UNIX for Dummies Questions & Answers

I want to add a crontab entry which should execute for every 4 hours and that 4 hours calculation should begin from the current time.

Normally if I set the crontab entry like this,

Code:

00 */4 30 05 * root date >>/tmp/cronout

The above will execute the date command for every 4 hours like 4,8,12,16,20,24 on 30th May, for example I start the crontab at 02:00:00. But if I start the crontab at 09:15:00, it should execute the crontab command after 4 hours like 13:15:00, then 17:15:00 like this.

Can any one please tell me, how to achieve this in crontab ?

Code:

15 */4 30 05 *   /usr/bin/date >>/tmp/cronout

If I start the above crontab at 9:00:00, it will execute the date command at 12:15:00, but I need this should be executed at 13:15:00 !

It wont work !

Is there any other way ?

You can setup cron to run @ specific start time only and use something like this :

Code:

15 9 * * * yourjob.sh

With yourjob.sh :

Code:

INTERVAL=14400 ## 4 hours in seconds
ITT=4 ## number of iterations
i=1
while (( i <= $ITT ))
do
date & ## i used date for test, this should be yourjb
sleep $INTERVAL
i=$(expr $i + 1)
done

You shouldn't loose to much seconds and it will run 9.15, 13.15, 17.15,21.15

I need to achieve this with the crontab entry itself, not using any other script.

In the crontab, I need to initialise the job to call another script for every 4 hours from the current time.

Time specified here is only for example.

Don't understand what triggers the start of this recursive process.
What would you type to cause this event?

@Ganeshwari, why do you have to use cron?

Don't use cron. It wasn't designed for that.

Just start a job, perhaps a wrapper, that runs your script every 4 hours.