How can i use Crontab, Crond Service

What is Crontab and Crond Service ?

Hi,
Crond service provides for using periodic task in linux to us. Ok, now how can i use crond service ?
This example may be helpful.

Open MPutty or Putty and connect your server (Centos, Redhat vs)
#crontab -e

Add those lines
*     *    *    *    * ls>>/var/log/yourLogFile

Upper line provides every minute execute ls command and send output to yourLogFile.

This * means that
minute     hour     day     month     dayofweek     command


# at 3:30 AM in the file /tmp/meminfo 
30 3 * * /root/scripts/yourScript.sh >> /tmp/meminfo 
# run custom script the second day of every month at 4:10 AM 
10 4 2 * * /root/scripts/yourScript.sh


When you finish your job in file, you must be start crond service this command
#service crond start