LinuxQuestions.org - about cron !!!

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Dear Friends ,

I m confused about the following usage of cron :

*/5 * * * * /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1

In the above example , what is the meaning of "/dev/null" AND what is the meaning of "2>&1" ?

My second question is , is it possible to run the above script in every five seconds ?

Waitig for ur kind rely . .. ..

/dev/null 2>&1 is where the cron script or command is sending/redirecting the STDOUT and STDERR.

For more details:
man stdout
man stderr

Cronjobs are limited to every minute. If you want something to run every 5 seconds, you'll need to write your own script.

Code:

#!/bin/sh

while true

do

  /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1

  sleep 5

done

Run that in the background, screen process or create your own startup script. ;)

But if you really wanted to do this within cron, you'd have to do something like this:

* * * * * /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
* * * * * sleep 5; /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
* * * * * sleep 10; /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
* * * * * sleep 15; /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
* * * * * sleep 20; /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
* * * * * sleep 25; /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
* * * * * sleep 30; /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
* * * * * sleep 35; /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
* * * * * sleep 40; /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
* * * * * sleep 45; /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
* * * * * sleep 50; /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
* * * * * sleep 55; /usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1

It's basically running the same script every minute but with the sleep command, delays the /usr/bin/mrtg command, so with those increments in seconds, would cause the command to run every 5 seconds. But I wouldn't recommend this way, especially every 5 seconds. Is there a reason you need to run every 5 seconds, is a minute at minimum not sufficient?