=1,numbersep=8pt, %
 		breaklines=true, %
-        frame=l, %
-        showtabs=false, %
+		frame=single, %
+		showtabs=false, %
 		basicstyle=\theListingFontFamily, %
 		showstringspaces=false, %
 		tabsize=4, %