问题
Is it possible to stop a PID from being reused?
For example if I run a job myjob in the background with myjob &, and get the PID using PID=$!, is it possible to prevent the linux system from re-using that PID until I have checked that the PID no longer exists (the process has finished)?
In other words I want to do something like:
myjob &
PID=$!
do_not_use_this_pid $PID
wait $PID
allow_use_of_this_pid $PID
The reasons for wanting to do this do not make much sense in the example given above, but consider launching multiple background jobs in series and then waiting for them all to finish.
Some programmer dude rightly points out that no 2 processes may share the same PID. That is correct, but not what I am asking here. I am asking for a method of preventing a PID from being re-used after a process has been launched with a particular PID. And then also a method of re-enabling its use later after I have finished using it to check whether my original process finished.
Since it has been asked for, here is a use case:
- launch multiple background jobs
- get PID's of background jobs
- prevent PID's from being re-used by another process after background job terminates
- check for PID's of "background jobs" - ie, to ensure background jobs finish
- [note if disabled PID re-use for the PID's of the background jobs those PIDs could not be used by a new process which was launched after a background process terminated]*
- re-enable PID of background jobs
- repeat
*Further explanation:
- Assume 10 jobs launched
- Job 5 exits
- New process started by another user, for example, they login to a tty
- New process has same PID as Job 5!
- Now our script checks for Job 5 termination, but sees PID in use by tty!
回答1:
You can't "block" a PID from being reused by the kernel. However, I am inclined to think this isn't really a problem for you.
but consider launching multiple background jobs in series and then waiting for them all to finish.
A simple wait (without arguments) would wait for all the child processes to complete. So, you don't need to worry about the
PIDs being reused.
When you launch several background process, it's indeed possible that PIDs may be reused by other processes.
But it's not a problem because you can't wait on a process unless it's your child process.
Otherwise, checking whether one of the background jobs you started is completed by any means other than wait is always going to unreliable.
回答2:
Unless you've retrieved the return value of the child process it will exist in the kernel. That also means that it's pid is bound to it and can't being re-used during that time.
回答3:
If you are afraid of reusing PID's, which won't happen if you wait as other answers explain, you can use
echo 4194303 > /proc/sys/kernel/pid_max
to decrease your fear ;-)
回答4:
Further suggestion to work around this - if you suspect that a PID assigned to one of your background jobs is reassigned, check it in ps to see if it still is your process with your executable and has PPID (parent PID) 1.
来源：https://stackoverflow.com/questions/40359901/bash-is-it-possible-to-stop-a-pid-from-being-reused