How to schedule cron job Linux

Schedule Crontab jobs

Crontab
It's a unix solaris utility that tasks to be run automatically in background at regular interval.
Daemon: crond

Accessibality:
/usr/lib/cron/cron.allow -> Use can use if your name exist in this file
In absence of this file it's necessary that your name does not exist in
/usr/lib/cron/cron.deny.
And If only cron.deny exist but empty then all users can use crontab.
But if neither exists only root can use crontab.

Commnad:
crontab -l ->List crontab file
crontab -e ->Edit crontab file
crontab -r ->Remove crontab file

====Crontab file fields

* * * * * 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)

Example:
0> Edit crontab for root 
# Login as root
# crontab -e


1>Edit crontab for user shukla
# crontab -u shukla -e

2>Display crontab defined by user shukla
# crontab -u shukla -l

3>Delete shukla's tmp file /home/shukla/dumps/*.* every day at 3:30pm
# crontab -u shukla -e
30 15 * * * * rm /home/shukla/dumps/*

4>Do above every monday-wednesday at 3pm but only in mnth of august
# crontab -u shukla -e
* 15 * * 8 1-3 rm /home/shukla/dumps/*

5>Do above on 1,23 of aug
* 15 1,23 * 8 1-3 rm /home/shukla/dumps/*

6>Do Above at 10:10AM and 6:20pm everyday...
10,20 10,18 * * * rm /home/shukla/dumps/*

7> Sample tricky examples
>Run Every hour
0 * * * * ....
>Run every min
* * * * * * ....

 >Run every 5 min
*/5 * * * *  .....

>Run ever 2 hour interval eg:00:05, 02:05
5 */2 * * *

>Ever month except april
* * * jan-mar,5-12 * * OR
* * * 1-3,5-12 * *

>On every reboot display msg alert
@reboot wall "PC is going to REBOOT"

>Ever hour same above --->this will run 1st sec of every hr equivalent to (0 * * * *)
@hourly wall "PC is going to REBOOT"

Some similar too:
@yearly --> (0 0 1 1 *)
@annually --> (0 0 1 1 *)
@monthly --> (0 0 1 * *)
@weekly --> (0 0 * * 0)
@daily --> (0 0 * * *)
@midnight --> (0 0 * * *)

--Shirish Shukla