Linux:Schedule a job with crontab

By default Linux will be running cron daemon in the background "ps -ef|grep crond" which is responsible to trigger the cron jobs.crond daemon is initiated by  init - /etc/init.d/crond
Jobs will be triggered with the help of crontab entry in "/etc/crontab".
Commands
To display list of crontab entires use "crontab -l"  in your shell 
To edit the crontab file use "crontab -e" in your shell 
Syntax

*    *    *    *    *  command to be executed
-    -    -    -    -
|    |    |    |    |
|    |    |    |    |
|    |    |    |    +----- day of week (0 - 7) (Sunday=0 or 7)
|    |    |    +---------- month (1 - 12)
|    |    +--------------- day of month (1 - 31)
|    +-------------------- hour (0 - 23)
+------------------------- min (0 - 59)

crontab environment setting

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/global/bin
MAILTO=abc@yyy.com
HOME=/

Eg.,

1. 30 00 * * Mon,Wed, Fri echo "On Mon,Wed,Fri at 00:30 "
2. 01 6-23/5 * * *   echo "On all days execute the command at 6:01 11:01 17:01 22:01 [6-23] "

More granularity can be done with the help of "/" 1-59/5 will run in multiples of 5

A look at default /etc/crontab file

 # cat /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

# 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

root run-parts helps if you need to execute each script in the mentioned directory

Log:

All cron jobs will be logged in /var/log/cron