subprocess - Python - how to execute shell commands with pipe, but without 'shell=True'?

Question

I have a case to want to execute the following shell command in Python and get the output,

echo This_is_a_testing | grep -c test

I could use this python code to execute the above shell command in python,

>>> import subprocess
>>> subprocess.check_output("echo This_is_a_testing | grep -c test", shell=True)
'1
'

However, as I do not want to use the "shell=True" option, I tried the following python code,

>>> import subprocess
>>> p1 = subprocess.Popen(["echo", "This_is_a_testing"], stdout=subprocess.PIPE)
>>> p2 = subprocess.Popen(["grep", "-c", "test"], stdin=p1.stdout)
>>> p1.stdout.close()
>>> p2.communicate()
(None, None)

I wonder why the output is "None" as I have referred to the descriptions in the webpage : http://docs.python.org/library/subprocess.html#subprocess.PIPE

Had I missed some points in my code ? Any suggestion / idea ? Thanks in advance.

Answer 1

Please look here:

>>> import subprocess
>>> p1 = subprocess.Popen(["echo", "This_is_a_testing"], stdout=subprocess.PIPE)
>>> p2 = subprocess.Popen(["grep", "-c", "test"], stdin=p1.stdout)
>>> 1
p1.stdout.close()
>>> p2.communicate()
(None, None)
>>>

here you get 1 as output after you write p2 = subprocess.Popen(["grep", "-c", "test"], stdin=p1.stdout), Do not ignore this output in the context of your question.

If this is what you want, then pass stdout=subprocess.PIPE as argument to the second Popen:

>>> p1 = subprocess.Popen(["echo", "This_is_a_testing"], stdout=subprocess.PIPE)
>>> p2 = subprocess.Popen(["grep", "test"], stdin=p1.stdout, stdout=subprocess.PIPE)
>>> p2.communicate()
('This_is_a_testing
', None)
>>>