PID of background function "$!" gives wrong value

Question

I am trying to capture the PID of a function executed in the background, but I seem to get the wrong number.

See the following script:

$ cat test1.sh 
#!/bin/bash

set -x

child() {
    echo "Child thinks is $$"
    sleep 5m 
}

child &
child_pid="$!"
echo "Parent thinks pid $child_pid"

sleep 3
kill -- -"$child_pid" # but it is wrong, get "No such process"

kill -- -"$$"
wait

I would expect the parent to terminate the child process of the function, but I get:

$ ./test1.sh 
+ child_pid=44551
+ echo 'Parent thinks pid 44551'
Parent thinks pid 44551
+ sleep 3
+ child
+ echo 'Child thinks is 44550'
Child thinks is 44550
+ sleep 5m
+ kill -- -44551
./test1.sh: line 15: kill: (-44551) - No such process
+ kill -- -44550
Terminated

I have read this question Get PID of a function executed in the background, but the answers seem to contradict what I am observing.

So how can I fix the above code, to get the correct PID of the function from the parent?

---

After some testing it seems that the command without the minus works

kill -- "$child_pid" 

But this isn't sufficient for my need, because I want to terminate any subprocesses of child too when I kill it.

Answer 1

$! gives the correct value.
$$ does not. Use $BASHPID instead

See man bash:

BASHPID

Expands to the process ID of the current bash process. This differs from $$ under certain circumstances, such as subshells that do not require bash to be re-initialized. Assignments to BASHPID have no effect.

Not sure, why your kill -- -PID is not working, cannot reproduce. You could use pkill -P PID instead.