How to get process ID of background process?
LinuxShellBackground ProcessPidLinux Problem Overview
I start a background process from my shell script, and I would like to kill this process when my script finishes.
How to get the PID of this process from my shell script? As far as I can see variable $!
contains the PID of the current script, not the background process.
Linux Solutions
Solution 1 - Linux
You need to save the PID of the background process at the time you start it:
foo &
FOO_PID=$!
# do other stuff
kill $FOO_PID
You cannot use job control, since that is an interactive feature and tied to a controlling terminal. A script will not necessarily have a terminal attached at all so job control will not necessarily be available.
Solution 2 - Linux
You can use the jobs -l
command to get to a particular jobL
^Z
[1]+ Stopped guard
my_mac:workspace r$ jobs -l
[1]+ 46841 Suspended: 18 guard
In this case, 46841 is the PID.
From help jobs
:
> -l Report the process group ID and working directory of the jobs.
jobs -p
is another option which shows just the PIDs.
Solution 3 - Linux
$$
is the current script's pid$!
is the pid of the last background process
Here's a sample transcript from a bash session (%1
refers to the ordinal number of background process as seen from jobs
):
$ echo $$
3748
$ sleep 100 &
[1] 192
$ echo $!
192
$ kill %1
[1]+ Terminated sleep 100
Solution 4 - Linux
An even simpler way to kill all child process of a bash script:
pkill -P $$
The -P
flag works the same way with pkill
and pgrep
- it gets child processes, only with pkill
the child processes get killed and with pgrep
child PIDs are printed to stdout.
Solution 5 - Linux
this is what I have done. Check it out, hope it can help.
#!/bin/bash
#
# So something to show.
echo "UNO" > UNO.txt
echo "DOS" > DOS.txt
#
# Initialize Pid List
dPidLst=""
#
# Generate background processes
tail -f UNO.txt&
dPidLst="$dPidLst $!"
tail -f DOS.txt&
dPidLst="$dPidLst $!"
#
# Report process IDs
echo PID=$$
echo dPidLst=$dPidLst
#
# Show process on current shell
ps -f
#
# Start killing background processes from list
for dPid in $dPidLst
do
echo killing $dPid. Process is still there.
ps | grep $dPid
kill $dPid
ps | grep $dPid
echo Just ran "'"ps"'" command, $dPid must not show again.
done
Then just run it as: ./bgkill.sh
with proper permissions of course
root@umsstd22 [P]:~# ./bgkill.sh
PID=23757
dPidLst= 23758 23759
UNO
DOS
UID PID PPID C STIME TTY TIME CMD
root 3937 3935 0 11:07 pts/5 00:00:00 -bash
root 23757 3937 0 11:55 pts/5 00:00:00 /bin/bash ./bgkill.sh
root 23758 23757 0 11:55 pts/5 00:00:00 tail -f UNO.txt
root 23759 23757 0 11:55 pts/5 00:00:00 tail -f DOS.txt
root 23760 23757 0 11:55 pts/5 00:00:00 ps -f
killing 23758. Process is still there.
23758 pts/5 00:00:00 tail
./bgkill.sh: line 24: 23758 Terminated tail -f UNO.txt
Just ran 'ps' command, 23758 must not show again.
killing 23759. Process is still there.
23759 pts/5 00:00:00 tail
./bgkill.sh: line 24: 23759 Terminated tail -f DOS.txt
Just ran 'ps' command, 23759 must not show again.
root@umsstd22 [P]:~# ps -f
UID PID PPID C STIME TTY TIME CMD
root 3937 3935 0 11:07 pts/5 00:00:00 -bash
root 24200 3937 0 11:56 pts/5 00:00:00 ps -f
Solution 6 - Linux
You might also be able to use pstree:
pstree -p user
This typically gives a text representation of all the processes for the "user" and the -p option gives the process-id. It does not depend, as far as I understand, on having the processes be owned by the current shell. It also shows forks.
Solution 7 - Linux
pgrep
can get you all of the child PIDs of a parent process. As mentioned earlier $$
is the current scripts PID. So, if you want a script that cleans up after itself, this should do the trick:
trap 'kill $( pgrep -P $$ | tr "\n" " " )' SIGINT SIGTERM EXIT
Solution 8 - Linux
I have run into this problem many times provisioning various infrastructure objects. Many times you need a temp proxy using kubectl or a temp port forward. I have found the timeout command to be a good solution for these, since it allows my script to be self contained and I can be assured that the process will end. I try to set small timeouts and rerun the script if I need still need it.