ORA-27100: shared memory realm already exists oracle
Phil is right - you need to see if you have any orphan Oracle processes and ipcs shared memory.

If you have more than one Oracle instance on your server, include the instance in your grep commands; more than likely, you may have some orphan Oracle processes still running.  Assuming your SID is custdev, you could do a 

ps -fu oracle | grep custdev


and then do a kill -9 PID (where PID is the process ID) on any of the PIDs that show up.