Question

I have a process in Perl that creates another one with the system command, I leave it on memory and I pass some variables like this:

my $var1 = "Hello";
my $var1 = "World";
system "./another_process.pl $var1 $var2 &";

But the system command only returns the result, I need to get the PID. I want to make something like fork. What should I do? How can I make something like fork but in diferent scripts?

Thanks in advance!

Answer 1

Perl has a fork function.

See perldoc perlfaq8 – How do I start a process in the background?

---

(contributed by brian d foy)

There’s not a single way to run code
in the background so you don’t have to
wait for it to finish before your
program moves on to other tasks.
Process management depends on your
particular operating system, and many
of the techniques are in perlipc.
Several CPAN modules may be able to
help, including
IPC::Open2
or
IPC::Open3
,
IPC::Run
,
Parallel::Jobs
,
Parallel::ForkManager
,
POE
,
Proc::Background
, and
Win32::Process
.

There are many other modules you might
use, so check those namespaces for
other options too. If you are on a
Unix-like system, you might be able to
get away with a system call where you
put an & on the end of the command:

    system("cmd &")

You can also try using
fork,
as described in
perlfunc
(although this is the same thing that
many of the modules will do for you).

STDIN, STDOUT, and STDERR are shared

Both the main process and the
backgrounded one (the “child” process)
share the same STDIN, STDOUT and
STDERR filehandles. If both try to
access them at once, strange things
can happen. You may want to close or
reopen these for the child. You can
get around this with opening a pipe
(see open) but on some systems this
means that the child process cannot
outlive the parent.

Signals

You’ll have to catch the SIGCHLD
signal, and possibly SIGPIPE too.
SIGCHLD is sent when the backgrounded
process finishes. SIGPIPE is sent when
you write to a filehandle whose child
process has closed (an untrapped
SIGPIPE can cause your program to
silently die). This is not an issue
with system("cmd&").

Zombies

You have to be prepared to “reap” the
child process when it finishes.
$SIG{CHLD} = sub { wait };
$SIG{CHLD} = ‘IGNORE’; You can also
use a double fork. You immediately
wait() for your first child, and the
init daemon will wait() for your
grandchild once it exits.

unless ($pid = fork) {
        unless (fork) { 
            exec "what you really wanna do";
            die "exec failed!";
  }

        exit 0;
    }

    waitpid($pid, 0);

See Signals in
perlipc
for other examples of code to do this.
Zombies are not an issue with
system("prog &").system("prog &").