Sometimes You can get “ORA-27101: shared memory realm does not exist ” error.
Details of error are as follows.
ORA-27101: shared memory realm does not exist
This error is related with Invalid ORACLE_HOME and ORACLE_SID or Database is really down.
Check your ORACLE_HOME and ORACLE_SID with echo command, or if database is down start it up again as follows.
SQL> startup ORACLE instance started. Total System Global Area 1224736768 bytes Fixed Size 1219112 bytes Variable Size 167773656 bytes Database Buffers 1040187392 bytes Redo Buffers 15556608 bytes Database mounted. Database opened.