Greatly improve the OMZ install.sh

Makes the installer more accomodating:
  - Allow headless installs (on a fresh system).
  - Set $SHELL to the correct value on chsh success.
  - Handle edge cases in regards to locating and using zsh.
  - Password entry prompt allows user to try again (after bad password).

tools/install.sh

@@ -80,19 +80,100 @@ main() {
   " ~/.zshrc > ~/.zshrc-omztemp
   mv -f ~/.zshrc-omztemp ~/.zshrc
 
-  # If this user's login shell is not already "zsh", attempt to switch.
-  TEST_CURRENT_SHELL=$(expr "$SHELL" : '.*/\(.*\)')
-  if [ "$TEST_CURRENT_SHELL" != "zsh" ]; then
+  # Do not prompt user unless STDIN file descriptor is bound to TTY,
+  if [ -t 0 ]; then
+    TEST_LOGIN_SHELL=$(expr "$SHELL" : '.*/\(.*\)')
+    if [ "$TEST_LOGIN_SHELL" = "zsh" ]; then
+      # No need to change login shell.
+      ENV_ZSH="$SHELL"
+    # Let's try to change the login shell.
+    else
+      # See what zsh (if any) they have in the current environment -- it's probably
+      # the zsh they prefer, assuming /etc/shells could have multiple zsh entries.
+      ENV_ZSH="$(which zsh)" # $(type -P zsh) would be better but not POSIX sh
+
+      # Will it blend?
+      if ! "$ENV_ZSH" -c 'exit 0' >/dev/null 2>&1; then
+        unset ENV_ZSH
+      fi
+
+      # Is the environment zsh also in the list of supported login shells?
+      if [ -n "$ENV_ZSH" ] && grep "^${ENV_ZSH}$" /etc/shells >/dev/null 2>&1; then
+        printf "${YELLOW}Found ${ENV_ZSH}${NORMAL} ${GREEN}in /etc/shells.${NORMAL}\n"
+        CHSH_ZSH="$ENV_ZSH"
+      else
+        # No? Then let's get the last zsh entry from the list of supported login shells.
+        # -- We know at least one is in there because of an earlier check in this script.
+        GREP_ZSH="$(grep /zsh$ /etc/shells | tail -1)"
+        # Will it blend?
+        if "$GREP_ZSH" -c 'exit 0' >/dev/null 2>&1; then
+          # Yep, let's use that zsh entry.
+          CHSH_ZSH="$GREP_ZSH"
+        else
+          # Nope, it's a bogus path.
+          printf "${RED}I can't change your shell automatically because there is a bad zsh entry in /etc/shells.${NORMAL}\n"
+          if [ -n "$ENV_ZSH" ]; then
+            # If the environment zsh is real, suggest they add that to the list of supported login shells.
+            printf "${BLUE}Consider adding $ENV_ZSH to /etc/shells and then manually change your default shell.${NORMAL}\n"
+          else
+            printf "${RED}No real zsh found in the current environment or /etc/shells.${NORMAL}\n"
+            printf "${RED}You might not have zsh properly installed...${NORMAL}\n"
+            # OKAY, IT'S GETTIN' WEIRD. BAIL OUT!
+            exit
+          fi
+        fi
+      fi
+
+      if [ -n "$CHSH_ZSH" ]; then
         # If this platform provides a "chsh" command (not Cygwin), do it, man!
         if hash chsh >/dev/null 2>&1; then
-      printf "${BLUE}Time to change your default shell to zsh!${NORMAL}\n"
-      chsh -s $(grep /zsh$ /etc/shells | tail -1)
-    # Else, suggest the user do so manually.
+          printf "${BLUE}Time to change your default shell to ${CHSH_ZSH}!${NORMAL}\n"
+          while true; do
+            set +e
+            trap 'CHSH_ZSH_ERRNO=$?' INT
+            CHSH_ZSH_STDERR="$(chsh -s "$CHSH_ZSH" 2>&1)"
+            CHSH_ZSH_ERRNO=${CHSH_ZSH_ERRNO-$?}
+            case $CHSH_ZSH_ERRNO in
+              0)
+                # Great. It worked.
+                # Update the $SHELL export for this session.
+                SHELL="$CHSH_ZSH"
+                break
+              ;;
+              130)
+                printf "${RED}Change of default login shell has been cancelled!${NORMAL}\n"
+                printf "${BLUE}To try again, run the following command at any time:\n${NORMAL}"
+                printf '    %schsh -s "%s"%s\n' "${BLUE}" "$CHSH_ZSH" "${NORMAL}"
+                break
+              ;;
+              *)
+                case "$CHSH_ZSH_STDERR" in
+                  *Credentials*)
+                    printf "${RED}Wrong password.${NORMAL} ${GREEN}Press CTRL-C to cancel.${NORMAL}\n"
+                  ;;
+                  *)
+                    # Probably best to fall back to the environment zsh if it exists.
+                    if [ -n "$ENV_ZSH" ]; then
+                      unset CHSH_ZSH
+                    fi
+                    break
+                  ;;
+                esac
+              ;;
+            esac
+            trap - INT
+            set -e
+          done
+        # Else, suggest the user change the login shell manually.
         else
           printf "I can't change your shell automatically because this system does not have chsh.\n"
-      printf "${BLUE}Please manually change your default shell to zsh!${NORMAL}\n"
+          printf "${BLUE}Please manually change your default shell to ${CHSH_ZSH}!${NORMAL}\n"
         fi
       fi
+    fi
+  else
+    printf "${BLUE}No TTY specified! Please manually change your default shell to zsh!${NORMAL}\n"
+  fi
 
   printf "${GREEN}"
   echo '         __                                     __   '
@@ -110,7 +191,12 @@ main() {
   echo 'p.p.s. Get stickers and t-shirts at http://shop.planetargon.com.'
   echo ''
   printf "${NORMAL}"
-  env zsh
+  
+  if [ -n "$CHSH_ZSH" ]; then
+    env "$CHSH_ZSH"
+  elif [ -n "$ENV_ZSH" ]; then
+    env "$ENV_ZSH"
+  fi
 }
 
 main