Installing a Crontab File

What is cron?

Crontab is a simple text file that holds a list of commands that are to be run at specified times.Cron is a that executes shell commands periodically on a given schedule. Cron is driven by a crontab, a configuration file that holds details of what commands are to be run along with a timetable of when to run them.

Description:

crontab -e [Edits a copy of the user's crontab file]

crontab -r [Remove the user's crontab files]

crontab -l [Lists the user's crontab file]

Crontab syntax:

A crontab file has six fields for specifying minute, hour, day of month, month, day of week and the command to be run at that interval. See below:

*     *     *     *     *  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)

Any line beginning with a ``#'' is a comment and is ignored.

Crontab examples:

* * * * *  #Runs every minute
*/2 * * * *  #Runs every 2 minutes
35 * * * *  #Runs at 35 minutes past the hour
30 5 * * *  #Runs at 5:30 am every day
45 18 * * *  #Runs at 6:45 pm every day
00 3 * * 0  #Runs at 3:00 am every Sunday
00 2 * * 6  #Runs at 2:00 am every Satuday
00 1 * * Sun  #Runs at 1:00 am every Sunday
30 5 1 * *  #Runs at 5:30 am on the first day of every month
00 0-23/2 03 08 *  #Runs every other hour on the 3rd of August


There are also special strings that can be used:
@reboot  #Runs at boot
@yearly  #Runs once a year [0 0 1 1 *]
@annually  #Runs once a year [0 0 1 1 *]
@monthly  #Runs once a month [0 0 1 * *]
@weekly  #Runs once a week [0 0 * * 0]
@daily  #Runs once a day [0 0 * * *]
@midnight  #Runs once a day [0 0 * * *]
@hourly  #Runs once an hour [0 * * * *]