Using Cron scheduler

Cron is a daemon that executes scheduled commands and is started automatically from /etc/init.d when multiuser run levels are started.

To list all cron jobs

crontab -l

To start a new crontab

crontab -e

Some systems will give you a choice of editor to edit the crontab file, if you want to set yours to a specific editor permanently type

export EDITOR=vi

And to set it for nano

export EDITOR=nano

Use the same process for your favorite editor

Once the file is open add a line containing the format below followed by the command you want to run.

Minute Hour Day of month Month Day Command

0-59    0-23        1-31         1-12    0-6  

0         20           13             *        6

For example to run a cron job command looking at the amount of disk space that is free(df -h)  at 11:30 *(every day of the month) * (every month) 1-5(day of week)

30 11 * * 1-5 df -h

To run every 20 mins from 7 - 5 on mon,wed,fri

0,20,40 7-17 * * 1,3,5 du -sh /var