problems with os.execute() return code

["ms2008vip" <ms2008vip@...>]

 Hello all,

 have a problem: 
    local stat = assert(os.execute("/usr/bin/pgrep -f 'tail -F /opt/aaa' >& /dev/null"))
    print(stat)  --> 0

But when I type this in bash, it returns 1, what happened? 


    
 				
--------------
ms2008vip
2013-06-30