Ouestion on Python Loop

[GOPI56]

Hi All,
I have a question on this python which i have attached below. My question is i want the python script "PIRoncall.py" script to be run once but i want the read file loop to continue,currently it keeps on duplicating and there is mutiple process of "PIRoncall.py" script. I have thought of using shell command for checking process existence (pgrep?) before launching PIRoncall.py instance but i am not sure how to proceed. Anyone please help ?

import time
import subprocess
import os

oldline="none"
while True:
   fh=open("status_mjpeg.txt","r")
   line = fh.read(5)
   print line
   fh.close()
   time.sleep(0.3)
   if oldline !=line:
       if line == "md_re":
            print "on"
            time.sleep(0.2)
            os.system(' /usr/bin/python PIRoncall.py &')
            time.sleep (2)
       elif line == "ready":
            print "on"
            os.system(' /usr/bin/python PIRoffcall.py')
            time.sleep(2)
       oldline = line