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

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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.

        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&")".

        You have to be prepared to "reap" the child process when it

            $SIG = sub { wait };

            $SIG = '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;

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


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