how to restore system user in oracle and lock it back

Following instructions will help you unlock the oracle db system user and lock it back when needed.
Those are the instructions:
1) login to a machine as user oracle
2) connect to sqlplus: sqlplus '/ as sysdba'
3) unlock user: alter user system account unlock;
4) change password: alter user system identified by manager;
5) verify connection: conn system/manager
6) restore password: ALTER USER system IDENTIFIED BY VALUES 'F2B917B76C7DFF0A';
7) lock user: alter user system account lock;
8) verify lock: conn system/manager
you should see the following message: ERROR: ORA-28000: the account is locked

how to locate oracle SID on linux / unix machine

Log into a UNIX or Linux machine as root.
execute: ps -ef |grep -i pmon
you should see something like that:
oracle 27161 1 0 Mar07 ? 00:03:53 ora_pmon_ODS
then ODS is a SID of that oracle.