Cron/at

From Linux 101, The beginner's guide to all things Linux.

Cron is a utility that allows events to be scheduled within Linux. An example of an event may be a script to update a log file or send an email out. In fact as long as it's executable, cron can run it. Cron is useful for scheduling recurring events.

The table that cron uses to schedule events is called the crontab and there is usually a crontab for each user including root. To view the crontab for the user that is currently logged in, you can type crontab -l and to view the crontab for a different user you can type crontab -u user -l replacing user with the actual user account in question. For example -

     crontab -u john -l
     00 09 * * Sat john updatedb

     30 00 01 * * root tar czf /root/homebak/backup.tar.gz /home >> /dev/null 2>&1

The crontab fields are exepected in this order:

     MM hh dd mm dow user path

where

     MM = minute (00-59)

     hh = hour (00-23)

     dd = day of month (01-31)

     dow = (0-6 or Sun, Mon, Tue etc)

     user = the user account to execute the cron job

     path = the path of the executable file to run

The above is an example of a user cron entry. There are also general cron entries that are for use within a non-user context and these are broken down into time periods. Linux uses the following directories for placing non-user cron files.

     /etc/cron.hourly

     /etc/cron.daily

     /etc/cron.weekly

     /etc/cron.monthly

If you view the files in these locations you can see several entries that have been added as part of the Linux installation process.

     [root@aiki cron.daily]# ls -l

     total 40

     lrwxrwxrwx  1 root root   28 Nov 13 18:03 00-logwatch -> ../log.d/scripts/logwatch.pl

     -rwxr-xr-x  1 root root  135 Mar 27  2004 00webalizer

     -rwxr-xr-x  1 root root  276 Feb 15  2004 0anacron

     -rwxr-xr-x  1 root root  180 Feb 15  2004 logrotate

     -rwxr-xr-x  1 root root  418 Mar 31  2004 makewhatis.cron

     -rwxr-xr-x  1 root root 1603 May  5  2004 prelink

     -rwxr-xr-x  1 root root  104 Apr 16  2004 rpm

     -rwxr-xr-x  1 root root   82 Apr 16  2004 slocate.cron

     -rwxr-xr-x  1 root root  100 Mar 25  2004 tetex.cron

     -rwxr-xr-x  1 root root  193 Feb 15  2004 tmpwatch

     -rwxr-xr-x  1 root root  136 May 11  2004 yum.cron

     [root@aiki cron.daily]#

Looking at one example file...

     [root@aiki cron.daily]# cat slocate.cron

     !/bin/sh

     . /etc/updatedb.conf

     renice +19 -p $$ >/dev/null 2>&1

     /usr/bin/updatedb

     [root@aiki cron.daily]#

As you can see, the slocate.cron file that cron runs will execute several scripts to update the 'locate' database for the slocate utility.
Type man cron for more information.