Set server output only in spool

Is it possible to set the server output only into the spool file and not in the shell ?

set serveroutput on
spool log.txt
exec MY_PROCEDURE();
spool off

Inside MY_PROCEDURE I have this :

DBMS_OUTPUT.put_line('Hello World');

I would like to put Hello World only in log.txt, not in the screen shell.

Answer

Simplest way is to pipe the unix output to /dev/null

$ sqlplus -S user/password @test.sql > /dev/null
$ cat test.sql 
set serveroutput on
set feedback off
spool log.txt
exec dbms_output.put_line('This is great!! and working');
spool off;
exit;
$ cat log.txt 
This is great!! and working                                                     
$