Today, when shutdown immediate shutdown the database, I got ORA-01089: immediate shutdown in progress – no operations are permitted error.
ps -ef|grep ora_ found that the various background processes of oracle are still running, indicating that the database was not shut down successfully.

The solution is as follows:
First exit and log back in at.
[oracle@localhost trace]$ sqlplus /nolog

SQL*Plus: Release 11.2.0.1.0 Production on Thu Sep 18 08:21:53 2014

Copyright (c) 1982, 2009, Oracle. All rights reserved.

@ 18-SEP-14>conn /as sysdba
Connected to an idle instance.
Next, use shutdown abort to force the database to shut down.
SYS@orcl 18-SEP-14>shutdown abort;
ORACLE instance shut down.
Finally, use startup force to open the database: SYS@orcl 18-SEP-14>shutdown abort; ORACLE instance shut down.
SYS@orcl 18-SEP-14>startup force;
ORACLE instance started.

Total System Global Area 849530880 bytes
Fixed Size 1339824 bytes
Variable Size 641732176 bytes
Database Buffers 201326592 bytes
Redo Buffers 5132288 bytes
Database mounted.
Database opened.

After opening the database, use shutdown immediate again to close the database.
SYS@orcl 18-SEP-14>shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
Done!