How to Check ORACLE environment variables

 
In Windows

echo ORACLE_SID = %ORACLE_SID%
echo ORACLE_HOME = %ORACLE_HOME%
echo ORA_NLS = %ORA_NLS%

In Linux
echo ORACLE_SID = $ORACLE_SID
echo ORACLE_HOME = $ORACLE_HOME
echo ORA_NLS = $ORA_NLS
 

You can change these Oracle environment variables as follows

SET ORACLE_SID=<NEW SID>

 

When you change ORACLE_SID you don't need to mention 
specifically the spfile or
 pfile path when you startup the Database.

Just startup command is innough to start the selected database.

eg.

 
SET ORACLE_SID=orcl
sqlplus/nolog

sqlplus> connect sys as sysdba

sqlplus>startup