how to get PID from subprocess library

[TheSaint]

hello,

I'm using to launch a program by subprocess.getstatusoutput. I'd like to
know whether I can get the program ID, in order to avoid another launch.

For clarity sake, I'm calling aria2 (the download manager for linux) and I
wouldn't like to call one more instance of it. So what will I use to find
the PID of the launched program?

 

[Kushal Kumaran]

hello,

I'm using to launch a program by subprocess.getstatusoutput. I'd like to
know whether I can get the program ID, in order to avoid another launch.

For clarity sake, I'm calling aria2 (the download manager for linux) and I
wouldn't like to call one more instance of it. So what will I use to find
the PID of the launched program?

The getstatusoutput function will only return when the command has
finished. 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.

 

[TheSaint]

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.

 

[Kushal Kumaran]

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.

 

[TheSaint]

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

I see. It's a step that I've to get on my account. Unfortunately I'll have
to study it some more.

BTW. I removed grep from the command. Python does it with the *in* statement

 

[GMail Felipe]

I see. It's a step that I've to get on my account. Unfortunately I'll have
to study it some more.

BTW. I removed grep from the command. Python does it with the *in* statement

Hi,

For the "ps" command, have you seen the psuti module?

The link to it is: http://code.google.com/p/psutil/

Regards,
Felipe.

 

[TheSaint]

GMail Felipe wrote:

For the "ps" command, have you seen the psuti module?

The link to it is: http://code.google.com/p/psutil/

You gave a brand new start
I bit of additional program to include into the package

 

[Anssi Saari]

self.handle= \
xmlrpclib.ServerProxy('http://localhost:%s/rpc' %int(self.numport))

Couldn't you just try to call something via this handle, like
self.handle.aria2.getVersion()? If there's an error, then start aria2
as a daemon and try again.

 

[TheSaint]

Couldn't you just try to call something via this handle, like
self.handle.aria2.getVersion()? If there's an error, then start aria2
as a daemon and try again.

Very good, you're right. Furthermore I should avoid to call that function
several times. I think to join it with __init__ function
The program on exit must tell aria2c to quit.