CRONTAB - Quick Reference

Crontab Commands

crontab -e	Edit your crontab file, or create one if it doesn’t already exist.
crontab -l	Display your crontab file.
crontab -r	Remove your crontab file.

Crontab syntax
A crontab file has five fields for specifying day , date and time followed by the command to be run at that interval.

*     *     *   *    *        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)

The value column can have a * or a list of elements separated by commas. An element is either a number in the ranges shown above or two numbers in the range separated by a hyphen (meaning an inclusive range).
  
Crontab Example
A line in crontab file like below removes the tmp files from /home/someuser/tmp each day at 6:30 PM.

     30     18     *     *     *         rm /home/someuser/tmp/*
  
Disable Email
By default cron jobs sends a email to the user account executing the cronjob. If this is not needed put the following command At the end of the cron job line.
 
    >/dev/null 2>&1

Generate log file
To collect the cron execution execution log in a file :
 
    30 18 * * * rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log