How to check if process with pid X is the one you expect

Question

In our application, we start some background processes. The pid of these processes is saved to a file.

pids values are re-used when the maximum is reached or when the system is rebooted.

How can I reliably check that the process with pid X is still the same process for which X was stored.

I've read https://serverfault.com/questions/366474/whats-a-proper-way-of-checking-if-a-pid-is-running and https://stackoverflow.com/questions/3043978/how-to-check-if-a-process-id-pid-exists but these solutions never check if the process that has pid X is the still same process as the one for which the pid was stored.

I need this info to reliably

• check the process is still running
• kill the process without risking to kill a different process that now has pid X

related https://serverfault.com/questions/279178/what-is-the-range-of-a-pid-on-linux-and-solaris

I will post my current solution. I'd like to know if it's a sensible approach and if there are better ways to do this.

Answer 1

I created following solution for this

My start script contains

nohup $commandline >> $TEMP/logfile 2>&1 &
MYPID=$!
echo $MYPID > pidfile
cat /proc/$MYPID/cmdline >> pidfile

My stop script reads the pidfile and checks if the cmdline corresponds to the one originally saved

[ -f pidfile ] || { echo no pidfile; exit 1; }
read pid < pidfile
psatstart=$(tail -1 pidfile)
psnow=$(</proc/$pid/cmdline)

if [[ "$psnow" == "$psatstart" ]]
then
    echo kill $pid
    kill $pid || { echo kill $pid failed; exit 2; }
    while ps -p $pid
    do
        sleep 1
    done
else
    echo pid $pid not the same, assume already ended
    echo S $psatstart
    echo N $psnow
fi
rm pidfile