about crontab

syntax : crontab -u (username) {-e | -l | -r }
-u : user executes crontab
-e : edit crontab list
-l : show contab list
-r : remove crontab

making crontab
m  h  dom  mon  dow  command
m : minute, 0 - 59, *
h : hour, 0 -23, *
dom : day of month, 1 - 31
mon : month, 1 - 12
dow :day of week, 0 - 6 (where 0=sunday, 1=monday)
command

examples :
30  *  *  * * /etc/backup.sh