linux_maintenance_crontab [My wiki]

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====Overview====
Crontab servers for jobs scheduling. In a nutshell, administrators put script there which they want to be executed at specific time or in specific frequency.
As such crontab is suitable for:

  * Backup
  * Housekeeping
  * Archiving
  * Monitoring

=====Configuration====
Configuration of crontab is fairly simply if we remember couple things. First let's check the syntax:

<sxh bash>
*     *     *   *    *        command to be executed
-     -     -   -    -
|     |     |   |    |
|     |     |   |    +----- day of week (0 - 6) (Sunday=0)
|     |     |   +------- month (1 - 12)
|     |     +--------- day of        month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)
</sxh>

Having seen that, a job in crontab will look as follows:


===Example===
<sxh bash>
30     18     *     *     *         rm /tmp/* >> /root/clean.log
</sxh>

This job, will execute each day at 18:30 and will delete everything in the temporary directory, and will write the log in /root/clean.log

 In order to create such a job, we have to execute the following: 

<sxh bash>
[root@mysqlmaster ~]# crontab -e <- "e" - stands for Edit
no crontab for root - using an empty one

~
30     18     *     *     *         rm /tmp/* >> /root/clean.log

:wq

</sxh>

To list all the jobs, we can issue the following command:

<sxh bash>
[root@mysqlmaster ~]# crontab -l
30     18     *     *     *         rm /tmp/* >> /root/clean.log
[root@mysqlmaster ~]#
</sxh>


**Crontab configurations are per user, so each user has his own crontab, be sure that your user has the correct permissions to the the job**