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Proc::Background - Generic interface to Unix and Win32 background process management


  use Proc::Background 'timeout_system';
  timeout_system($seconds, $command, $arg1, $arg2);
  timeout_system($seconds, "$command $arg1 $arg2");
  my $proc1 = Proc::Background->new($command, $arg1, $arg2) || die "failed";
  my $proc2 = Proc::Background->new("$command $arg1 1>&2") || die "failed";
  if ($proc1->alive) {
  say 'Ran for ' . ($proc1->end_time - $proc1->start_time) . ' seconds';
    autodie => 1,           # Throw exceptions instead of returning undef
    cwd => 'some/path/',    # Set working directory for the new process
    exe => 'busybox',       # Specify executable different from argv[0]
    command => [ $command ] # resolve ambiguity of command line vs. argv[0]
  # Set initial file handles
    stdin => undef,                # /dev/null or NUL
    stdout => '/append/to/fname',  # will try to open()
    stderr => $log_fh,             # use existing handle
    command => \@command,
  # Automatically kill the process if the object gets destroyed
  my $proc4 = Proc::Background->new({ autoterminate => 1 }, $command);
  $proc4    = undef;  # calls ->terminate


This is a generic interface for placing processes in the background on both Unix and Win32 platforms. This module lets you start, kill, wait on, retrieve exit values, and see if background processes still exist.


new [options] command, [arg, [arg, ...]]
new [options] 'command [arg [arg ...]]'

This creates a new background process. Just like system(), you can supply a single string of the entire command line, or individual arguments. The first argument may be a hashref of named options. To resolve the ambiguity between a command line vs. a single-element argument list, see the command option below.

By default, the constructor returns an empty list on failure, except for a few cases of invalid arguments which call croak.

For platform-specific details, see "IMPLEMENTATION" in Proc::Background::Unix or "IMPLEMENTATION" in Proc::Background::Win32, but in short:


This implementation uses fork/exec. If you supply a single-string command line, it is passed to the shell. If you supply multiple arguments, they are passed to exec. In the multi-argument case, it will also check that the executable exists before calling fork.


This implementation uses the Windows CreateProcess API. If you supply a single-string command line, it derives the executable by parsing the command line and looking for the first element in the PATH, appending ".exe" if needed. If you supply multiple arguments, the first is used as the exe and the command line is built using Win32::ShellQuote. To let Windows search for the executable, pass option { exe => undef }.



This module traditionally has returned undef if the child could not be started. Modern Perl recommends the use of exceptions for things like this. This option, like Perl's autodie pragma, causes all fatal errors in starting the process to die with exceptions instead of returning undef. (module-usage errors or other problems prior to launching the process may still 'croak' regardless of this setting)


You may specify the command as an option instead of passing the command as a list. A string value is considered a command line, and an arrayref value is considered an argument list. Using this option resolves the ambiguity in the plain-list constructor between a command line vs. a single-element argument list.


Specify the executable. This can serve two purposes: on Win32 it avoids the need to parse the commandline, and on Unix it can be used to run an executable while passing a different value for $ARGV[0].

stdin, stdout, stderr

Specify one or more overrides for the standard handles of the child. The value should be a Perl filehandle with an underlying system fileno value. As a convenience, you can pass undef to open the NUL device on Win32 or /dev/null on Unix. You may also pass a plain-scalar file name which this module will attmept to open for reading or appending.

(for anything more elaborate, see IPC::Run instead)

Note that on Win32, none of the parent's handles are inherited by default, which is the opposite on Unix. When you specify any of these handles on Win32 the default will change to inherit the rest from the parent.


Specify a path which should become the child process's current working directory. The path must already exist.


If you pass a true value for this option, then destruction of the Proc::Background object (going out of scope, or script-end) will kill the process via ->terminate. Without this option, the child process continues running. die_upon_destroy is an alias for this option, used by previous versions of this module.



The command (string or arrayref) that was passed to the constructor.


The path to the executable that was passed as an option to the constructor, or derived from the command.


Return the value that the Perl function time() returned when the process was started.


Returns the process ID of the created process. This value is saved even if the process has already finished.


Return 1 if the process is still active, 0 otherwise. This makes a non-blocking call to wait to check the real status of the process if it has not been reaped yet.


Boolean whether the process is thought to be stopped. This does not actually consult the operating system, and just returns the last known status from a call to suspend or resume. It is always false if alive is false.


Returns the exit code of the process, assuming it exited cleanly. Returns undef if the process has not exited yet, and 0 if the process exited with a signal (or TerminateProcess). Since 0 is ambiguous, check for exit_signal first.


Returns the value of the signal the process exited with, assuming it died on a signal. Returns undef if it has not exited yet, and 0 if it did not die to a signal.


Return the value that the Perl function time() returned when the exit status was obtained from the process.


This writeable attribute lets you enable or disable the autoterminate option, which could also be passed to the constructor.


  $exit= $proc->wait; # blocks forever
  $exit= $proc->wait($timeout_seconds); # since version 1.20

Wait for the process to exit. Return the exit status of the command as returned by wait() on the system. To get the actual exit value, divide by 256 or right bit shift by 8, regardless of the operating system being used. If the process never existed, this returns undef. This function may be called multiple times even after the process has exited and it will return the same exit status.

Since version 1.20, you may pass an optional argument of the number of seconds to wait for the process to exit. This may be fractional, and if it is zero then the wait will be non-blocking. Note that on Unix this is implemented with "alarm" in Time::HiRes before a call to wait(), so it may not be compatible with scripts that use alarm() for other purposes, or systems/perls that resume system calls after a signal. In the event of a timeout, the return will be undef.


Pause the process. This returns true if the process is stopped afterward. This throws an excetion if the process is not alive and autodie is enabled.


Resume a paused process. This returns true if the process is not stopped afterward. This throws an exception if the process is not alive and autodie is enabled.

terminate, terminate(@kill_sequence)

Reliably try to kill the process. Returns 1 if the process no longer exists once terminate has completed, 0 otherwise. This will also return 1 if the process has already exited.

@kill_sequence is a list of actions and seconds-to-wait for that action to end the process. The default is TERM 2 TERM 8 KILL 3 KILL 7 . On Unix this sends SIGTERM and SIGKILL; on Windows it just calls TerminateProcess (graceful termination is still a TODO).

Note that terminate() (formerly named die()) on Proc::Background 1.10 and earlier on Unix called a sequence of:

  ->die( ( HUP => 1 )x5, ( QUIT => 1 )x5, ( INT => 1 )x5, ( KILL => 1 )x5 );

which wasn't what most people need, since SIGHUP is open to interpretation, and QUIT is almost always immediately fatal and generates a coredump. The new default should accomodate programs that acknowledge a second SIGTERM, and give enough time for it to exit on a laggy system while still not holding up the main script too much.

die is preserved as an alias for terminate.

This throws an exception if the process has been reaped and autodie is enabled.


timeout_system timeout, command, [arg, [arg...]]
timeout_system 'timeout command [arg [arg...]]'

Run a command for timeout seconds and if the process did not exit, then kill it.

In a scalar context, timeout_system returns the exit status from the process. In an array context, timeout_system returns a two element array, where the first element is the exist status from the process and the second is set to 1 if the process was killed by timeout_system or 0 if the process exited by itself.

The exit status is the value returned from the wait() call. If the process was killed, then the return value will include the killing of it. To get the actual exit value, divide by 256.

If something failed in the creation of the process, the subroutine returns an empty list in a list context, an undefined value in a scalar context, or nothing in a void context.


The following behaviors aren't ideal, but are preserved for backward-compatibility.

Commandline vs. Single Argv[]

->new($x) is treated as a command line. In ->new({ exe => $y }, $x), $x is treated as $ARGV[0]. Use ->new({ command => ... }) (scalar vs. arrayref) to dis-ambiguate.

Win32 Argv Quoting

This is a bug in Windows, not this module. It is not possible to universally convert an @ARGV into a commandline, because each Win32 program performs its own command line parsing, and cmd.exe and find.exe deviate from the majority of other executables. Those things could be improved with hieuristics, which this module doesn't have.

Win32 exe determination

If you don't specify an absolute path for option exe, this module manually searches the %PATH% looking for the executable, and is less thorough than the native Windows shell behavior. Use { exe => undef } to get the naive Windows exe search. (but you need Win32::Process version 0.17 or newer)


This module only supports TerminateProcess, which is equivalent to SIGKILL, not SIGTERM. SIGTERM could be emulated by calling taskkill.exe, or using windows messages. Patches welcome.



IPC::Run is a much more complete solution for running child processes. It handles dozens of forms of redirection and pipe pumping, and should probably be your first stop for any complex needs.

However, also note the very large and slightly alarming list of limitations it lists for Win32. Proc::Background is a much simpler design and should be more reliable for simple needs.


If you are running on Win32, this article by Daniel Colascione helps describe the problem you are up against for passing argument lists: Everyone quotes command line arguments the wrong way

This module gives you parsing / quoting per the standard CommandLineToArgvW behavior. But, if you need to pass arguments to be processed by cmd.exe then you need to do additional work.


  • Blair Zajac <>

  • Michael Conrad <>


  • Florian Schlichting <>

  • Kevin Ryde <>

  • Salvador Fandiño <>

  • Sven Kirmess <>


version 1.32


This software is copyright (c) 2023 by Michael Conrad, (C) 1998-2009 by Blair Zajac.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.