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Full Discussion: Pid file and process check
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
Old 07-17-2019
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
 

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LEARN ABOUT LINUX

pid

pid(n)                                                         Tcl Built-In Commands                                                        pid(n)

__________________________________________________________________________________________________________________________________________________

NAME

       pid - Retrieve process identifiers

SYNOPSIS

       pid ?fileId?
_________________________________________________________________

DESCRIPTION

       If  the  fileId  argument is given then it should normally refer to a process pipeline created with the open command.  In this case the pid
       command will return a list whose elements are the process identifiers of all the processes in the pipeline, in order.   The  list  will  be
       empty if fileId refers to an open file that is not a process pipeline.  If no fileId argument is given then pid returns the process identi-
       fier of the current process.  All process identifiers are returned as decimal strings.

EXAMPLE

       Print process information about the processes in a pipeline using the SysV ps program before reading the output of that pipeline:

              set pipeline [open "| zcat somefile.gz | grep foobar | sort -u"]
              # Print process information
              exec ps -fp [pid $pipeline] >@stdout
              # Print a separator and then the output of the pipeline
              puts [string repeat - 70]
              puts [read $pipeline]
              close $pipeline

SEE ALSO

       exec(n), open(n)

KEYWORDS

       file, pipeline, process identifier

Tcl                                                                     7.0                                                                 pid(n)
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