Subj : Re: Remove Oldest Process Every Minute. How To?
To   : comp.os.linux
From : nobody
Date : Thu Aug 26 2004 11:23 pm

Juul <just@freemail.nl> spewed this unto the Network: 
> I've build a picturegrabber in PHP that starts every minute from within a
> shellscript that runs as a cronjob.
> 
> The PHP-script needs about 5 minutes to complete and sometimes it will
> hang. That's why I wrote a shellscript for it.
> 
> Every minute a new session of the PHP-script starts. There can be more than
> 1 process of the PHP-script active but not more than 6. So the old
> instances can complete when a new instance of the PHP-script starts. When
> the 6th process starts then all processes of the PHP-script would be killed
> by the shellscript. This is my problem...
> 
> By killing all the processes of the PHP-script it kills also the just
> started processes and I actually want to kill only the oldest process when
> the 6th process starts. How can I do this?

Create a directory for your PHP scripts to put PID files in. In this
directory, each PHP script will put a file that contains the PID
in the name:

<?php

function makepidfile() {
  $pid = getmypid();
  system("/bin/touch /local/www/data/pids/$pid.pid");
}

?>


You can then use the file-creation times to determine which PID is the
oldest. The shell script might look like this:

#!/bin/sh

LS=/bin/ls
GREP=/bin/grep
WC=/usr/bin/wc
PHP=/usr/bin/php
AWK=/usr/bin/awk
PATH=/local/www/data/cgi-bin
HEAD=/usr/bin/head

# If your PHP script fails to remove the PID file after a
# normal (not killed by this script) termination, and
# another process running as the same user gets the same
# PID, then this script will accidentally kill the
# wrong process. However, the stale .pid file will get
# removed.

while [ $($LS /local/www/data/pids/ | $WC -l) -ge 6 ]; do
    $OLDEST = $($LS -r --sort=time /local/www/data/pids | $HEAD -n 1)
    kill -9 $(basename $OLDEST .pid)
    rm -f $OLDEST
done

.