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A command-line prompt with timeout and countdown

Have you ever started a long operation and walked away from the computer, and come back half an hour later only to find that the process is hung up waiting for some user input? It’s a sub-optimal user experience, and in many cases it can be avoided by having the program choose a default if the user doesn’t respond within a certain amount of time. One example of this UI technique in the wild is powering off your computer – most modern operating systems will pop up a dialogue to confirm or cancel the shutdown, with a countdown until the shutdown proceeds automatically.

This article is about how to achieve the same effect in command-line programs using Ruby.

Let’s start with the end result. We want to be able to call our method like this:

puts ask_with_countdown_to_default("Do you like pie?", 30.0, false)

We pass in a question, a (possibly fractional) number of seconds to wait, and a default value. The method should prompt the user with the given question and a visual countdown. If the user types ‘y’ or ‘n’, it should immediately return true or false, respectively. Otherwise when the countdown expires it should return the default value.

Here’s a high-level implementation:

def ask_with_countdown_to_default(question, seconds, default)
  with_unbuffered_input($stdin) do
    countdown_from(seconds) do |seconds_left|
      write_then_erase_prompt(question, seconds_left) do
        wait_for_input($stdin, seconds_left % 1) do
          case char = $stdin.getc
          when ?y, ?Y then return true
          when ?n, ?N then return false
          else                  # NOOP
          end
        end
      end
    end
  end
  return default
ensure
  $stdout.puts
end                             # ask_with_countdown_to_default

Let’s take it step-by-step.

By default, *NIX terminals operate in “canonical mode”, where they buffer a line of input internally and don’t send it until the user hits RETURN. This is so that the user can do simple edits like backspacing and retyping a typo. This behavior is undesirable for our purposes, however, since we want the prompt to respond as soon as the user types a key. So we need to temporarily alter the terminal configuration.

  with_unbuffered_input($stdin) do

We use the POSIX Termios library, via the ruby-termios gem, to accomplish this feat.

def with_unbuffered_input(input = $stdin)
  old_attributes = Termios.tcgetattr(input)
  new_attributes = old_attributes.dup
  new_attributes.lflag &= ~Termios::ECHO
  new_attributes.lflag &= ~Termios::ICANON
  Termios::tcsetattr(input, Termios::TCSANOW, new_attributes)

  yield
ensure
  Termios::tcsetattr(input, Termios::TCSANOW, old_attributes)
end                             # with_unbuffered_input

POSIX Termios defines a set of library calls for interacting with terminals. In our case, we want to disable some of the terminal’s “local” features – functionality the terminal handles internally before sending input on to the controlling program.

We start by getting a snapshot of the terminal’s current configuration. Then we make a copy for our new configuration. We are interested in two flags: “ECHO” and “ICANON”. The first, ECHO, controls whether the terminal displays characters that the user has types. The second controls canonical mode, which we explained above. After turning both flags off, we set the new configuration and yield. After the block is finished, or if an exception is raised, we ensure that the original terminal configuration is reinstated.

Now we need to arrange for a countdown timer.

    countdown_from(seconds) do |seconds_left|

Here’s the implementation:

def countdown_from(seconds_left)
  start_time   = Time.now
  end_time     = start_time + seconds_left
  begin
    yield(seconds_left)
    seconds_left = end_time - Time.now
  end while seconds_left > 0.0
end                             # countdown_from

First we calculate the wallclock time at which we should stop waiting. Then we begin looping, yielding the number of seconds left, and then when the block returns recalculating the number. We keep this up until the time has expired.

Next up is writing, and re-writing, the prompt.

      write_then_erase_prompt(question, seconds_left) do

This method is implemented as follows:

def write_then_erase_prompt(question, seconds_left)
  prompt_format = "#{question} (y/n) (%2d)"
  prompt = prompt_format % seconds_left.to_i
  prompt_length = prompt.length
  $stdout.write(prompt)
  $stdout.flush

  yield

  $stdout.write("\b" * prompt_length)
  $stdout.flush
end                             # write_then_erase_prompt

We format and print a prompt, flushing the output to insure that it is displayed immediately. The prompt includes a count of the number of seconds remaining until the query times out. In order to make it a nice visually consistent length, we use a fixed-width field for the countdown (“%2d”). Note that we don’t use

puts

to print the prompt – we don’t want it to advance to the next line, because we want to be able to dynamically rewrite the prompt as the countdown proceeds.

After we are done yielding to the block, we erase the prompt in preparation for the next cycle. In order to erase it we create and output string of backspaces (“\b”) the same length as the prompt.

Now we need a way to wait until the user types something, while still periodically updating the prompt.

        wait_for_input($stdin, seconds_left % 1) do

We pass

wait_for_input

an input stream and a (potentially fractional) number of seconds to wait. In this case we only want to wait until the next second-long “tick” so that we can update the countdown. So we pass in the remainder of dividing seconds_left by 1. E.g. if seconds_left was 5.3, we would set a timeout of 0.3 seconds. After 3/10 of a second of waiting for input, the wait would time out, the prompt would be erased and rewritten to show 4 seconds remaining, and then we’d start waiting for input again.

Here’s the implementation of

wait_for_input

:

def wait_for_input(input, timeout)
  # Wait until input is available
  if select([input], [], [], timeout)
    yield
  end
end                             # wait_for_input

We’re using

Kernel#select

to do the waiting. The parameters to

#select

are a set of arrays – one each for input, output, and errors. We only care about input, so we pass the input stream in the first array and leave the others blank. We also pass how long to wait until timing out.

If new input is detected,

select

returns an array of arrays, corresponding to the three arrays we passed in. If it times out while waiting, it returns

nil

. We use the return value to determine whether to execute the given block or note. If there is input waiting we yield to the block; otherwise we just return.

While it takes some getting used to, handling IO timeouts with

select

is safer and more reliable than using the

Timeout

module. And it’s less messy than rescuing

Timeout::Error

every time a read times out.

Finally, we need to read and interpret the character the user types, if any.

          case char = $stdin.getc
          when ?y, ?Y then return true
          when ?n, ?N then return false
          else                  # NOOP
          end

If the user types ‘y’ or ‘n’ (or uppercase versions of the same), we return

true

or

false

, respectively. Otherwise, we simply ignore any characters the user types. Typing characters other than ‘y’ or ‘n’ will cause the loop to be restarted.

Note the use of character literals like

?y

to compare against the integer character code returned by

IO#getc

. We could alternately use

Integer#chr

to convert the character codes into single-character strings, if we wanted.

Wrapping up, we make sure to return the default value should the timeout expire without any user input; and we output a newline to move the cursor past our prompt.

  return default

And there you have it; a yes/no prompt with a timeout and a visual countdown. Static text doesn’t really capture the effect, so rather than include sample output I’ll just suggest that you try the code out for yourself (sorry, Windows users, it’s *NIX-only).

Full source for this article at: http://gist.github.com/148765

Written by avdi

July 16, 2009 at 10:45 pm

4 Responses

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  1. This is nice, but I need a cross-platform solution that works on Windows!😦

    Jeb

    July 17, 2009 at 3:05 pm

  2. Maybe Windows 7 will be POSIX compatible, who knows?😉

    wayward

    July 17, 2009 at 4:43 pm

  3. It should not be hard to get this working on Windows. Since the Windows shell always gets its input character-by-character, there's no need for the Termios step. The rest should be the same or very similar.

    avdi

    July 18, 2009 at 10:22 pm

  4. […] You can alter these default terminal device behaviours using the Ruby “termios” gem. […]


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