This script depends on Bash-only features, and so should not be executed with sh.
diff --git a/contrib/gitflow-installer.sh b/contrib/gitflow-installer.sh
index 33dbe58..bb803f4 100644
--- a/contrib/gitflow-installer.sh
+++ b/contrib/gitflow-installer.sh
@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 # git-flow make-less installer for *nix systems, by Rick Osborne
 # Based on the git-flow core Makefile: