Only try to change shell on interactive sessions.

install.sh

@@ -5,10 +5,12 @@ set errexit
 
 function prerequisites {
   if which zsh > /dev/null ; then
+    if [[ $- == *i* ]] ; then
       if [[ `getent passwd $USER | cut -d: -f7` != */zsh ]] ; then
         echo 'Enter password to change shell.' >&2
         chsh -s `which zsh`
       fi
+    fi
     install_git https://github.com/robbyrussell/oh-my-zsh.git $HOME/.oh-my-zsh
   else
     echo "ZSH not found!" >&2