Recipe 15.6 Reading Single Characters from the Keyboard

15.6.1 Problem

You want to read a single character from the keyboard. For instance, you've displayed a menu of one-character options, and you don't want to require users to press the Enter key to make their selection.

15.6.2 Solution

Use the CPAN module Term::ReadKey to put the terminal into cbreak mode, read characters from STDIN, and then put the terminal back into its normal mode:

use Term::ReadKey;

ReadMode 'cbreak';
$key = ReadKey(0);
ReadMode 'normal';

15.6.3 Discussion

Term::ReadKey can put the terminal into many modes—cbreak is just one of them. cbreak mode makes each character available to your program as it is typed (see Example 15-1). It also echoes the characters to the screen; see Recipe 15.10 for an example of a mode that does not echo.

Example 15-1. sascii

  #!/usr/bin/perl -w
  # sascii - Show ASCII values for keypresses
  
  use Term::ReadKey;
  ReadMode('cbreak');
  print "Press keys to see their ASCII values.  Use Ctrl-C to quit.\n";
  
  while (1) {
      $char = ReadKey(0);
      last unless defined $char;
      printf(" Decimal: %d\tHex: %x\n", ord($char), ord($char));
  }
  
  ReadMode('normal');

Using cbreak mode doesn't prevent the terminal's device driver from interpreting end-of-file and flow-control characters. If you want to be able to read a real Ctrl-C (which normally sends a SIGINT to your process) or a Ctrl-D (which indicates end-of-file under Unix), you want to use raw mode.

An argument of 0 to ReadKey indicates that we want a normal read using getc. If no input is available, the program will pause until there is some. We can also pass -1 to indicate a non-blocking read, or a number greater than 0 to indicate the number of seconds to wait for input to become available; fractional seconds are allowed. Non-blocking reads and timed-out reads return either undef when no input is available or a zero-length string on end-of-file.

Recent versions of Term::ReadKey also include limited support for non-Unix systems.

15.6.4 See Also

The getc and sysread functions in Chapter 29 of Programming Perl, and in perlfunc(1); the documentation for the Term::ReadKey module from CPAN; Recipe 15.8; Recipe 15.9