Your example looks a little garbled; to connect to sqlplus via the command line, with a user sys and password 123456:
sqlplus sys/123456 as sysdba
or
sqlplus "sys/123456 as sysdba"
prior to Oracle 10. If you are already inside sqlplus (as I assume from the fact that your example starts with a SQL>), you use the connect command:
SQL> connect sys/123456 as sysdba
In all cases, if you haven't set the environment variable ORACLE_SID, you need to specify that after the password, like this:
sqlplus sys/123456@<mydbname> as sysdba
where is either of the form /, if you're using Oracle 10 or later, or a valid entry from your tnsnames.ora file (located in $ORACLE_HOME/network/admin) for all versions.