diff --git a/tools/install.sh b/tools/install.sh
index 348fcfc8..d7dcb069 100755
--- a/tools/install.sh
+++ b/tools/install.sh
@@ -34,7 +34,7 @@ check_optional_args() {
     case "$arg" in
       --batch)
         LAUNCH_ZSH_AFTER=0
-        printf "${BLUE}--batch:${NORMAL} zsh will not be started after installation\n"
+        printf "%s\n" "${BLUE}--batch:${NORMAL} zsh will not be started after installation"
         ;;
       *)
         printf "${YELLOW}Unrecognized argument: ${BLUE}${arg}${NORMAL}\n"
@@ -101,7 +101,7 @@ main() {
   mv -f ~/.zshrc-omztemp ~/.zshrc
 
   # If this user's login shell is not already "zsh", attempt to switch.
-  TEST_CURRENT_SHELL=$(expr "$SHELL" : '.*/\(.*\)')
+  TEST_CURRENT_SHELL=$(expr "${SHELL:=/bin/false}" : '.*/\(.*\)')
   if [ "$TEST_CURRENT_SHELL" != "zsh" ]; then
     # If this platform provides a "chsh" command (not Cygwin), do it, man!
     if hash chsh >/dev/null 2>&1; then