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 DEBIAN
dpid.h
dpid.h(3) libdaemon dpid.h(3)
NAME
dpid.h -
Contains an API for manipulating PID files.
SYNOPSIS
#include <sys/types.h>
Defines
#define DAEMON_PID_FILE_KILL_WAIT_AVAILABLE 1
This variable is defined to 1 iff daemon_pid_file_kill_wait() is supported.
Typedefs
typedef const char *(* daemon_pid_file_proc_t )(void)
Prototype of a function for generating the name of a PID file.
Functions
const char * daemon_pid_file_proc_default (void)
A function for creating a pid file name from daemon_pid_file_ident.
int daemon_pid_file_create (void)
Creates PID pid file for the current process.
int daemon_pid_file_remove (void)
Removes the PID file of the current process.
pid_t daemon_pid_file_is_running (void)
Returns the PID file of a running daemon, if available.
int daemon_pid_file_kill (int s)
Kills a running daemon, if available.
int daemon_pid_file_kill_wait (int s, int m)
Similar to daemon_pid_file_kill() but waits until the process died.
Variables
const char * daemon_pid_file_ident
Identification string for the PID file name, only used when daemon_pid_file_proc is set to daemon_pid_file_proc_default().
daemon_pid_file_proc_t daemon_pid_file_proc
A function pointer which is used to generate the name of the PID file to manipulate.
Detailed Description
Contains an API for manipulating PID files.
Definition in file dpid.h.
Define Documentation
#define DAEMON_PID_FILE_KILL_WAIT_AVAILABLE 1
This variable is defined to 1 iff daemon_pid_file_kill_wait() is supported. Since:
0.3
See also:
daemon_pid_file_kill_wait()
Definition at line 82 of file dpid.h.
Typedef Documentation
typedef const char*(* daemon_pid_file_proc_t)(void)
Prototype of a function for generating the name of a PID file.
Definition at line 37 of file dpid.h.
Function Documentation
int daemon_pid_file_create (void)
Creates PID pid file for the current process. Returns:
zero on success, nonzero on failure
Examples:
testd.c.
pid_t daemon_pid_file_is_running (void)
Returns the PID file of a running daemon, if available. Returns:
The PID or negative on failure
Examples:
testd.c.
int daemon_pid_file_kill (int s)
Kills a running daemon, if available. Parameters:
s The signal to send
Returns:
zero on success, nonzero on failure
int daemon_pid_file_kill_wait (int s, int m)
Similar to daemon_pid_file_kill() but waits until the process died. This functions is new in libdaemon 0.3. The macro
DAEMON_PID_FILE_KILL_WAIT_AVAILABLE is defined iff libdaemon supports this function.
Parameters:
s The signal to send
m Seconds to wait at maximum
Returns:
zero on success, nonzero on failure (timeout condition is considered a failure)
Since:
0.3
See also:
DAEMON_PID_FILE_KILL_WAIT_AVAILABLE
Examples:
testd.c.
const char* daemon_pid_file_proc_default (void)
A function for creating a pid file name from daemon_pid_file_ident. Returns:
The PID file path
int daemon_pid_file_remove (void)
Removes the PID file of the current process. Returns:
zero on success, nonzero on failure
Examples:
testd.c.
Variable Documentation
const char* daemon_pid_file_ident
Identification string for the PID file name, only used when daemon_pid_file_proc is set to daemon_pid_file_proc_default(). Use
daemon_ident_from_argv0() to generate an identification string from argv[0]
Examples:
testd.c.
daemon_pid_file_proc_t daemon_pid_file_proc
A function pointer which is used to generate the name of the PID file to manipulate. Points to daemon_pid_file_proc_default() by default.
Author
Generated automatically by Doxygen for libdaemon from the source code.
Version 0.14 17 Jan 2010 dpid.h(3)