You can do that with xscreensaver.
From xscreensaver man:
--watch Prints a line each time the screensaver changes state: when the screen blanks, locks, unblanks, or when the running hack is
changed. This option never returns; it is intended for use by shell
scripts that want to react to the screensaver in some way. An example
of its output would be:
BLANK Fri Nov 5 01:57:22 1999
RUN 34
RUN 79
RUN 16
LOCK Fri Nov 5 01:57:22 1999
RUN 76
RUN 12
UNBLANK Fri Nov 5 02:05:59 1999
The above shows the screensaver activating, running three different
hacks, then locking (perhaps because the lock-timeout went off) then
unblanking (because the user became active, and typed the correct
password.) The hack numbers are their index in the `programs' list
(starting with 1, not 0, as for the --select command.)
For example, suppose you want to run a program that turns down the
volume on your machine when the screen blanks, and turns it back up
when the screen un-blanks. You could do that by running a Perl program
like the following in the background. The following program tracks the
output of the --watch command and reacts accordingly:
#!/usr/bin/perl
my $blanked = 0;
open (my $in, "xscreensaver-command -watch |") || die;
while (<$in>) {
if (m/^(BLANK|LOCK)/) {
if (!$blanked) {
system ("sound-off");
$blanked = 1;
}
} elsif (m/^UNBLANK/) {
system ("sound-on");
$blanked = 0;
}
}
Note that LOCK might come either with or without a preceding BLANK
(depending on whether the lock-timeout is non-zero), so the above
program keeps track of both of them.
