ORA-01031 insufficient privileges sysdba login

Error
$ sqlplus /nolog
SQL>conn sys/xxx as sysdba;
error: "ORA-01031: insufficient privileges" 

Solution

$ORACLE_SID=oracle
$export ORACLE_SID
$ORACLE_HOME=/db/oracle/9.2.0
$export ORACLE_HOME
$cd /db/oracle/9.2.0/dbs
$mv orapworacle9i orapworacle9i_bk
$cd /db/oracle/9.2.0/bin
$orapwd file=$ORACLE_HOME/dbs/orapw$ORACLE_SID password=xxx entries=40
$ sqlplus /nolog
SQL>conn sys/xxx as sysdba;
Connected to an idle instance.
Check the metalink Document

SYSDBA and SYSOPER Privileges in Oracle [ID 50507.1]
https://support.oracle.com/CSP/main/article?cmd=show&type=NOT&id=50507.1