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\n' 

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\n', None) >>>