Visit Our UNIX and Linux User Community


Pid file and process check


 
Thread Tools Search this Thread
Top Forums UNIX for Beginners Questions & Answers Pid file and process check
# 1  
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
# 2  
Old 07-17-2019
I don't know of any OS that randomly picks the next PID to use when creating a new process. But, it is theoretically possible to do it that way.

Most of the systems I have worked on have a maximum PID value that is a configuration parameter. They start out assigning PIDs from 1 and increment the PID they want to assign by one every time a new process is created (e.g., by fork()), but before assigning that value, they check to see if that PID is currently in use. If it is in use, the OS tries again with the next possible PID until it finds one that is not in use. Using this method, you won't run into problems with PID recycling unless there are so many processes running on your system that you don't have many possible PIDs available that aren't in use.

Furthermore, if you have a ps utility that conforms to POSIX requirements, you can ask ps to just print the fields you want to see and not have to worry about some unexpected value in a field you aren't interested in containing what looks like a PID. (Look at your man page for the ps -o format option.)
These 3 Users Gave Thanks to Don Cragun For This Post:
# 3  
Old 07-18-2019
/var/run/ is supposed to be writable and volatile: to be cleaned during system boot, or an in-memory file system (e.g. tmpfs).
The daemon process should create the pidfile, and remove it when it is killed, in an exit handler that captures all kill signals. (Please do not kill -9 the daemon process!)
Further, an init script can check for an existing pidfile, if yes then check for validity and refuse to start a second daemon process. If the daemon-process runs as non-root, the init script creates a subdirectory and chowns it to the daemon user.

AIX generates random and big (7 or 8 digits) pids.
These 2 Users Gave Thanks to MadeInGermany For This Post:

Previous Thread | Next Thread
Test Your Knowledge in Computers #145
Difficulty: Easy
Unix popularized the hierarchical file system with arbitrarily nested subdirectories, originally introduced by Multics.
True or False?

10 More Discussions You Might Find Interesting

1. Shell Programming and Scripting

Shell script to report file size, pid and also kill the process

Hi All, Looking for a quick LINUX shell script which can continuously monitors the flle size, report the process which is creating a file greater than certain limit and also kill that process. Can someone please help me on this? (4 Replies)
Discussion started by: vasavimacherla
4 Replies

2. AIX

How to get process name from PID?

HI, i used ps -ef | grep 3539052 | grep -v grep and i got a output like ths root 3539052 3407918 0 May 07 - 709:31 /usr/sbin/syslogd but what i need is instead of full path /usr/sbin/syslogd i want only the process name that is 'syslogd' here. (3 Replies)
Discussion started by: sumanthupar
3 Replies

3. UNIX for Dummies Questions & Answers

Check who is writing to a file? (pid)

Hello, is there a way to check who is writing to a file? (pid) Like someone starts a ftp transfer, is uploading a file and I'd need to know which pid is actually writing to it. Or is there a way to check which file a process is currently accessing? Greetings and thanks for all your... (4 Replies)
Discussion started by: TehOne
4 Replies

4. Shell Programming and Scripting

Check process running Status with PID

Good day I am fairly new to Shell Scripting. I want a script to check if a process is up by checking the process's PID and then return a value for when it's running which would be 0. If it isn't running it should give any other value that 0. Any Help is appreciated Regards (9 Replies)
Discussion started by: 1nsyz1on
9 Replies

5. Programming

[C] Process pid by name

Hi I use linux OS. I've already written a function that allow me to get the process name by pid. (searching in /proc). Now I'd like to perform the inverse task.I mean get the process pid by its name. I could write a function that search in every folder in /proc for the process name, but i... (2 Replies)
Discussion started by: Dedalus
2 Replies

6. Shell Programming and Scripting

getting pid from a process

Hi all, i was able to redirect pid of process to a file in the following way ps aux|awk '$11 == "/Applications/ProjectX/DServer" >> /Applications/ProjectX/DServer.pid it works fine but if one folder name caontains space its not working like below ps aux|awk '$11 == "/Applications/Project\... (1 Reply)
Discussion started by: kirankumars
1 Replies

7. Shell Programming and Scripting

Get an PID of particular process

Hi I have written a shell script to find and kill the particular process. Here in shell script i have written the code like cnt = $(ps -ef | grep Shree) echo $cnt I am getting the output root 2326 2317 0 14:39:46 pts/1 0:28 Shree -f fdc.fbconf FDCapp.fbapp Here I want to... (2 Replies)
Discussion started by: Shreedhar Naik
2 Replies

8. UNIX for Dummies Questions & Answers

Need to get pid of a process and have to store the pid in a variable

Hi, I need to get the pid of a process and have to store the pid in a variable and i want to use this value(pid) of the variable for some process. Please can anyone tell me how to get the pid of a process and store it in a variable. please help me on this. Thanks in advance, Amudha (7 Replies)
Discussion started by: samudha
7 Replies

9. Solaris

getting pid of process

hi all, Is there a simple script anyone could through out to me, to find the pid of a process given the name. I actually need to bind this pid to a processor set. I would probably put these comamns in a shell script which would have. a) kick start the executable b) get the pid c) bind it to a... (10 Replies)
Discussion started by: Naanu
10 Replies

10. Shell Programming and Scripting

Process PID

Hi Friends :p I have a little problem please help me out. I have a Unix based OS Sun Server having oracle 8i as database on it. The server has one client with windows OS. The client uses developer 2000 (GUI) to run query and run processes. I want to know how can I know the PID of a process run... (3 Replies)
Discussion started by: vanand420
3 Replies

Featured Tech Videos