looping process

[ust]

I am not too understand shell script writing , could someone can help me ,

I want to have a shell script for testing , the process will run repeatly (looping), I want to test the CPU time , I have the below script to test :

while x=0
do
cp /tmp/abc.txt /tmp/def.txt
done

the process is non-stop running , but it is not fit my requirement , because the above script will generate a new process after the old process was completed , what I want is a SAME process is running a non-stop process (same PID ) ,

More clearly , I just want to have a script to run a process ( may be a looping ) , this process need to use the same PID . In my previous script , after the cp process finished , it will generate another process ( so the PID changed ) so not fit my requirement.
could someone can post the script ? thx in advance.

[Santa's little helper]

I'm not sure I understand.You want a process A which will run a process B and test B's execution time , is that
right ? Which process has to be non-stop ?