how to use crontab to excute tasks on time

like windows has scheduled tasks.linux has crontab.
it’s a tool for excute tasks on time.

how to install crontab?we can use yum.
yum install vixie-cron crontabs
then run it
service crond start
we can add it into autostart when we open the computer.
chkconfig --level 345 crond on

how to use crontab
run following command to start crontab edit mode.
crontab -e

crontabs’ format

Example of job definition:
 .---------------- minute (0 - 59)
 | .------------- hour (0 - 23)
 | | .---------- day of month (1 - 31)
 | | | .------- month (1 - 12) OR jan,feb,mar,apr …
 | | | | .---- day of week (0 - 6) (Sunday=0 or 7) or sun,mon,tue,wed,thu,fri,sat
 | | | | |
 * * * * * user-name command to be executed

In addition to numbers, there are several special symbols that are “*” ,”/” and “-“.
“*” representing all the numbers in the range of values,”/”representing each meaning,”*/5″representing every five units,”-“representing from a number to a number, and”,”separating several discrete numbers.

for example:
30 2 * * * sh xx.sh run “sh xx.sh” on 2:30am everyday.
*/10 * * * 1-5 sh xx.sh from Monday to Friday, run task every ten minutes.
0 17 1,5 * * sh xx.sh every month,on the first and fifth day,when 17:00pm,run the task.

we can use crontab -l to check the scheduled tasks.