Creating child processes in bash

The following example can be used to create a Bash script that demonstrates forking and shows how to kill the parent process while also killing the child process.

Example

#!/bin/bash

# Fork a true child process using a subshell
(
    # This is the child process
    trap "echo 'Child process $$ terminated'; exit" SIGTERM
    echo "Child process $$ started."
    while true; do
        echo "Child process $$ is running..."
        sleep 2
    done
) &

trap "echo 'Parent process $$ terminating...'; kill $!; wait $!; exit" SIGTERM

# Parent process
echo "Parent process $$ managing child process $!"
while true; do
    echo "Parent process $$ is running..."
    sleep 2
done

Steps to Test

Save the script as fork_example.sh and make it executable:
```
 chmod u+x ./fork_example.sh
```
Run the script:
```
 ./fork_example.sh
```
Using another terminal, find the process IDs of the parent and child:
```
 ps aux | grep fork_example.sh
```
Kill the parent process:
```
 kill <parent_pid>
```

Explanation

$$ is a special parameter for getting the PID of the script itself.
$! is a special variable in Bash for getting the PID of the most recently started background process.
The trap command in the child process ensures that it can clean up properly when receiving a SIGTERM.
When the parent process is killed, the child process—being a direct descendant—is also terminated, since it has no parent to manage it.

This example demonstrates how process hierarchies in Unix-like systems work and how trap and SIGTERM can be used to clean up child processes.