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Date: Thu, 8 Jan 2015 13:24:52 +1100
Subject: Re: Playing with threads
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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On Thu, Jan 8, 2015 at 1:00 PM, Ganesh Pal <ganesh1pal@gmail.com> wrote:
> I'm trying to use threads  to achieve the below work flow
>
> 1. Start a process , while its running grep for a string 1
> 2. Apply the string 1 to the command in step 1 and exit step 2
> 3. Monitor the stdout of step1 and print success if the is pattern  found

If by "grep" you mean "search", then this would be best done by
spawning a subprocess and monitoring its output.

https://docs.python.org/3/library/subprocess.html

Set stdout=PIPE, then read from the stdout member, and you'll be
reading the program's output. Something like this:

with subprocess.Popen(["find","/","-name","*.py"],stdout=subprocess.PIPE)
as findpy:
    for line in findpy.stdout:
        # do something with line

You'll get lines as they're produced, and can use standard string
manipulation on them. I don't know what your step 2 is, but you can
set stdin to be a pipe as well, and then you just write to your end
and the process will receive that on its standard input.

No threads needed; just your Python process and the subprocess.

ChrisA