Logging on to RMAN reports ORA-12162: TNS: netservicenameisincorrectlyspecified Error

Login RMAN reports ORA-12162: TNS: net service name is incorrectly specified Error

[Oracle @ localhost admin] $ date
Tue Apr 22 22 18:06:16 CST 2014
[Oracle @ localhost admin] $ rman target/
// There is a strange ora-12162 error, the first response is TNS, but the second response tells me that this is a simple error, pay attention to the environment variables

Recovery Manager: Release 11.2.0.1.0-Production on Tue Apr 22 18:08:11 2014

Copyright (c) 1982,200 9, Oracle and/or its affiliates. All rights reserved.

RMAN-00571: ========================================================== ==============================
RMAN-00569: ==================== error message stack follows ==========================
RMAN-00571: ========================================================== ==============================
RMAN-00554: initialization of internal recovery manager package failed
RMAN-04005: error from target database:
ORA-12162: TNS: net service name is incorrectly specified
[Oracle @ localhost admin] $ echo $ ORACLE_HOME // check ORACLE_HOME and set it correctly.
/U01/app/oracle/product/11.2.0/db_1
[Oracle @ localhost admin] $ echo $ ORACLE_SID // check ORACLE_SID and the setting is blank.

[Oracle @ localhost ~] $ Export ORACLE_SID = PROD // manually inject ORACLE_SID
[Oracle @ localhost ~] $ Rman target // Start RMAN.

Recovery Manager: Release 11.2.0.1.0-Production on Tue Apr 22 18:13:40 2014

Copyright (c) 1982,200 9, Oracle and/or its affiliates. All rights reserved.

Connected to target database: PROD (DBID = 239015079)

RMAN>