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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GCORE(1) General Commands Manual GCORE(1)
NAME
gcore - get core image of running process
SYNOPSIS
gcore [-s][-c core] pid
DESCRIPTION
gcore creates a core image of each specified process, suitable for use with adb(1). By default the core image is written to the file
<pid>.core.
The options are:
-c Write the core file to the specified file instead of <pid>.core.
-s Stop the process while creating the core image and resume it when done. This makes sure that the core dump will be in a consistent
state. The process is resumed even if it was already stopped. Of course, you can obtain the same result by manually stopping the
process with kill(1).
The core image name was changed from core.<pid> to <pid>.core to prevent matching names like core.h and core.c when using programs such as
find(1).
FILES
<process-id>.core The core image.
BUGS
If gcore encounters an error while creating the core image and the -s option was used the process will remain stopped.
Swapped out processes and system processes (the swapper) may not be gcore'd.
4.2 Berkeley Distribution April 15, 1994 GCORE(1)