Pid file and process check Post: 303036934

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Top Forums UNIX for Beginners Questions & Answers Pid file and process check Post 303036934 by baris35 on Wednesday 17th of July 2019 07:24:39 PM

07-17-2019

Registered User

Pid file and process check

Hello,
I am running ubuntu14.04
What I am trying to do is restart a process with a shell when pid is dead.
I restored pid nr in a file and check with ps aux | grep -v grep | grep $(cat *.pid)| awk '{ print $2 }'
While surfing on google, I have found an answer saying that restoring pid in a file for this purpose is not a good way.

Quote:

Let me add some information on why not to use PID files. While they are very popular; they are also very flawed and there's no reason why you wouldn't just do it the correct way.

Consider this:

PID recycling (killing the wrong process):

/etc/init.d/foo start: start foo, write foo's PID to /var/run/foo.pid
A while later: foo dies somehow.
A while later: any random process that starts (call it bar) takes a random PID, imagine it taking foo's old PID.
You notice foo's gone: /etc/init.d/foo/restart reads /var/run/foo.pid, checks to see if it's still alive, finds bar, thinks it's foo, kills it, starts a new foo.
PID files go stale. You need over-complicated (or should I say, non-trivial) logic to check whether the PID file is stale, and any such logic is again vulnerable to 1..

What if you don't even have write access or are in a read-only environment?

It's pointless overcomplication; see how simple my example above is. No need to complicate that, at all.

See also: Are PID-files still flawed when doing it 'right'?

By the way; even worse than PID files is parsing ps! Don't ever do this.

ps is very unportable. While you find it on almost every UNIX system; its arguments vary greatly if you want non-standard output. And standard output is ONLY for human consumption, not for scripted parsing!
Parsing ps leads to a LOT of false positives. Take the ps aux | grep PID example, and now imagine someone starting a process with a number somewhere as argument that happens to be the same as the PID you stared your daemon with! Imagine two people starting an X session and you grepping for X to kill yours. It's just all kinds of bad.
If you don't want to manage the process yourself; there are some perfectly good systems out there that will act as monitor for your processes. Look into runit, for example.

Do you believe that bold part of the quote is correct?
How come a new process can have a pid nr of an old dead process?

Thanks in advance
Boris

baris35

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LEARN ABOUT DEBIAN

cpulimit

CPULIMIT(1)							   User commands						       CPULIMIT(1)

NAME

       cpulimit -- limits the CPU usage of a process

SYNOPSIS

       cpulimit [TARGET] [OPTIONS...]

DESCRIPTION

       TARGET must be exactly one of these:

       -p, --pid=N
	      pid of the process

       -e, --exe=FILE
	      name of the executable program file

       -P, --path=PATH
	      absolute path name of the executable program file

       OPTIONS

       -b, --background
	      run cpulimit in the background, freeing up the terminal

       -c, --cpu
	      specify the number of CPU cores available. Usually this is detected for us.

       -l, --limit=N
	      percentage of CPU allowed from 1 up. Usually 1 - 100, but can be higher on multi-core CPUs. (mandatory)

       -v, --verbose
	      show control statistics

       -z, --lazy
	      exit if there is no suitable target process, or if it dies

       -h, --help
	      display this help and exit

EXAMPLES

       Assuming you have started `foo --bar` and you find out with top(1) or ps(1) that this process uses all your CPU time you can either

       # cpulimit -e foo -l 50
	      limits the CPU usage of the process by acting on the executable program file (note: the argument "--bar" is omitted)

       # cpulimit -p 1234 -l 50
	      limits the CPU usage of the process by acting on its PID, as shown by ps(1)

       # cpulimit -P /usr/bin/foo -l 50
	      same as -e but uses the absolute path name

       # /usr/bin/someapp

       # cpulimit -p $! -l 25 -b
	      Useful for scripts where you want to throttle the last command run.

       # cpulimit -l 20 firefox
	      Launch Firefox web browser and limit its CPU usage to 20%

       # cpulimit -c 2 -p 12345 -l 25
	      The -c flag sets the number of CPU cores the program thinks are available. Usually this is detected for us, but can be over-ridden.

NOTES

       o   cpulimit  always  sends  the  SIGSTOP  and SIGCONT signals to a process, both to verify that it can control it and to limit the average
	   amount of CPU it consumes.  This can result in misleading (annoying) job control messages that indicate that the job has  been  stopped
	   (when  actually it was, but immediately restarted).	This can also cause issues with interactive shells that detect or otherwise depend
	   on SIGSTOP/SIGCONT.	 For example, you may place a job in the foreground, only to see it immediately stopped and restarted in the back-
	   ground.  (See also <http://bugs.debian.org/558763>.)

       o   When  invoked  with the -e or -P options, cpulimit looks for any process under /proc with a name that matches the process name argument
	   given.  Furthermore, it uses the first instance of the process found.  To control a specific instance of a process, use the	-p  option
	   and provide a PID.

       o   The current version of cpulimit assumes the kernel HZ value 100.

AUTHOR

       This manpage was written for the Debian project by gregor herrmann <gregoa@debian.org> but may be used by others.

cpulimit							     June 2012							       CPULIMIT(1)

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