Recipe 15.7 Ringing the Terminal Bell

15.7.1 Problem

You want to sound an alarm on the user's terminal.

15.7.2 Solution

Print the "\a" character to sound a bell:

print "\aWake up!\n";

Or use the "vb" terminal capability to show a visual bell:

use Term::Cap;

$OSPEED = 9600;
eval {
    require POSIX;
    my $termios = POSIX::Termios->new( );
    $termios->getattr;
    $OSPEED = $termios->getospeed;
};

$terminal = Term::Cap->Tgetent({OSPEED=>$OSPEED});
$vb = "";
eval {
    $terminal->Trequire("vb");
    $vb = $terminal->Tputs('vb', 1);
};

print $vb;                                  # ring visual bell

15.7.3 Discussion

The "\a" escape is the same as "\cG", "\007", and "\x07". They all correspond to the ASCII BEL character and cause an irritating ding. In a crowded terminal room at the end of the semester, this beeping caused by dozens of vi novices all trying to get out of insert mode at once can be maddening. The visual bell is a workaround to avoid irritation. Based upon the polite principle that terminals should be seen and not heard (at least, not in crowded rooms), some terminals let you briefly reverse the foreground and background colors to give a flash of light instead of an audible ring.

Not every terminal supports the visual bell, which is why we eval the code that finds it. If the terminal doesn't support it, Trequire will die without having changed the value of $vb from "". If the terminal does support it, the value of $vb will be set to the character sequence to flash the bell.

There's a better approach to the bell issue in graphical terminal systems like xterm. Many of these let you enable the visual bell from the enclosing application itself, allowing all programs that blindly output a chr(7) to become less noisy.

15.7.4 See Also

The section on "String Literals" in Chapter 2 of Programming Perl or the section on "Quote and Quote-like Operators" in perlop(1); the documentation for the standard Term::Cap module