$ export ORACLE_SID=<MYDB>
$ sqlplus /nolog
SQL> connect / as sysdba
SQL> Shutdown
SQL> Startup mount
SQL> Alter database archivelog;
SQL> alter database open;
SQL> archive log list;
Database log mode Archive Mode
Automatic archival Enabled
Archive destination /oradump/<MYDB>
Oldest online log sequence 7
Next log sequence to archive 7
Current log sequence 9