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In windows, i solved it like this.
1. Open cmd.
2. Type sqlplus "/as sysdba"
3. created my own user and made him dba.
create user shashi identified by oracle;
grant connect,resource,dba to shashi.
4. Now i am able to login to oracle with shashi/oracle.
Try this in Linux> terminal.
For more info,
thanks for the reply,
but i am able to login as sysdba on the same terminal but unable to connect remotely as sysdba. I guess this is related to some OS permission issues..
you need to have password file and ownership of password file then u can access as "sys as sysdba"