Connect to Target database and Catalog Database at the same time

Set the environment variable for target database. To check whether you have set the variables correctly, log into sqlplus to verify;

SQL> select name, open_mode from v$database;

NAME      OPEN_MODE
--------- --------------------
COSP1      READ WRITE



Now, exist from sqlplus and execute this command on OS prompt.

$ rman target / catalog rmancat/rmancat@rmandb

Recovery Manager: Release 11.2.0.1.0 - Production on Mon Feb 24 14:44:17 2014

Copyright (c) 1982, 2009, Oracle and/or its affiliates.  All rights reserved.

connected to target database: COSP1 (DBID=147596350)
connected to recovery catalog database

RMAN>

Note: rmancat/rmancat is the username and password for my recovery catalog database rmandb