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 OPENSOLARIS
setpgid
setpgid(2) System Calls setpgid(2)
NAME
setpgid - set process group ID
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
int setpgid(pid_t pid, pid_t pgid);
DESCRIPTION
The setpgid() function sets the process group ID of the process with ID pid to pgid.
If pgid is equal to pid, the process becomes a process group leader. See Intro(2) for more information on session leaders and process group
leaders.
If pgid is not equal to pid, the process becomes a member of an existing process group.
If pid is equal to 0, the process ID of the calling process is used. If pgid is equal to 0, the process specified by pid becomes a process
group leader.
RETURN VALUES
Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.
ERRORS
The setpgid() function will fail if:
EACCES The pid argument matches the process ID of a child process of the calling process and the child process has successfully executed
one of the exec family of functions (see exec(2)).
EINVAL The pgid argument is less than (pid_t) 0 or greater than or equal to PID_MAX, or the calling process has a controlling terminal
that does not support job control.
EPERM The process indicated by the pid argument is a session leader.
EPERM The pid argument matches the process ID of a child process of the calling process and the child process is not in the same ses-
sion as the calling process.
EPERM The pgid argument does not match the process ID of the process indicated by the pid argument, and there is no process with a
process group ID that matches pgid in the same session as the calling process.
ESRCH The pid argument does not match the process ID of the calling process or of a child process of the calling process.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Interface Stability |Standard |
+-----------------------------+-----------------------------+
|MT-Level |Async-Signal-Safe |
+-----------------------------+-----------------------------+
SEE ALSO
Intro(2), exec(2), exit(2), fork(2), getpid(2), getsid(2), attributes(5), standards(5)
SunOS 5.11 28 Dec 1996 setpgid(2)