Perl Cookbook

Reading from the Keyboard

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 the user to press Enter to make their selection.

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';

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 Section 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 ...