Fixed - ORA-30013: undo tablespace 'UNDOTBS1' is currently in use

how to drop and recreate undo tablespace in oracle 11g

/dev/mapper/vg_root-lv_oracle           6.8G  6.3G  155M  98% /u01

[root@oracle-db-server11 XE]# pwd
/u01/app/oracle/oradata/XE
[root@oracle-db-server11 XE]# ls -ltrh
total 4.8G
-rw-r-----. 1 oracle dba  21M Sep  6 00:23 temp.dbf
-rw-r-----. 1 oracle dba 581M Sep  6 04:25 system.dbf
-rw-r-----. 1 oracle dba 101M Sep  6 04:25 users.dbf
-rw-r-----. 1 oracle dba 791M Sep  6 04:30 sysaux.dbf
-rw-r-----. 1 oracle dba 3.3G Sep  6 04:32 undotbs1.dbf
-rw-r-----. 1 oracle dba 9.4M Sep  6 04:33 control.dbf

SQL>  drop tablespace UNDOTBS1 including contents and datafiles;
 drop tablespace UNDOTBS1 including contents and datafiles
*
ERROR at line 1:
ORA-30013: undo tablespace 'UNDOTBS1' is currently in use


SQL> select tablespace_name,file_name from dba_data_files;

TABLESPACE_NAME
------------------------------
FILE_NAME
--------------------------------------------------------------------------------
USERS
/u01/app/oracle/oradata/XE/users.dbf

SYSAUX
/u01/app/oracle/oradata/XE/sysaux.dbf

UNDOTBS1
/u01/app/oracle/oradata/XE/undotbs1.dbf

SQL> alter tablespace undotbs1  offline;
alter tablespace undotbs1  offline
*
ERROR at line 1:
ORA-30042: Cannot offline the undo tablespace

SQL> create undo tablespace undotbs2 datafile '/u01/app/oracle/oradata/XE/undotbs2.dbf' size 100M;

Tablespace created.

SQL> alter system set undo_tablespace=undotbs2;

System altered.

SQL> alter system set undo_management=MANUAL scope=spfile;

System altered.

SQL> exit
Disconnected from Oracle Database 11g Express Edition Release 11.2.0.2.0 - 64bit Production
[root@oracle-db-server11 XE]# /etc/init.d/oracle-xe stop
Stopping oracle-xe (via systemctl):                        [  OK  ]
[root@oracle-db-server11 XE]# /etc/init.d/oracle-xe start
Starting oracle-xe (via systemctl):                        [  OK  ]

[root@oracle-db-server11 XE]# sqlplus system/oracle@xe;

SQL*Plus: Release 11.2.0.2.0 Production on Wed Sep 6 05:12:15 2017

Copyright (c) 1982, 2011, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Express Edition Release 11.2.0.2.0 - 64bit Production

SQL> show parameter undo_tablespace;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
undo_tablespace                      string      UNDOTBS2


SQL> select owner, segment_name, tablespace_name, status from dba_rollback_segs order by 3;

OWNER  SEGMENT_NAME                   TABLESPACE_NAME
------ ------------------------------ ------------------------------
STATUS
----------------
SYS    SYSTEM                         SYSTEM
ONLINE

PUBLIC _SYSSMU10_2569484742$          UNDOTBS1
OFFLINE

PUBLIC _SYSSMU1_3789641169$           UNDOTBS1
OFFLINE


If any one the above segment is online then change it to offline 
SQL>alter rollback segment "_SYSSMU10_2569484742$" offline;


Now Drop 

SQL>  drop tablespace UNDOTBS1 including contents and datafiles;

Tablespace dropped.


SQL> exit