This guide aims to help you understand cryptic Crontab Syntax which is often hard to decipher at first glance, this guide will help you understand how to use and understand your crontab file. 

Crontab Syntax Chart

#---------------- Minute (0 - 59)
  |  #------------- Hour (0 - 23)
  |  |  #---------- Day of the Month (1 - 31)
  |  |  |  #------- Month (1 - 12)
  |  |  |  |  #---- Day of the Week (0 - 6) (Sunday=0 or 7)
  |  |  |  |  |
  *  *  *  *  *  example-script.sh

An example of a typical cronjob:

Min     Hour     Day Of Month     Month     Day of Week     Command / Script
30      *        *                *         1-5             echo "Live Long and prosper \\//"

The above cronjob would run every half an hour from Monday to Friday.

The following table should help you understand crontabs six field format, starting at the left and working across to the actual command / script.

Field Possible Values
Minute                     0-59 (* means every minute at the start of each minute)
Hour                        0-23 (* means every hour at the start of every hour)
Day of Month          0-31 (* means every day)
Month                      1-12 (* means every month, you can use month names if you prefer)
Day of Week           0-7 (0 and 7 both mean Sunday, again you can use names – see below)

An alternative to the six field crontab syntax:

@daily         last | mail user@domain.com

Here is a list of the most common @ options for Cron:

@reboot                -       This runs the Cron job when the machine is started up or if the Cron daemon is restarted

@midnight            -       This runs the Cron job once a day at midnight, it’s the equivalent of 0 0 * * *

@daily                    -       Does exactly the same as @midnight

@weekly                -       This runs a Cron job once a week on a Monday morning and is the equivalent of 0 0 * * 0

@monthly             –       This runs a Cron job once a month on the first day of every month at midnight and is the same as 0 0 1 * *

@annually             –       Runs a Cron job once a year at midnight on the first day of the first month and is the equivalent of 0 0  1 1 *

@yearly                  -       This is the same as annually

If you have your scripts ready there are a number of directories you can drop your scripts into located in the /etc directory such as /etc/cron.daily