Reading Command line output Python

Question

I have a problem where I am issuing a command using python and then taking in the values to create a list of services.

serviceList = subprocess.Popen(command, shell=True, stdout =subprocess.PIPE).stdout.read()
print serviceList

command is a working command that works perfectly when I copy and paste it into cmd, giving me a list of services and their status.

If I run this command it just returns nothing. When I print out serviceList it is blank.

I am using python 2.7

• Stuart: And what happens when you enter this in python?
• shar: oh sorry i completely forgot to put that in. it just gives me a blank space, or an empty list
• martineau: Try print serviceList.communicate()
• shar: gives me the error str object has no attribute communicate, I will try this with os instead of subprocess Edit: still gives the same error
• khelwood: I think you need to post more context for your code.

Answer 1

You must use communicate() method instead of stdout.read() to get the value of serviceList.

Even the Python docs recommend it.

Warning: Use communicate() rather than .stdin.write, .stdout.read or .stderr.read to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.

Try this:

proc = subprocess.Popen(command, shell=True, stdout =subprocess.PIPE)
serviceList = proc.communicate()[0]
print serviceList

communicate() returns a tuple (stdoutdata, stderrdata). Here, i assign the first element of the tuple to serviceList.

• shar: This was the method that worked for me. I tried this beforehand but forgot a crucial thing. In my command string i should have put an 'r' in the beginning "r'C:\Python27\A..." is how my command should have started instead of "'C:\Python27\A"

Answer 2

If the program simply prints out a bunch of information then exits, an easier way (also no way for it to deadlock due to full buffer) to read output would be to call:

process = subprocess.Popen(command) # only call shell=True if you *really need it
stdoutdata, stderrdata = process.communicate() # blocks until process terminates

docs:

*Calling shell=True with external input opens your code to shell injection attacks, and should be used with caution

Answer 3

To save the standard output, add output = serviceList.stdout.readlines() to your code.

• shar: gives me the error that str object has no attribute stdout
• Jeremy: When you run the command serviceList = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE).stdout.read() are you sure that you are adding quotation marks around the command that you're trying to run? It's working fine for me in my python interpreter.

Answer 4

There's also the subprocess function check_output() which blocks and returns the output of the process as a byte-string. If you want to avoid blocking, you could make a function that calls this and use it as that target for a new Thread() e.g.

import subprocess
import threading
def f():
 print subprocess.check_output([command])
threading.Thread(target=f).start()