Of course I still use Perl

Imagine this scenario: you ran a program to do some (potentially) long-running task. The program has a --verbose or --debug command-line option to print progress report to stdout and/or log file. This time you forgot to issue the command-line option and 10 minutes into it you wonder how the program is doing. You could start over by terminating the program and run it again. But it would be nice if we can get the running program to report its progress.

We can of course. One of the ways is to use Unix signals. We can use either SIGUSR1 or SIGUSR2, which are reserved for application purposes. Here's a sample program:

#!/usr/bin/env perl

use 5.010;

use strict;

use warnings;

use Progress::Any;

my $progress = Progress::Any->get_indicator(task => '');

my $verbose = @ARGV && $ARGV[0] eq '--verbose';

$SIG{USR1} = sub {

    if ($progress->target) {

        say sprintf("task progress: %d/%d (%d secs remaining)",

                    $progress->pos, $progress->target, $progress->remaining);

    } else {

        say "task not running";

    }

};

my $end = int(20+100*rand());

$progress->target($end);

for (1..$end) {

    sleep 2;

    $progress->update(pos => $_);

    if ($verbose) {

        say sprintf("(verbose) task progress: %d/%d (%d secs remaining)",

                $_, $end, $progress->remaining);

    }

}

$progress->finish;

say "Finished";

If we invoke the program with --verbose, it will nicely report its progress on the stdout:

%  ./program --verbose
(verbose) task progress: 1/47 (0 secs remaining)
(verbose) task progress: 2/47 (45 secs remaining)
(verbose) task progress: 3/47 (58 secs remaining)
...

However, if we left out the --verbose option, it will not output anything until finished. However, we can send a signal to it and it will print something like:

$ kill -USR1 %1
task progress: 2/70 (125 secs remaining)
% kill -USR1 %1
task progress: 10/70 (110 secs remaining)

The good ol' dd program also employs the same trick. If you send SIGUSR1 to it, it will print I/O statistics like how much data it has copied and the transfer speed.

Aside from the above technique, the HUP signal is also traditionally used. For example, if verbosity/logging-level is controlled from a config file, the HUP signal can be used to make our program re-read the config file and re-set the level. And thus we can temporarily increase logging level without having to start over or drop connections.