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ORA-27101: shared memory realm does not exist

Cause: Unable to locate shared memory realm

Action: Verify that the realm is accessible

There may be 2 scenarios, which may lead you to encounter ORA-27101

1) Invalid ORACLE_HOME and ORACLE_SID

2) Database is down

[oracle@192 ~]$ sqlplus scott/tiger
SQL*Plus: Release 10.2.0.1.0 - Production on Wed May 22 16:48:05 2013
Copyright (c) 1982, 2005, Oracle.  All rights reserved.
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux Error: 2: No such file or directory

Scenario 1: Invalid ORACLE_HOME and ORACLE_SID:

ORA-27101 is usually due to the invalid ORACLE_HOME and ORACLE_SID parameters at OS level. You should investigate that ORACLE_HOME and ORACLE_SID are set correctly, and ORACLE_HOME should be without trailing slash.

[oracle@192 ~]$ echo $ORACLE_HOME
/data/app/oracle/product/10.2.0/db_1

[oracle@192 ~]$ echo $ORACLE_SID
testdb

In our case ORACLE_SID was not correct, we need to export ORACLE_SID to the valid one which is ora19c

[oracle@192 ~]$ export ORACLE_SID=ora10g
[oracle@192 ~]$ sqlplus scott/tiger
SQL*Plus: Release 10.2.0.1.0 - Production on Wed May 22 16:52:11 2013
Copyright (c) 1982, 2005, Oracle.  All rights reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options

Scenario 2: Database is down:

You may also face it when the database is down, to solve ORA-27101 you may have to start the database only

[oracle@192 ~]$ sqlplus sys/sys as sysdba
SQL*Plus: Release 10.2.0.1.0 - Production on Wed May 22 17:07:38 2013
Copyright (c) 1982, 2005, Oracle.  All rights reserved.
Connected to an idle instance.

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.

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