#!/usr/bin/perl
#
# Outputs the ID of the active window
# in C hex format, e.g. 0x602f9a.
# If no window is active, it outputs 0x0.
#
use strict; use warnings;

{
    $_ = `xprop -root _NET_ACTIVE_WINDOW`; # e.g. _NET_ACTIVE_WINDOW(WINDOW): window id # 0x602f9a
    /id \# (0x[0-9a-f]+)$/ or die;
    my $active_window_ID = $1; # e.g. 0x602f9a
    print "$active_window_ID\n";
}