oracle@localhost admin]$ rman target /
Recovery Manager: Release 11.2.0.4.0 – Production on Tue Apr 22 18:08:11 2018
Copyright (c) 1982, 2009, 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 $ORACLE_HOME // check ORACLE_HOME
/u01/app/oracle/product/11.2.0/db_1
[oracle@localhost admin]$ echo $ORACLE_SID //check ORACLE_SID

[oracle@localhost ~]$ export ORACLE_SID=orcl //ORACLE_SID

[oracle@localhost ~]$ rman target/ // RMAN run successfully
Recovery Manager: Release 11.2.0.4.0 – Production on Tue Apr 22 18:13:40 2018
Copyright (c) 1982, 2009, Oracle and/or its affiliates.All rights reserved
connected to target database: PROD (DBID=239015079)

RMAN>