[Tutor] os command

[Hugo González Monteverde]

In UNIX, you use the fork() exec() technique for starting a new process, 
  from the very first process(init) onwards. The os.system() uses  a shell
to do that, or you may do it yourself (in your script)

A "command" is just an executable file running(process), unless you've 
got a library function that does just that, then you could do what you 
must without creating a new process.

Probably your best bet would be to use the subprocess module. It was 
designed around the limitations of the previous methods.

Take a look at what oyu can do with it at:

http://www.python.org/doc/2.4.2/lib/module-subprocess.html

For example, the pid of the command would be available at the pid 
atribute of a subprocess instance.

p.pid

Hugo	

Johan Geldenhuys wrote:
> Sorry for the late reply,
> But is it necessary to use a child process? I don't want to execute the 
> command in another process. What happens with the parent process and how 
> do I execute my command in the parent process?
> 
> Thanks
> 
> Hugo González Monteverde wrote:
> 
>> Hi,
>>
>> os.system will return the errorval of the application. You need to
>>
>> 1) get the pid of the child process
>> 2) kill it using os.kill(os.SIGTERM)
>> 3) reap the killed process
>>
>> This is all in unix/linux, of course.
>>
>> what I do (untested, please check order of args and correct usage of 
>> exec):
>>
>> pid = os.fork()
>>
>> if pid == 0: #child process
>>     os.execvp("tcpdump", "tcpdump", "-n",  "-i",  "eth0")
>>
>> else:   #parent
>>     time.sleep(5)
>>     os.kill(pid, os.SIGTERM)
>>     os.waitpid(pid, 0)   #wait for process to end
>>
>>
>>
>> Johan Geldenhuys wrote:
>>
>>> I have script that calls a system command that I want to run for 5 
>>> minutes.
>>> """
>>> import os
>>> cmd = 'tcpdump -n -i eth0'
>>> os.system(cmd)
>>> """
>>>
>>> I can start a timer after the cmd is issued, but I don't know how to 
>>> send a control signal to stop the command after I issued it. This is 
>>> normally from the shell  ^c.
>>>
>>> This cmd my run in more than one thread on different interfaces and I 
>>> don't whant all of then stopped at once. I think the best way is to 
>>> make a thread for each interface where the cmd can be issued and 
>>> stopped without the danger of stopping he wrong thread.
>>>
>>> Can anybody help?
>>>
>>> Thanks
>>>
>>> Johan
>>> _______________________________________________
>>> Tutor maillist  -  Tutor at python.org
>>> http://mail.python.org/mailman/listinfo/tutor
>>>
>>
>