Cron is a utility that schedules tasks to run automatically in the background at the interval specified by the cron daemon, which then executes these tasks. Crontab is the configuration file that contains a list of items to be run and their schedule, so it is in this file that the time is configured. A cronjob is the name of a single job or task.
*/5 * * * * /usr/sbin/ntpdate -u time.nist.gov
In this example, it is running the ntpdate, which synchronizes the time at an interval of five minutes against time.nist.gov.
You can edit the crontab file by using the command
or by accessing the file via an editor such as vi.
We have a number of cron jobs the most common are:
* * * * root /opt/ma/netmail/sbin/cleancores -d 2>&1 >/dev/null
This does a clean up of the core files (mplus-cleancores)
* * * * root /opt/ma/netmail/sbin/ndswatchdog 2>&1 >/dev/null
This does a status check on the Novell Directory Services (mplus-ndswatchdog)
* * * * root /opt/ma/ooo/bin/test-ooo.sh 2>&1 >/dev/null
This runs a test on the attachment policy service (mplus-ooo)
0 0 * * 0 root /opt/novell/eDirectory/bin/ndsrepair -U -A no -F /root/ndsrepair.log >> /dev/null 2>&1
This runs a repair of the Novell Directory Services (mplus-ndsrepair)
0 * * * * root /opt/ma/netmail/sbin/nsrlupdate 2>&1 >/dev/null
his runs an update to the spam engine (mplus-nsrlupdate)
/7 * * * * root /opt/ma/netmail/sbin/rblcheck 2>&1 >/dev/null
This runs the rblcheck tool, which re-registers your system with the service (mplus-rblcheck)
This disables the root user from receiving system mail.
Any output from the commands above is essentially deleted by the job itself (2>&1 >/dev/null - output sent to dev/null).