From: Mark Hubbart <discord@...>
Date: 2004-08-26T06:26:06+09:00
Subject: Re: easy way to check if a pid is running?


On Aug 25, 2004, at 2:01 PM, Gennady wrote:

> George Mochrie wrote:
>> hi
>> A quick question:
>> Is there an easy way to check if a process is running given it's PID? 
>> There seems to be nothing like Process.exists?(pid). If I was root I 
>> could use Process.kill(0,pid) to test, but the program needs to run 
>> as a plain user. Do
>
> Not only root can do kill(0, pid), any user can send signal (or 0) to 
> his/her own processes.
>
> On systems that have /proc you can also check for presence of a 
> directory named like pid there.

You can't send a signal, but you get a different error:

mark@eMac% irb --prompt xmp

Process.kill 0, 1
Errno::EPERM: Operation not permitted
         from (irb):2:in `kill'
         from (irb):2

Process.kill 0, 512
Errno::ESRCH: No such process
         from (irb):3:in `kill'
         from (irb):3

Process.kill 0, 419
     ==>1

process 1 is owned by root, 512 doesn't exist, 419 was mine.

So, Process.exists? could be:

def Process.exists?(pid)
   Process.kill 0, pid
   return true
rescue Errno::ESRCH
   return false
rescue Exception
   return true
end
     ==>nil

Process.exists? 1
     ==>true

Process.exists? 512
     ==>false

Process.exists? 419
     ==>true

>> I have to parse the output from ps -e?
>> Thanks in advance :)
>
> Gennady.
>