ksh script to run rman backup

I know how to run man backup manually when I login to rman.

But, I want to have the ksh script to run rman so that I can schedule cron jobs to run the ksh script.

I am not sure howI should connect to rman through ksh script. My scripts as follows, but it does not seem to work.

cd /backups
clear

echo " "
echo " ***  Daily Oracle Full RMAN Backup  Starting  ***"
echo " ***  Daily Oracle Full RMAN Backup  Starting  ***">/backups/daily_bkup.rpt

echo " "
echo " ***             Run RMAN backup             ***"
echo " ***             Run RMAN backup             ***">>/backups/daily_bkup.rpt
echo " "

RMAN
connect catalog username/password@catalog