如何从PL/SQL运行操作系统命令,如CHMOD？

我如何chmod从PL/SQL块运行命令？

dbms_scheduler方式:

begin    
    dbms_scheduler.create_job (
       job_name    => 'ext_tasks_job',
       job_type    => 'executable',
       job_action  => '/bin/sh',
       number_of_arguments => 2,
       auto_drop   => true);    
    dbms_scheduler.set_job_argument_value ('ext_tasks_job', 1, '-c');
    dbms_scheduler.set_job_argument_value ('ext_tasks_job', 2, 'chmod 777 /somedir/some.file');
    dbms_scheduler.run_job ('ext_tasks_job'); 
end; 

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