Stages of Oracle Startup and Shutdown

Instance Startup, DB Mount, DB Open
(DB Close, DB Unmount, Instance Shutdown)

startup
startup restrict
startup nomount
startup mount
startup read only

startup force restrict
startup force mount
startup force nomount

shutdown (shutdown normal)
shutdown immediate
shutdown abort
shutdown transactional

e.g.

$>sqlplus / as sysdba
SQL> startup;

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$>sqlplus / as sysdba
SQL>startup nomount;

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$>sqlplus / as sysdba
SQL>starup mount;

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$>sqlplus / as sysdba
SQL>shutdown immediate;

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-Suresh