Wednesday, 16 May 2012

Cron in Linux

               Cron job are used to schedule commands to be executed periodically. You can setup commands or scripts, which will repeatedly run at a set time

#edit cron table
crontab -e

MIN HOUR DOM MON DOW CMD

   *        *        *        *        *    command to be executed

    OR

* * * * *    USERNAME    command to be executed
|  |  | |  |
|  |  | |  |----- Day of week (0 - 6) (Sunday=0)
|  |  | |-------- Month (1 - 12)
|  |  |---------- Day of month (1 - 31)
|  |------------- Hour (0 - 23)
|---------------- Minute (0 - 59)

#List
crontab -l
crontab -u username -l

#Erase
crontab -r
crontab -r -u username

#run every 10 minutes
*/10 * * * * command

#stop receiving email output
0 3 * * * /root/backup.sh >/dev/null 2>&1

#Backup
crontab -u root(username) -l > /home/cron.backup
0 12 * * * /usr/bin/top -n 1 -b -S >> /home/cron.backup

#Version
rpm -qa | grep cron
rpm -qil version

#Schedule
Keyword     Equivalent
@yearly      0 0 1 1 *
@daily        0 0 * * *
@hourly      0 * * * *
@reboot      Run at startup.

The asterisk (*) : specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to every hour or an asterisk in the month field would be equivalent to every month.

The comma (,) : specifies a list of values, for example: "1,5,10".

The dash (-) : specifies a range of values, for example: "5-10" days , which is equivalent to typing "5,6,7,8,9,10" using the comma operator.


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