ORA-01102 Cannot mount database in Exclusive mode

1. Verify that there is not a "sgadef.dbf" file in the directory
"ORACLE_HOME/dbs".

% ls $ORACLE_HOME/dbs/sgadef.dbf

If this file does exist, remove it.

% rm $ORACLE_HOME/dbs/sgadef.dbf

2. Verify that there are no background processes owned by "oracle"

% ps -ef | grep ora_ | grep $ORACLE_SID

If background processes exist, remove them by using the Unix
command "kill". For example:

% kill -9

3. Verify that no shared memory segments and semaphores that are owned
by "oracle" still exist

% ipcs -b

If there are shared memory segments and semaphores owned by "oracle",
remove the shared memory segments

% ipcrm -m

and remove the semaphores

% ipcrm -s

NOTE: The example shown above assumes that you only have one
database on this machine. If you have more than one
database, you will need to shutdown all other databases
before proceeding with Step 4.

4. Verify that the "$ORACLE_HOME/dbs/lk" file does not exist

5. Startup the instance