Run a shell script as a different user

If you need to run a shell script as apache while logged in as root you can use the following.

/bin/su -s /bin/bash -c '/var/www/html/sync/cron_email.sh' apache

Breaking it down:
 /bin/su : switch user
 -c "/path/to..." : command to run
 - : option to su, make it a login session (source profile for the user)
 apache : user to become

If the target user to run has nologin shell defined, then you can use the -s option to specify the shell.