Oracle startup error (could not open parameter file)

I installed the Oracle 11g server on Ubuntu 12.10. But I can't start the server. When I start up the server, the following error occurs: SQL> startup ORA-01078: failure in processing system parameters LRM-00109: could not open parameter file '/u01/app/oracle/product/11.2.0/xe/dbs/initXE.ora' How can I solve this issue? I think the connect failed because target host or object does not exist. How can a target host be specified? Running: $ ls -latr /u01/app/oracle/product/11.2.0/xe/dbs/initXE.ora returns: -rw-r--r-- 1 oracle dba 2637 Mar 13 17:07 /u01/app/oracle/product/11.2.0/xe/dbs/initxe.ora Running echo $ORACLE_SID displays nothing - an empty line.