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Readline

To use this module, do require('readline'). Readline allows reading of a stream (such as STDIN) on a line-by-line basis.

Note that once you've invoked this module, your node program will not terminate until you've closed the interface, and the STDIN stream. Here's how to allow your program to gracefully terminate:

var rl = require('readline');

var i = rl.createInterface(process.stdin, process.stdout, null);
i.question("What do you think of node.js?", function(answer) {
  // TODO: Log the answer in a database
  console.log("Thank you for your valuable feedback.");

  // These two lines together allow the program to terminate. Without
  // them, it would run forever.
  i.close();
  process.stdin.destroy();
});

rl.createInterface(input, output, completer)

Takes two streams and creates a readline interface. The completer function is used for autocompletion. When given a substring, it returns [[substr1, substr2, ...], originalsubstring].

Also completer can be run in async mode if it accepts two arguments:

function completer(linePartial, callback) { callback(null, [['123'], linePartial]); }

createInterface is commonly used with process.stdin and process.stdout in order to accept user input:

var readline = require('readline'),
  rl = readline.createInterface(process.stdin, process.stdout);

rl.setPrompt(prompt, length)

Sets the prompt, for example when you run node on the command line, you see >, which is node's prompt.

rl.prompt()

Readies readline for input from the user, putting the current setPrompt options on a new line, giving the user a new spot to write.

rl.question(query, callback)

Prepends the prompt with query and invokes callback with the user's response. Displays the query to the user, and then invokes callback with the user's response after it has been typed.

Example usage:

interface.question('What is your favorite food?', function(answer) {
  console.log('Oh, so your favorite food is ' + answer);
});

rl.close()

Closes tty.

rl.pause()

Pauses tty.

rl.resume()

Resumes tty.

rl.write()

Writes to tty.

Event: 'line'

function (line) {}

Emitted whenever the in stream receives a \n, usually received when the user hits enter, or return. This is a good hook to listen for user input.

Example of listening for line:

rl.on('line', function (cmd) {
  console.log('You just typed: '+cmd);
});

Event: 'close'

function () {}

Emitted whenever the in stream receives a ^C or ^D, respectively known as SIGINT and EOT. This is a good way to know the user is finished using your program.

Example of listening for close, and exiting the program afterward:

rl.on('close', function() {
  console.log('goodbye!');
  process.exit(0);
});

Here's an example of how to use all these together to craft a tiny command line interface:

var readline = require('readline'),
  rl = readline.createInterface(process.stdin, process.stdout),
  prefix = 'OHAI> ';

rl.on('line', function(line) {
  switch(line.trim()) {
    case 'hello':
      console.log('world!');
      break;
    default:
      console.log('Say what? I might have heard `' + line.trim() + '`');
      break;
  }
  rl.setPrompt(prefix, prefix.length);
  rl.prompt();
}).on('close', function() {
  console.log('Have a great day!');
  process.exit(0);
});
console.log(prefix + 'Good to see you. Try typing stuff.');
rl.setPrompt(prefix, prefix.length);
rl.prompt();

Take a look at this slightly more complicated example, and http-console for a real-life use case.

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