[Tutor] Understand subprocess poll

[jarod_v6 at libero.it]

Dear all!!
I want to use subprocess for run some programs But I need to be sure the
program end before continue with the other:

subprocess.call("ls")
cmd1 = i
p1 = subprocess.Popen(cmd1,shell=True,stdout=subprocess.PIPE)

while True:
if p1.poll() is None:
time.sleep(3)

pass
if p1.poll()==0:
print '#'
break
if p1.poll() is not None and p1.poll() != 0:
raise Exception('Error building Alignment using star with hg19
database')



This are not working. How can I do?
thanks in advance for the precious help
bw,