SQL & PL/SQL

HI I have a simple procedure in my oracle 11g database which is as follows.i am calling this procedure from a shell script and i am trying to redirect this dbmsoutput into a log file in the unix shell script.
but after the script executes when i cat the log file it says SP2-0103: Nothing in SQL buffer to run. why is it the dbmsoutput not redirected to the logfile?

procedure

CREATE OR REPLACE procedure display_line as 
begin 
dbms_output.put_line(' this is displayed in output file');
end;
/

unix script

sqlplus -s scott/tiger@ORCL<<!>>$PWD/abc.log
set server output on
spool $PWD/abc.log
exec display_line
/
spool off

!

Regards
Raj