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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 CENTOS

nsenter

NSENTER(1)							   User Commands							NSENTER(1)

NAME

       nsenter - run program with namespaces of other processes

SYNOPSIS

       nsenter [options] [program] [arguments]

DESCRIPTION

       Enters the namespaces of one or more other processes and then executes the specified program.  Enterable namespaces are:

       mount namespace
	      mounting	and unmounting filesystems will not affect rest of the system (CLONE_NEWNS flag), except for filesystems which are explic-
	      itly marked as shared (by mount --make-shared).  See /proc/self/mountinfo for the shared flag.

       UTS namespace
	      setting hostname, domainname will not affect rest of the system (CLONE_NEWUTS flag).

       IPC namespace
	      process will have independent namespace for System V message queues, semaphore sets and shared memory segments (CLONE_NEWIPC flag).

       network namespace
	      process will have independent IPv4 and IPv6 stacks, IP routing tables, firewall rules, the /proc/net  and  /sys/class/net  directory
	      trees, sockets etc.  (CLONE_NEWNET flag).

       PID namespace
	      children	will  have  a  set of PID to process mappings separate from the nsenter process (CLONE_NEWPID flag).  nsenter will fork by
	      default if changing the PID namespace, so that the new program and its children share the same PID namespace and are visible to each
	      other.  If --no-fork is used, the new program will be exec'ed without forking.

       See the clone(2) for exact semantics of the flags.

       If program is not given, run ``${SHELL}'' (default: /bin/sh).

OPTIONS

       Argument  with  square brakets, such as [file], means optional argument.  Command line syntax to specify optional argument --mount=/path/to
       /file.  Please notice the equals sign.

       -t, --target pid
	      Specify a target process to get contexts from.  The paths to the contexts specified by pid are:

	      /proc/pid/ns/mnt	  the mount namespace
	      /proc/pid/ns/uts	  the UTS namespace
	      /proc/pid/ns/ipc	  the IPC namespace
	      /proc/pid/ns/net	  the network namespace
	      /proc/pid/ns/pid	  the PID namespace
	      /proc/pid/root	  the root directory
	      /proc/pid/cwd	  the working directory respectively

       -m, --mount [file]
	      Enter the mount namespace.  If no file is specified enter the mount namespace of the target process.  If file is specified enter the
	      mount namespace specified by file.

       -u, --uts [file]
	      Enter the UTS namespace.	If no file is specified enter the UTS namespace of the target process.	If file is specified enter the UTS
	      namespace specified by file.

       -i, --ipc [file]
	      Enter the IPC namespace.	If no file is specified enter the IPC namespace of the target process.	If file is specified enter the IPC
	      namespace specified by file.

       -n, --net [file]
	      Enter the network namespace.  If no file is specified enter the network namespace of the target process.	If file is specified enter
	      the network namespace specified by file.

       -p, --pid [file]
	      Enter the PID namespace.	If no file is specified enter the PID namespace of the target process.	If file is specified enter the PID
	      namespace specified by file.

       -r, --root [directory]
	      Set the root directory.  If no directory is specified set the root directory to the root directory of the target process.  If direc-
	      tory is specified set the root directory to the specified directory.

       -w, --wd [directory]
	      Set the working directory.  If no directory is specified set the working directory to the working directory of the  target  process.
	      If directory is specified set the working directory to the specified directory.

       -F, --no-fork
	      Do not fork before exec'ing the specified program.  By default when entering a pid namespace enter calls fork before calling exec so
	      that the children will be in the newly entered pid namespace.

       -V, --version
	      Display version information and exit.

       -h, --help
	      Print a help message.

SEE ALSO

       setns(2), clone(2)

AUTHOR

       Eric Biederman <ebiederm@xmission.com>

AVAILABILITY

       The nsenter command is part of the util-linux package and is available  from  Linux  Kernel  Archive  <ftp://ftp.kernel.org/pub/linux/utils
       /util-linux/>.

util-linux							   January 2013 							NSENTER(1)
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