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Warm welcome to all linux newbies.. You will find a lot of interesting stuffs here which you can understand easily. Experienced guys please excuse..This is just a beginners guide....:)

Cron jobs tutorial for beginners

Cron is a utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon. These tasks are often termed as cron jobs. Crontab (CRON TABle) is a file which contains the schedule of cron entries to be executed at specified times.

commands:

crontab filename Install filename as your crontab file.
crontab -e --> Edit your crontab file.
crontab -l --> Show your crontab file.
crontab -r --> Remove your crontab file.
crontab -v --> Display the last time you edited your crontab file

Inorder to setup a cron you just need to specify the task and the time at which it should be executed. Definitely it should be in the format as below. It can be a little tough at the beginning, but will be much easier if you learn how to specify the time at which a particular task should be executed. Workout with examples to get more idea about cron jobs.

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)

ie,
minute hour day month day-of-week command-line-to-execute
0-59 0-23 1-31 1-12 0-7

In a linux server you can find cron entry for a user at /var/spool/cron/username


examples:

if you want a certain command to run at 5.30 am, you will have to code it as:
30 5 * * * command to execute

If you want something run at 8 pm everyday, it has to be coded as:
0 20 * * * command to execute (20 is 8 pm in the 24-hour time format)

If you want to set the cron job every sunday at midnight 11.30 PM
30 23 * * 0 command to execute (0--represents the Sunday)

If you want to run the task at 1am and 2am only from Monday to Friday:
* 1,2 * * 1-5 command to execute

If you want to execute a cronjob at 4 am every Sunday:
00 4 * * 0 command to execute

To execute a cronjob at 4:42 am every 1st of the month
42 4 1 * * command to execute

Cron also supports 'step' values.

A value of */2 in the date of month field means that the command runs every two days and */5 in the hours field would mean the command runs every 5 hours.

Difference between */5 * * * * and 5 * * * *

*/5 * * * * command to execute -->> This will execute the cronjob in every 5
minutes.

5 * * * * command to execute -->> This will execute cron job in 5th minute of every hour

If both the day of month and day of week are specified, the command will be executed when either of the events happen.

* 12 16 * 1 command to execute -->> command will be executed on every Monday and every 16th

Cron also accepts lists in the fields. Lists can be in the form, 1,2,3 (meaning 1 and 2 and 3) or 1-3 (also meaning 1 and 2 and 3)

59 11 * * 1,2,3,4,5 command to execute -->> will execute the command at 11:59 Monday, Tuesday, Wednesday, Thursday and Friday

to be continued...

ref:
http://www.unixgeeks.org/security/newbie/unix/cron-1.html
http://en.wikipedia.org/wiki/Cron
http://www.aota.net/Script_Installation_Tips/cronhelp.php3
http://www.thesitewizard.com/general/set-cron-job.shtml
https://www.simplehelix.com/hosting/knowledgebase.php?action=displayarticle&catid=4&id=77