UI errors if a TTY is required and not given

lib/vagrant/errors.rb

@@ -323,6 +323,11 @@ module Vagrant
       error_key(:ssh_unavailable_windows)
     end
 
+    class UIExpectsTTY < VagrantError
+      status_code(73)
+      error_key(:ui_expects_tty)
+    end
+
     class VagrantInterrupt < VagrantError
       status_code(40)
       error_key(:interrupted)

lib/vagrant/ui.rb

@@ -51,6 +51,9 @@ module Vagrant
       def ask(message, opts=nil)
         super(message)
 
+        # We can't ask questions when the output isn't a TTY.
+        raise Errors::UIExpectsTTY if !$stdin.tty?
+
         # Setup the options so that the new line is suppressed
         opts ||= {}
         opts[:new_line] = false if !opts.has_key?(:new_line)
@@ -60,7 +63,7 @@ module Vagrant
         say(:info, message, opts)
 
         # Get the results and chomp off the newline
-        STDIN.gets.chomp
+        $stdin.gets.chomp
       end
 
       # This is used to output progress reports to the UI.

templates/locales/en.yml

@@ -141,6 +141,11 @@ en:
         Port: %{port}
         Username: %{username}
         Private key: %{key_path}
+      ui_expects_tty: |-
+        Vagrant is attempting to interface with the UI in a way that requires
+        a TTY. Most actions in Vagrant that require a TTY have configuration
+        switches to disable this requirement. Please do that or run Vagrant
+        with TTY.
       vagrantfile_exists: |-
         `Vagrantfile` already exists in this directory. Remove it before
         running `vagrant init`.