How can I execute an external Windows command and instantly return in Perl?

Question

I've tried using system() with fork(), tried exec(), and still not getting what I need.

I want to write a Perl script which executes, let's say, a different Perl script 5 times in a row (sending it different parameter values), but have it run concurrently. I realize I could turn my script into a .pm file and reference it but I'd prefer to keep the child script independent of the parent...

• system() works, but executes the commands consecutively (makes sense per doc)
• exec() doesnt work - it only executes the first method (makes sense per doc)
• I added a fork() to the child perl script and then tried using system()
• this did not work either.
• backtick command ' documents says it works the same as system()...

Isn't there a simple way in Perl (im using WindowsXP) to execute a process, and not care about the return values or antyhing and just continue on into the next line of the parent script?

Answer 1

You need to fork in the parent, then exec in the new process. It goes like this, assuming A is the original script, and B is the one you want to do 5 times:

                A1
fork in A1   -> A1 A2
exec B in A2 -> A1 B2
fork in A1   -> A1 A3 B2
exec B in A3 -> A1 B3 B2

etc.

Answer 2

you can do it like this (fork in the parent, exec in the child):

for my $cmd qw(command1 command2 command3) {
       exec $cmd unless fork
}

the way that exec $cmd unless fork works is that fork will return a true value in the parent (the process id) and will return a false value in the child, thus exec $cmd only gets run if fork returns false (aka, in the child).

or if you want to keep tabs on the process as it runs concurrently:

my @procs;

for my $cmd qw(cmd1 cmd2 cmd3) {

   open my $handle, '-|', $cmd or die $!;

   push @procs, $handle;
}

then you can read from an element of @procs if you need to.

Or take a look at one of the many cpan modules like Forks::Super that handle the details of fork management.

Answer 3

On Windows, you can give the super-secret 1 flag to system, IIRC.

system 1, @cmd;

A quick google search for this question on perlmonks gives: http://www.perlmonks.org/?node_id=639814

Hope that helps.

Answer 4

A module is overkill for this job. You want to fork to create a new process and then exec to run a command in the new process. To run five commands, you need something like:

 for (my $i=0; $i<5; $i++) {
     if (fork() == 0) {
         exec($command[$i]);   # runs command, doesn't return
     }
 }

Answer 5

A very lightweight approach.

Windows:

foreach my $cmd (@cmds)
{
    `start $cmd`;
}

Unix:

foreach my $cmd (@cmds)
{
     `$cmd &`;
}