You are missing something in your environment varaibles, this will show what the problem is:
set `sqlplus -s $user_name/$password@$tns<<EOS > /full/path/to/my/directory/oracle.log
exec dbms_job.broken(118,TRUE);
exit;
EOS`
Read the log for the error. Why did you use set `command` if there is nothing else in the script? That is, instead of just a here doc. Plus the syntax is "set - `command` ", if I understand what you are doing.