ORA-27101: shared memory realm does not exist

 Issue

When i tried to connect with the database then i got the following error.
ORA-27101: shared memory realm does not exist

Error

SQL*Plus: Release 12.1.2 – Production on Mon May 07 23:14:54 2018
Copyright (c) 1982, 2015, Oracle. All rights reserved.
Enter user-name: username
Enter password:
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist

Cause
According to Oracle docs error
ORA-27101: shared memory realm does not exist
Cause: Unable to locate shared memory realm
Action: Verify that the realm is accessible

Solution
1. Resolve this issue with setting the ORACLE_HOME, ORACLE_SID, LD_LIBRARY_PATH and PATH environment variable.

On Linux
Check the environment value for the ORACLE_HOME and ORACLE_SID.

echo $ORACLE_SID
ORCL1
echo $ORACLE_HOME
/opt/app/oracle/12102

--Since the ORACLE_SID value is incorrect export the correct value and try connecting again.

$export ORACLE_SID=ORCL
$sqlplus
SQL*Plus: Release 12.1.2 – Production on Mon May 07 23:30:41 2018
Copyright (c) 1982, 2015, Oracle. All rights reserved.
Enter user-name: vst_dba
Enter password:
Connected.

On windows
Check the environment variable setting and correct the value it its wrong.

From control panel:
Right click on My Computer --> Properties--> Advance System Settings --> Environment Variables --> Check ORACLE_HOME or ORACLE_SID variable.

From Command prompt:
echo %ORACLE_HOME%
D:oraclexeapporacleproduct11.2.0server
echo %ORACLE_SID%
XE

2. You can also check the spfile or pfile, both are present with SID name.

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