The code we use

examples/orphaned.pl

use strict;
use warnings;
use 5.010;


say "In Main: PID: $$ PPID: ", getppid();

my $pid = fork();
if ($pid) {
    say $pid;
    sleep 1;
    say "In Parent: PID: $$ PPID: ", getppid();
    sleep 1;
    exit();
} else {
    say "In Child: PID: $$ PPID: ", getppid();
    sleep 3;
    say "In Child: PID: $$ PPID: ", getppid();
}

Run the following commands in a Linux terminal:

echo $$
perl orphaned.pl

5209

In Main: PID: 5378 PPID: 5209
5379
In Child: PID: 5379 PPID: 5378
In Parent: PID: 5378 PPID: 5209
In Child: PID: 5379 PPID: 1

echo $$

The first command is still a shell command. In the shell the $$ variable contains the process ID of the current shell. It was 5209 when I ran this.

Perl PID and PPID

In Perl too the variable $$ contains the process ID of the current process. In addition the getppid() function returns the process ID of the parent process. If you run a script from a Linux terminal then this will be the shell process running in that terminal.

So in the first output line of our Perl script we see that the current process ID is 5378 (this is the perl process) and the PID of the parent process (sometimes called PPID) is 5209. That is the PID of the shell where we ran our program.

Forking script

Using the fork system call Perl can duplicate a process and from that point there will be two almost identical copies of the original process. The difference is that the new process that was spawned will get a new PID. In this copy of the process the fork function will return 0 so in our code the internal $pid variable will be 0. This process is usually called the child-process.

The original process will retain its PID and here fork will return the PID of the newly created child-process. In our example this is 5379.

The calls to sleep are there to ensure the order of the printing and to make sure that the first time we print from the child process the parent is still around and when the child prints again the parent is already gone.

The first time the child process prints In Child: PID: 5379 PPID: 5378 it shows its own PID 5379 which is the same number the parent received from fork and the process ID of its parent. 5378 which is the PID of the main process. Then it goes to sleep and let's the parent process print In Parent: PID: 5378 PPID: 5209 and then exits.

Finally, when the child process prints again In Child: PID: 5379 PPID: 1 it original parent is gone so now it prints 1 as the PID of the parent process. That it, the init process became the parent of this orphaned process.