RMAN does not display the output

If you specify the LOG option at the command line, then RMAN displays command input but does not display the RMAN output.
The easiest way to send RMAN output both to a log file and to standard output is to use the UNIX tee command or its equivalent on another operating system.
For example:
% rman tee rman.log
RMAN>

SQL> select * from dual where 1=0;
no rows selected
SQL>

I found a good one by using NOT EXISTS clause

SQL> select * from dual where 1=0
UNION
select 'no data found' from dual where not exists (select * from dual where 1=0);
DUMMY
no data found
1 row selected.
SQL>

In this way, both input and output are visible within the RMAN command-line interface.