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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)?

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  • can you check for special "control word" something like "quit"? Commented Dec 16, 2014 at 14:08
  • 1
    related: Stop reading process output in Python without hang? Commented Dec 16, 2014 at 15:29
  • calling p = Popen(["prog"], stdout=PIPE) DOES NOT order Popen to read until EOF. Commented Dec 16, 2014 at 15:31
  • @J.F.Sebastian Your link works like a charm. Could lock down this question and add reference to: stackoverflow.com/questions/4417962/… Commented Dec 17, 2014 at 8:08

2 Answers 2

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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.

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1 Comment

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.
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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()
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6 Comments

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.
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.
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
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.
@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
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