First post here and I must say I'm kinda amateur with programming in general... I'm using python 2.7.5 subprocess module on osx to read to output of an utility called dns-sd.
The goal is to find out what SSH file servers are running on my network. Using "dns-sd -B _ssh._tcp ." works fine in the following use:
from sys import *
from subprocess import *
class ProcessNAS(object):
def __init__ (self, name):
self.name = name
self.status = False
self.process = None
def StartCheck(self):
print "Checking for NAS..."
stdout.flush()
self.process = Popen( ["dns-sd", "-B", "_ssh._tcp", "."], stdout=PIPE )
while True:
line = self.process.stdout.readline()[:-1]
print line
if "Add" in line and self.name in line:
self.status = True
print "NAS '" + self.name + "' is available."
elif "Rmv" in line and self.name in line:
self.status = False
print "NAS '" + self.name + "' is unavailable."
newCheckNAS = ProcessNAS("Drobo-FS")
newCheckNAS.StartCheck()
Note that this is a "live" utility and if the Python script keep running, new lines that dns-sd outputs will automatically be display by print. Here is a typical output:
Checking for NAS...
Browsing for _ssh._tcp
DATE: ---Tue 02 Jul 2013---
19:44:30.670 ...STARTING...
Timestamp A/R Flags if Domain Service Type Instance Name
19:48:07.061 Add 2 4 local. _ssh._tcp. Drobo-FS
NAS 'Drobo-FS' is available.
The problem I have is that as soon as I do newCheckNAS.StartCheck(), the rest of the program just wait for the dns-sd utility to complete to continue. But this live utility will never stop, it need to continue monitoring in the background.
I looked at various threading, multiprocessing or even pybonjour modules but I don't really understand how they work... I guess I have to start a thread that runs this utility and another thread to listen to it?
Here is a very basic implementation of your class running as a new process. This will allow you to continue execution within your main script/process but you will need to use some sort of interprocess communication (ie. a shared memory data structure described in multiprocessing
docs) if you want to do anything with the output of the child process within the parent.
from sys import *
from subprocess import *
import multiprocessing
class ProcessNAS(multiprocessing.Process):
def __init__ (self, name):
self.name_ = name
self.status = False
self.process = None
super(ProcessNAS, self).__init__(target=self.StartCheck)
def StartCheck(self):
print "Checking for NAS..."
stdout.flush()
self.process = Popen( ["dns-sd", "-B", "_ssh._tcp", "."], stdout=PIPE )
while True:
line = self.process.stdout.readline()[:-1]
print line
if "Add" in line and self.name_ in line:
self.status = True
print "NAS '" + self.name_ + "' is available."
elif "Rmv" in line and self.name_ in line:
self.status = False
print "NAS '" + self.name_ + "' is unavailable."
newCheckNAS = ProcessNAS("Drobo-FS")
newCheckNAS.start() # <-- instead of StartCheck(), call start()
import time
time.sleep(4)
newCheckNAS.terminate() # <-- now have two processes - able to kill the child