CRON is a very powerful tool in the *nix world. It is used to schedule jobs, tasks, or scrips. Any action performed on the command line can be executed via CRON.

Each user has their own crontab file which they can use to run their own scheduled tasks. The permissions for these tasks are identical to the users permissions at the command line. The system also has a series of cron related files and folders. These are /etc/crontab, /etc/cron.hourly, /etc/cron.daily /etc/cron.daily, /etc/cron.monthly. The items above with the '.' in the name are directories where scrips can be stored or linked to. The are run in the order that they appear in the directory. The directory contents are actionable by cron through the commands below which are found in /etc/crontab
# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

The start of the file is shown blow and is there to pass parameters to the cron process before it starts to action on any of the line items. Each item is pretty self explanatory.
Issuing Commands
Each command line argument that cron encounters has to include when cron is to run the command line. Below is the format and syntax of the cron timetable.
.---------------- minute (0 - 59)  
|  .------------- hour (0 - 23) 
|  |  .---------- day of month (1 - 31) 
|  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ... 
|  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7)  OR sun,mon,tue,wed,thu,fri,sat
|  |  |  |  | 
*  *  *  *  *  command to be executed 
Multiple date/time values
There are several ways of specifying multiple date/time values in a field:
  • The comma (',') operator specifies a list of values, for example: "1,3,4,7,8" (space inside the list must not be used)
  • The dash ('-') operator specifies a range of values, for example: "1-6", which is equivalent to "1,2,3,4,5,6"
  • The asterisk ('*') operator specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to 'every hour' (subject to matching other specified fields).
There is also an operator which some extended versions of cron support, the slash ('/') operator (called "step"), which can be used to skip a given number of values. For example, "*/3" in the hour time field is equivalent to "0,3,6,9,12,15,18,21".

Here is a simple example for rotating log files for Squint.
# squint squid reports
# Daily at 3am
00 03 * * *     root /usr/local/bin/ daily
# Weekly, on Mondays
00 01 * * 1     root /usr/local/bin/ weekly
# Monthly, on the first day of the month
00 02 1 * *     root /usr/local/bin/ monthly
Notice that root appears in the fifth field. This is a requirement of some implementations of cron.

More info
Issue the command man cron or search in Google, for more detailed information.

The most common version of cron was written by Paul Vixie. The most recent implementations should conform to IEEE Std1003.2-1992

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