editing crontab

meniscus · 12-11-2006, 07:05 AM

The following code excecutes rrd_hddtemp.pl every 1 min

Code:

*/1 * * * * root /usr/local/bin/rrd_hddtemp.pl > /dev/null

Can i modify it to run every 1 sec?

acid_kewpie · 12-11-2006, 07:14 AM

every second??? if you want to do someting once a second you are not doing it right... somethign runnign that fast needs to be programmed correctly. you realise that every time you run a cron job you start up a brand new environment, source profile configs and such, that's a massive overhead to do once a second and then through away... update your perl script to run as a loop or something. hddtemp sounds like a temperature related thing yeah? you really thing a harddrive can change it's temperature significantly once a second???

meniscus · 12-11-2006, 07:32 AM

Yes but ive edited that script to monitor CPU temperature 2. A CPU could burn up in 1 min if a fan failed or something couldnt it?

Every 10 secs then? It cant be that much load on the CPU can it?

acid_kewpie · 12-11-2006, 08:19 AM

depends how well or badly written the script is... if a cpu is going to burn up within a minute, seems you'd need to also react to it even sooner don't you think? can you do anything within 10 seconds?

either way cron works on a per minute basis. by definition, if you need to do somethign mroe often, a generic extneral scheduler is not going to be a good way to do it.

matthewg42 · 12-11-2006, 08:40 AM

AFAIK, cron's smallest time unit is a minute. I wouldn't advise invoking running your perl program every second - this will start a new perl process and compile the script every execution, which is going to be horribly inefficient.

It would be much better to have a loop inside your script which does whatever it is you want to do, then sleeps for a second. For example:

Code:

#!/usr/bin/perl -w

use strict

while (1) {
    do_stuff();
    sleep 1;
} 

sub do_stuff {
    ...
}

If you're worried about such a program getting killed, you could have a cron job which tests if the program is running and re-launches it if necessary, or, if you have root access, have a respawn entry in your inittab file. Be very careful when putting stuff in inittab. You can make your system un-bootable if you mess this up.

meniscus · 12-11-2006, 09:33 AM

ok thanks. Ill steer clear of initab 2!