Gather constant output from process (Python)

Question

I have a program that generates constant output (hcitool lescan):

CC:41:00:4D:AA:AA Name1
CC:41:00:4D:AA:BB Name2
CC:41:00:4D:AA:CC Name3

I would like to parse this output in Python constantly, but after few seconds I want to kill the process. As it has to be terminated manually (by pressing CTRL-C), I cannot use subprocess.check_value(["prog"]). Also calling p = subprocess.Popen(["prog"], stdout=subprocess.PIPE) isn't good, as it orders Popen to read this to the EOF. Also, such call hangs the Python script.

My question is: How can I start a program in Python (can be limited to Linux environment) and terminate it after few seconds while gathering it's output (from stdout)?

can you check for special "control word" something like "quit"? — Vor, Dec 16 '14 at 14:08
related: [Stop reading process output in Python without hang?](http://stackoverflow.com/a/4418891/4279) — jfs, Dec 16 '14 at 15:29
calling `p = Popen(["prog"], stdout=PIPE)` DOES NOT order `Popen` to read until `EOF`. — jfs, Dec 16 '14 at 15:31
@J.F.Sebastian Your link works like a charm. Could lock down this question and add reference to: http://stackoverflow.com/questions/4417962/stop-reading-process-output-in-python-without-hang — Melon, Dec 17 '14 at 08:08

Jonathan Davies · Answer 1 · 2014-12-16T15:12:00.743

Depending on what your program is doing, you could use a couple of methods.

The first one would be to put the process in a while loop and check for the MAC address in the lescan output file.

import os

tag = 0
while tag != "00:11:22:33:44:55":
    open("file.txt","w").close()
    os.system("hcitool lescan> file.txt"+" & pkill --signal SIGINT hcitool")
    f = open("file.txt","r")
    read = f.read()
    if "00:11:22:33:44:55" in read:
        print "found"
        tag = "00:11:22:33:44:55"
print "program finished"
os._exit(0)

The second solution would be if you are using lescan and hcidump at the same time.

import time
import os
import subprocess

d = subprocess.Popen(["sudo hcitool lescan --duplicates & sudo hcidump -w dump.txt"], stdout=subprocess.PIPE, stderr=subprocess.STDOUT, shell=True)
time.sleep(3)
dump = os.system("sudo hcidump -r dump.txt>scan.txt")
#read the file like the previous solution I shown you
print "program finished"
os._exit(0)

The best way I found to do this was to output your readings to a file and the scan the file for any specific thing your are looking for.

Comment back if you need anything specific, though I think I have covered your question.

1. your 1st solution with `os.system()` call kills `hcitool` *immediately*. It is not very useful. 2. I don't know what are you trying to achieve by using `hcidump` here but don't use `stdout=PIPE` unless you read from the pipe 3. Don't use `shell=True` unless you need it. 4. Remove `os._exit(0)`, it does the wrong thing here e.g., it doesn't flush stdout and it is unnecessary here. 5. [here're possible alternative solutions](http://stackoverflow.com/a/4418891/4279). — jfs, Dec 16 '14 at 15:56

Moose · Answer 2 · 2014-12-16T16:22:03.387

0

This should work as long as the subprocess you are running are continuously producing output. You can end it anytime with Ctrl-C.

(Incorporated J.F. Sebastian's sensible suggestions)

import subprocess

p = subprocess.Popen(['hcitool', 'lescan'],stdout=subprocess.PIPE,bufsize=1)

try:
    for line in iter(p.stdout.readline, ""):
        print line, # Do watever processing you want to do here
except KeyboardInterrupt:
    p.kill()

p.wait()
p.stdout.close()

edited Dec 16 '14 at 16:22

answered Dec 16 '14 at 15:07

Moose

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1
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This is bad coding practice, @Melon clearly stated he wanted to have a timer and then for the program to stop, after a certain amount of seconds. MEANING, that the program should be AUTOMATED with NO keyboard input. – Jonathan Davies Dec 16 '14 at 15:10
That is not how I read his questions. I interpreted it as he wanted to be able to stop it after a few seconds. Maybe I misunderstood. Either way, I don't see what that has to do with coding practices. – Moose Dec 16 '14 at 15:17
Killing a program with CTRL-C is bad practice, all programs should be self-efficient and terminate at the right time. `os._exit(0)` quits any program after it has finished its processes, without the use of CTRL-C. @Moose – Jonathan Davies Dec 16 '14 at 15:21
You are not killing the Python program. It can happily continue to run. You are sending an interrupt, telling it to kill the subprocess. I can see lots of use for that. – Moose Dec 16 '14 at 15:31
@JonathanDavies: OP mistakenly thinks that `Ctrl-C` is mandatory to kill the child process in a few seconds. Therefore @Moose answer is (incorrect but) possible interpretation of the question – jfs Dec 16 '14 at 15:33
@Moose: 1. don't use `shell=True` and the list argument (drop `shell=True`). 2. use `print line,` (note: the trailing comma) to avoid doubling newlines. 3. Add `p.wait()` at the end, to avoid zombies. 4. Add `bufsize=1` -- [it might make a difference sometimes](http://stackoverflow.com/a/17698359/4279). 5. Add `p.stdout.close()` after the loop for proper cleanup. – jfs Dec 16 '14 at 15:36

Gather constant output from process (Python)

2 Answers2