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Perl Programming Language

FAQ 8.12 How do I start a process in the background?


This is an excerpt from the latest version perlfaq8.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

--------------------------------------------------------------------

8.12: How do I start a process in the background?

    Several modules can start other processes that do not block your Perl
    program. You can use IPC::Open3, Parallel::Jobs, IPC::Run, and some of
    the POE modules. See CPAN for more details.

    You could also use

            system("cmd &")

    or you could use fork as documented in "fork" in perlfunc, with further
    examples in perlipc. Some things to be aware of, if you're on a
    Unix-like system:

    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
        "open"ing a pipe (see "open" in perlfunc) 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 &")".

--------------------------------------------------------------------

The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.

If you'd like to help maintain the perlfaq, see the details in
perlfaq.pod.

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