fish_greeting: cat todo.txt only if file exists

fish_cfg/functions/fish_greeting.fish

@@ -3,5 +3,9 @@ function fish_greeting
     #
     nagios_status
     #
-    echo; echo; cat ~/todo.txt
+    if test -f ~/todo.txt
+        echo
+        echo
+        cat ~/todo.txt
+    end
 end