How to get PID from forked child process in shell script

Question

I believe I can fork 10 child processes from a parent process.

Below is my code:

#/bin/sh
fpfunction(){
    n=1
    while (($n<20))
    do
        echo "Hello World-- $n times"
        sleep 2
        echo "Hello World2-- $n times"
        n=$(( n+1 ))
    done
}

fork(){
    count=0
    while (($count<=10))
    do
        fpfunction &
        count=$(( count+1 ))
    done
}

fork

However, how can I get the pid from each child process I just created?

Solution 1

The PID of a backgrounded child process is stored in $!.

fpfunction &
child_pid=$!
parent_pid=$$

For the reverse, use $PPID to get the parent process's PID from the child.

fpfunction() {
    local child_pid=$$
    local parent_pid=$PPID
    ...
}

---

Also for what it's worth, you can combine the looping statements into a single C-like for loop:

for ((n = 1; n < 20; ++n)); do
do
    echo "Hello World-- $n times"
    sleep 2
    echo "Hello World2-- $n times"
done

Solution 2

From the Bash manual:

!

Expands to the process ID of the most recently executed background (asynchronous) command.

i.e., use $!.