how to get PID from subprocess library

[Kushal Kumaran]

On Sat, May 21, 2011 at 6:20 PM, TheSaint <nobody at nowhere.net.no> wrote:
> Kushal Kumaran wrote:
>
>> That's how it is able to give you the status.  So, if you
>> are using getstatusoutput, you will have only one instance of your
>> command running.
>
> My intent is to launch only one program instance, which will goes as daemon.
> To avoid a second call I'd like rather to use Python than
> ==============================code=========================================
>    def start(self):
>        '''try to start aria2c as a daemon and return its handle to where it
> can
>        proceed to issue commands'''
>
>        # aria2c is running, then don't try it again
>        if (chkout('ps -A |grep aria2c')[0] > 0):
>            try:
>                chkout(self.ARIA_CMD)
>            except:
>                raise SystemExit('aria2c is not working as deamon')
>        elif self.handle: return self.handle
>        # everything is good, it will return an handle
>        self.handle= \
>        xmlrpclib.ServerProxy('http://localhost:%s/rpc' %int(self.numport))
>        return self.handle
> ==============================code=========================================
>
> Here I've named subprocess.getstatusoutput as chkout, I'm calling 2 more
> programs to find whether there's a running instance of aria2c. I think it's
> not nice, python libraries should get the matter done.
>

Unfortunately, because of the way daemons work, you will not be able
to do this (there's some trickery with ptrace and similar tools, but
surely there must be simpler ways for you).

Here's a very simplified view of a typical daemon's startup:

- your process (let's call it pid1) starts a program which says it
will daemonize (let's call it pid2).

- pid2 forks an additional process (pid3), then pid2 exits

- pid1 gets the exit status of its own child (pid2)

Accordingly, even if you get a PID, it will only be pid2, which is not
the PID of the daemon process.  You will need some other way of
getting at the daemon's PID.

You could look for a way to make aria2c not become a daemon and use
subprocess.Popen to start it.  That gives you the PID and ways to see
if the process is still running.

-- 
regards,
kushal