ORA-01034: ,ORA-27101: shared memory realm does not exist

ORA-01034 : ORACLE not available
ORA-27101 : shared memory realm does not exist
Cause of the Problem:
This problem happens whenever ORACLE_SID or ORACLE_HOME is not properly set. Or for normal users whenever oracle database is not started. For remote users there may be a problem in listener.
Solution of the Problem:
A)For Local Connections:
1)Verify the ORACLE_SID and ORACLE_HOME is set properly. You can check the variable in UNIX by,
If it is set incorrectly then set on UNIX by
$ export ORACLE_SID=db_name_here (on ksh,sh)
$ setenv ORACLE_SID=db_name_here (on csh)
Remember that ORACLE_SID is case sensitive on UNIX.
2)Verify the database instance is running.
On UNIX you can verify by,
SQL>!ps -ef |grep smon
It will return a row as ora_smon_your_database_name
If it does not return such row then your database is not started. So, start it after setting proper sid.
$sqlplus / as sysdba
On windows system press CTRL+ALT+DEL and see the process and look for oracle.exe. If it is not present there then from right click my computer>select manage>services>and select oracle database service and start it.
B)For Remote Connections:
1)Verify the ORACLE_HOME is set correctly for all listed Oracle databases. If the ORACLE_HOME points to the different oracle version software, but the database was created using a different version, then this error can occur.
2)Review the listener.ora file and it is properly set. Like if you set ORACLE_HOME path inside listener ended by slash(/) then the error will come.
(ORACLE_HOME = /oracle/app/oracle/product/10.2.0/)
(ORACLE_HOME = /u01/app/oracle/product/10.2.0)
3)If listener.ora use SID then verify that you have set properly ORACLE_SID. Also be sure about listener services by issuing lsnrctl services.
4)Verify the database is running on server machine while you connect as normal user.

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