No stdout from killed subprocess

Question

i have a homework assignment to capture a 4way handshake between a client and AP using scapy. im trying to use "aircrack-ng capture.pcap" to check for valid handshakes in the capture file i created using scapy

i launch the program using Popen. the program waits for user input so i have to kill it. when i try to get stdout after killing it the output is empty.

i've tried stdout.read(), i've tried communicate, i've tried reading stderr, and i've tried it both with and without shells

check=Popen("aircrack-ng capture.pcap",shell=True,stdin=PIPE,stdout=PIPE,stderr=PIPE)
check.kill()    
print(check.stdout.read())

Answer 1

While you shouldn't do this (trying to rely on hardcoded delays is inherently race-condition-prone), that the issue is caused by your kill() being delivered while sh is still starting up can be demonstrated by the problem being "solved" (not reliably, but sufficient for demonstration) by tiny little sleep long enough let the shell start up and the echo run:

import time
from subprocess import Popen, PIPE

check=Popen("echo hello && sleep 1000", shell=True, stdin=PIPE, stdout=PIPE, stderr=PIPE)
time.sleep(0.01) # BAD PRACTICE: Race-condition-prone, use one of the below instead.
check.kill()    
print(check.stdout.read())

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That said, a much better-practice solution would be to close the stdin descriptor so the reads immediately return 0-byte results. On newer versions of Python (modern 3.x), you can do that with DEVNULL:

import time
from subprocess import Popen, PIPE, DEVNULL

check=Popen("echo hello && read input && sleep 1000", 
            shell=True, stdin=DEVNULL, stdout=PIPE, stderr=PIPE)
print(check.stdout.read())

...or, with Python 2.x, a similar effect can be achieved by passing an empty string to communicate(), thus close()ing the stdin pipe immediately:

import time
from subprocess import Popen, PIPE

check=Popen("echo hello && read input && sleep 1000", 
            shell=True, stdin=PIPE, stdout=PIPE, stderr=PIPE)
print(check.communicate('')[0])

Answer 2

Never, and I mean, never kill a process as part of normal operation. There's no guarantee whatsoever how far it has proceeded by the time you kill it, so you cannot expect any specific results from it in such a case.

• To explicitly pass nothing to a subprocess as input to prevent hanging when it tries to read stdin:

  • connect its stdin to /dev/null (nul in Windows) as per run a process to /dev/null in python :

    p=Popen(<...>, stdin=open(os.devnull))  #or stdin=subprocess.DEVNULL in Python 3.3+
    

  • or use stdin=PIPE and <process>.communicate() without arguments -- this will pass an empty stream

• Use <process>.communicate(), or use subprocess.check_output() instead of Popen to read output reliably

  • A process, in the general case, is not guaranteed to output any data at any particular moment due to I/O buffering. So you need to read the output stream after the process completes to be sure you've got everything.
  • At the same time, you need to keep reading the stream in the meantime if the process can produce enough output to fill an I/O buffer¹. Otherwise, it will hang waiting for you to read the buffered data. If both stdout and stderr are PIPEs, you need to read them both, in parallel -- i.e. in different threads.
    • communicate() and check_output (that uses the former under the hood) achieve this by reading stdout and stderr in two separate threads.

• Prefer convenience functions to Popen for common use cases -- in your case, check_output -- as they take care of all the aforementioned caveats for you.

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¹_{Pipes are fully buffered and a typical buffer size is 64KB}