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Re: Identifying whether a process is running

by snoopy (Deacon)
on Nov 09, 2009 at 23:15 UTC ( #806070=note: print w/replies, xml ) Need Help??

in reply to Identifying whether a process is running

If you already know the process id(s), and own the processes, the kill builtin function might be of use.

From the doco:

   kill SIGNAL, LIST
        $cnt = kill 1, $child1, $child2
        kill 9, @goners;

    If SIGNAL is zero, no signal is sent to the process, but the
    kill(2) system call will check whether it's possible to send
    a signal to it (that means, to be brief, that the process is
    owned by the same user, or we are the super user).  This is
    a useful way to check that a child process is alive.
  • Comment on Re: Identifying whether a process is running

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Re^2: Identifying whether a process is running
by astroboy (Chaplain) on Nov 10, 2009 at 20:51 UTC
    Alas, this didn't work for me. I'm trying to see from a web page whether a daemon that should be running as root is actually there. The apache user can't issue a kill 0 to root's process (well it can, but it doesn't report the result correctly)

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[hippo]: Understood. I'll have to go through the code and see if it's doing anything fancy with ties, dual-vars or non-scalars. In the end, it's probably a bug though.
[Corion]: Aaah - you should be able to do this with overload, but I would hit somebody really hard if they constructed objects that are true but the empty string, and you not knowing about the domain knowledge where this makes sense
[Eily]: you could tie a variable into not having the same value each time, if you like to make people who try to debug your code facepalm
[Corion]: perl -wle 'package o; use overload q("") => sub {warn "str"; ""}, bool => sub{warn "bool"; 1}; package main; my $o={}; bless $o => o; print "Yay" if ($o && !length($o))'
[Corion]: But people writing such code should document the objects they construct and why it makes sense for an object to be invisible as string while being true in a boolean context
[hippo]: That's equal parts clever and horrendous.
[Eily]: the overload version wouldn't return true with "$x" && !length $x though, I guess
[hippo]: The more I look at this code, the more $x is a plain old scalar and the more this condition will never be true. I'm calling it a bug at this point.
[hippo]: Thanks for your input which has soothed my sanity (a little)

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