special characters in sql*plus

the following characters are replaces by:
 @ => ORACLE_SID
 ? => ORACLE_HOME

SQL> spool test_@.log
SQL> prompt hi
hi
SQL> spool off
SQL> !ls -rlt test*
-rw-r--r--   1 oravcms    oravcms        136 Mar  7 14:32 test_VCMST.log

Oracle Expert » How to spot duplicate datafiles