Start and stop database

Normal sequence
nohup echo "startup" | sqlplus "/ as sysdba" &
sql>startup nomount;
sql>alter database mount;
sql>alter database open;

RAC
shutdown transactional local

Restrict
Only user with RESTRICTED SESSION privilege can logon
Enable with:
sql>startup restrict;
or
sql>alter system enable restricted session;
Disable with:
sql>alter system disable restricted session;

Read only or read/write
sql>ALTER DATABASE OPEN READ ONLY;
sql>ALTER DATABASE OPEN READ WRITE; 

Quiesce
#Only dba operations allowed
#In a RAC environment this affect all instances
sql>ALTER SYSTEM QUIESCE RESTRICTED;
sql>ALTER SYSTEM UNQUIESCE;
#Check instance status
sql>select active_state from v$instance;

You can safely perform OS datafiles backup

sql>alter system suspend;

sql>alter system resume;

Check status using:

sql>SELECT DATABASE_STATUS FROM V$INSTANCE;