What is cron?
Cron is the name of program that enables unix users to execute commands or
scripts (groups of commands) automatically at a specified time/date. It is
normally used for sys admin commands, like makewhatis, which builds a
search database for the man -k command, or for running a backup script,
but can be used for anything.
An experienced Linux sysadmin knows the importance of running the routine maintenance jobs in the background automatically. Linux Cron utility is an effective way to schedule a routine background job at a specific time and/or day on an on-going basis.
Cron job pattern
Cron format is a simple, yet powerful and flexible way to define time and frequency of various actions. nnCron make active use of cron format in both classic and extended modes.
Traditional (inherited from Unix) cron format consists of five fields separated by white spaces:
<Minute> <Hour> <Day_of_the_Month> <Month_of_the_Year> <Day_of_the_Week>
nnCron can use both traditional and “enhanced” version of cron format, which has an additional (6th) field: <Year>:
<Minute> <Hour> <Day_of_the_Month> <Month_of_the_Year> <Day_of_the_Week> <Year>
A user can select the format he would like to use by selecting or unselecting the Year field checkbox on General tab in Options dialog (which can be opened by doublecliking the nnCron icon in system tray). By default, nnCron uses the enhanced format.
The following graph shows what it consists of:
* * * * * *
| | | | | |
| | | | | +– Year (range: 1900-3000)
| | | | +—- Day of the Week (range: 1-7, 1 standing for Monday)
| | | +—— Month of the Year (range: 1-12)
| | +——– Day of the Month (range: 1-31)
| +———- Hour (range: 0-23)
+———— Minute (range: 0-59)
Examples:
| Expression | Description |
| * * * * * | Each minute |
| 59 23 31 12 5 * | One minute before the end of year if the last day of the year is Friday |
| 59 23 31 DEC Fri * | Same as above (different notation) |
| 45 17 7 6 * * | Every year, on June 7th at 17:45 |
| 45 17 7 6 * 2001,2002 | Once a year, on June 7th at 17:45, if the year is 2001 or 2002 |
| 0,15,30,45 0,6,12,18 1,15,31 * 1-5 * | At 00:00, 00:15, 00:30, 00:45, 06:00, 06:15, 06:30, 06:45, 12:00, 12:15, 12:30, 12:45, 18:00, 18:15, 18:30, 18:45, on 1st, 15th or 31st of each month, but not on weekends |
| */15 */6 1,15,31 * 1-5 * | Same as above (different notation) |
| 0 12 * * 1-5 * (0 12 * * Mon-Fri *) | At midday on weekdays |
| * * * 1,3,5,7,9,11 * * | Each minute in January, March, May, July, September, and November |
| 1,2,3,5,20-25,30-35,59 23 31 12 * * | On the last day of year, at 23:01, 23:02, 23:03, 23:05, 23:20, 23:21, 23:22, 23:23, 23:24, 23:25, 23:30, 23:31, 23:32, 23:33, 23:34, 23:35, 23:59 |
| 0 9 1-7 * 1 * | First Monday of each month, at 9 a.m. |
| 0 0 1 * * * | At midnight, on the first day of each month |
| * 0-11 * * * | Each minute before midday |
| * * * 1,2,3 * * | Each minute in January, February or March |
| * * * Jan,Feb,Mar * * | Same as above (different notation) |
| 0 0 * * * * | Daily at midnight |
| 0 0 * * 3 * | Each Wednesday at midnight |
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