How does sqlplus know which SID to connect for Oracle 10g Express ?

After Oralce 10g express was installed and web console (http://oracle-xe-host-name:8080/apex) worked fine, how about now you apply sqlplus to connect the absolutely only one instance 'XE' in the same machine ? Since there is no Oracle 10g express client installed, sqlplus has no idea where TNS administration is located, it's therefore no way to connect instance 'XE'. Please try the followings to solve:

• For Windows: Set system variable TNS_ADMIN equal to %ORACLE_XE_SERVER_HOME%\NETWORK\ADMIN
  
• For Linux: Copy tnsnames.ora from $ORACLE_XE_SERVER_HOME/networ/admin to /etc