[Tutor] Process problem

Alberto Troiano albertito_g at hotmail.com
Thu Jun 16 22:21:53 CEST 2005


You weren't pedantic at all
I needed the explanation 'cause I don't like to make things I don't 
understand so now I will add the code to mine



<html><div><IMG height=12 src="http://graphics.hotmail.com/emvamp.gif" 

>From: "Michael P. Reilly" <arcege at gmail.com>
>Reply-To: "Michael P. Reilly" <arcege at gmail.com>
>To: Alberto Troiano <albertito_g at hotmail.com>
>Subject: Re: [Tutor] Process problem
>Date: Thu, 16 Jun 2005 10:40:09 -0400
>On 6/16/05, Alberto Troiano <albertito_g at hotmail.com> wrote:
> >
> > Hey
> >
> > What its that for?
> >
> > can you explain me what it does? and the way to implementing it here:
> > os.system("python2.2 /root/levantamuertos.py &")
> > and what headaches are you talking about?
> >
>The 'nohup' command is to make sure that the child process sticks around
>after the parent (and grandparents) have died.
>When a process creates a child, the child is tied to the parent and when 
>parent exits, all the child processes get sent a specific signal (with
>os.kill()) telling them that the parent has died. The default procedure 
>they receive this signal is to exit.
>A little history: back in the days of only modems, you would dial into a
>UNIX mainframe from a terminal and if the modem disconnected, the system 
>you) would not want all your processes hanging around afterward. For this
>reason, the UNIX system would send a "hangup" signal (SIGHUP) to the login
>shell and all its child processes. Shells added a "nohup" (no hangup)
>command so certain processes could survive if the user wanted.
>When you say "nohup" it tells the process to ignore the default procedure
>and to ignore the SIGHUP (hangup signal). It does not put the process in 
>background so you still need the "&" and it does not change stdout and
>stderr (that is why I put those in my e-mail too).
>In python, you would just say:
>os.system("nohup python2.2 /root/levantamuertos.py >/dev/null 2>&1 &")
>This should handle most headaches. The headaches that I mentioned are going
>to be very subtle.
>    1. One is that sometimes developers have changed shells without
>    telling users. It has happened in the past that "&" has had an implicit
>    "nohup" in one version of a shell and in the next release, the implict
>    "nohup" is no longer there. It is better to be explicit and clear... it 
>    helps readability.
>    2. A second headache comes from the output streams (stdout and
>    stderr). You want to be running a number of these
>levantamuertos.pyprocess at the same time. Did you know that the
>output of your cron job is
>    captured and e-mailed to you? And by default, the output of child 
>    (levantamuertos.py) would be the same as cron job, usually only on
>    error? So you might get a lot of mixed output from the different
>    levantamuertos.py processes running at the same time. If you send all
>    the output to /dev/null. Or to specific log files (based on PID), then 
>    may not have this headache.
>I hope this helps and wasn't too pedantic. I get that way sometimes.
>There's so many different worlds,
>So many different suns.
>And we have just one world,
>But we live in different ones.

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