Need explanation on Perl Fork

Shell Programming and Scripting
1

Hi,

I am breaking my head to understand the below line of code in perl:

next if $pid = fork;    # Parent goes to next server.
    die "fork failed: $!" unless defined $pid;

Can anyone please explain me in detail as I am a slow learner?

Thanks,
Ali.

2

If fork succeeds, it returns the process ID of the child process in the variable $pid (of the parent process).
So, the parent will have some (non-zero) value in $pid and the child process will have a 0 value in $pid . And the next statements will be attempted to be executed by both the parent and the child processes.

If fork fails, $pid will be undef .

So, the first statement says "if forking succeeds, go to the next loop block iteration (applicable for the parent process only as only the parent has a non-zero value in $pid )".

So, the next statement ( die "fork failed: $!" unless defined $pid; ) is executed only by:

1) the parent process, if forking failed in the first statement, or
2) the child process, if forking is successful.

In 1), the parent process will die since $pid will be undef.
In 2), the child process will not die (that is, it will continue with the next set of statements) as for the child, $pid will be 0 (which is a defined value).

3

Elixir i have question too ... by wat u explained ..suppose if i want to run a date command on ten remote nodes i should place my command here ?

#!/usr/bin/perl
use strict;
use warnings;
print "Starting main program\n";
my @childs;
for ( my $count = 1; $count <= 10; $count++) {
        my $pid = fork();
        if ($pid) {
        # parent
        #print "pid is $pid, parent $$\n";
        push(@childs, $pid);
        } elsif ($pid == 0) 
            {
                 */ Does the remote command go here /*

                exit 0;
             } 
           else {
                die "couldnt fork: $!\n";
        }

}
 
foreach (@childs) {
        my $tmp = waitpid($_, 0);
         print "done with pid $tmp\n";
 }
 
print "End of main program\n";