We can shutdown Oracle database in three ways

1) Shutdown normal

2) Shutdown immediate

3)Shutdown abort

4) Shutdown Transactional

sqlplus / as sysdba

shutdown Normal

Shutdown Normal: In this mode ,Oracle will wait for in process transaction to complete. Since it waits on all in process transaction,   this can take long time. Also new user are allowed to connect after this command is issued. It completes only after all the user are disconnected. It performs a clean shutdown of database and no recovery is required after the database is opened

Shutdown Immediate: In this mode, Oracle will terminate all the connection.It will also rollback all uncommitted transaction.Current client SQL statements being processed by the database are allowed to complete.No new connection are allowed after the command is issued. This can hang sometimes as pmon may not be able to terminate few sessions. In that case we can look for the alert log of the database,it will show the active session.We can terminate the process shutdown to proceed

Tue Aug 10 11:00:27 2015
Active call for process 164671 user ‘oracle’ program
SHUTDOWN: waiting for active calls to complete.
Tue Aug 10 11:00:27 2015
SHUTDOWN: Active sessions prevent database close operation

we can look for the process id 164671 on the Unix level

ps -ef|grep 164671

It will be LOCAL process, we can kill it

kill -9 164671

The shutdown should proceed

It can also hangs sometimes because of large transaction to rollback and temporary segment cleanup.You can following query to check about the transaction rollback

select sum(used_ublk) from v$transaction;

Shutdown abort: This is hard crash of the database. This is fastest way to shutdown database.All the process are killed immediately. It is like your server crashed.  Crash recovery is performed after the database is startup

Shutdown Transactional: Session which are doing transaction are allowed to complete. Basically it allows to shutdown of database without interruption to the clients. No new transaction are allowed after the command is started

 

There are the following steps of shutting down the database.

1)Close the database.
2) Unmount the database.
3)Shut down the instance.

1) Close a database:
When we close the database, oracle writes all the data and recovery data in the SGA to the datafiles and redo log files. Next oracle close all online datafiles and redo log files.
Any offline datafiles of any offline tablespace have been closed already.

2) Unmount the database:

After the database has been closed, Oracle unmount the database to dissociate it from its instance. At this point, instance remains in the memory of the computer.
After the database is unmounted, Oracle closes the control files of the database.

3) Shut down an Instance:

The final step in the shut down the database is the shutting down an instance, The SGA is removed from the memory and the background processes are terminated.

Related:How to find the semaphore for the Database Instance