ORA-01102: cannot mount database in EXCLUSIVE mode

Problem:

I had used DBCA to create Oracle database and after doing all; had started the database from SQL and got the error message:

SQL> startup
ORACLE instance started.

Total System Global Area 1603411968 bytes
Fixed Size                  2213776 bytes
Variable Size             402655344 bytes
Database Buffers         1191182336 bytes
Redo Buffers                7360512 bytes
ORA-01102: cannot mount database in EXCLUSIVE mode

I looked for the pmon process;

[oracle@geoppdb01 ~]$ ps -ef|grep pmon
oracle   11758     1  0 Jan05 ?        00:00:00 ora_pmon_mydb
oracle   17754     1  0 14:17 ?        00:00:00 ora_pmon_MYDB
oracle   17795 17715  0 14:19 pts/1    00:00:00 grep pmon

Cause:

ORACLE_SID environment variable is case sensitive and in the .bash_profile I had set this as uppercase, despite the fact that  it was lower case.

Solution:

I had first changed the ORACLE_SID variable as lowercase, and then shutt down the two instances and startup solved my problem.

SQL> startup
ORACLE instance started.

Total System Global Area 1603411968 bytes
Fixed Size                  2213776 bytes
Variable Size             402655344 bytes
Database Buffers         1191182336 bytes
Redo Buffers                7360512 bytes
Database mounted.
Database opened.
SQL>