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John Eaglesham


Spawn::Safe - Fork and exec a process "safely".


A basic example:

 use Spawn::Safe;
 use Data::Dumper;
 my $results = spawn_safe({ argv => [ 'ls', '-al', '/var/' ], timeout => 2 });
 die Dumper $results;

As a replacement for backticks:

 use Spawn::Safe;
 # $output = `ls -al /var/`;
 $output = spawn_safe(qw{ ls -al /var/ })->{stdout};


Spawn::Safe is a module designed to make "safe" calls to outside binaries easier and more reliable. Spawn::Safe never invokes a shell (unless the shell is explicitly requested), so escaping for the shell is not a concern. An optional timeout is made available, so scripts will not hang forever, and the caller is able to retrieve both stdout and stderr. An optional string can be passed to the executed program's standard input stream.



Spawn (via fork and exec) the specified binary and capture its output.


If passed a single scalar, spawn_safe will assume that to be the the target binary, and execute it without a limit on runtime.

If passed an array, spawn_safe will execute the first element of the array as the target binary, with the remaining elements passed as parameters to the target binary, without a limit on runtime.

The preferred mode is to pass in a single hash reference. When called this way, the following keys are available:

  • argv

    Either a string containing the name of the binary which will be called with no parameters:

     my $r = spawn_safe({ argv => 'ls' });

    Or an array reference containing the binary and all of its parameters:

     my $r = spawn_safe({ argv => [ 'ls', '-al' ] });
  • timeout

    The amount of time, in seconds, the binary will be allowed to run before being killed and a timeout error being returned. If false (or is otherwise undefined or unset), the timeout will be infinite.

  • env

    A hash reference containing the new environment for the executed binary. If false (or otherwise undefined or unset), it will default to the current environment. You must specify the complete environment, as the current environment will be overwritten as a whole. To alter only one variable, a copy of the enviornment must be made, altered, and then passed in as a whole, eg:

     my %new_env = %ENV;
     $new_env{'TMP'} = '/var/tmp/';
     my $r = spawn_safe({ argv => 'ls', env => \%new_env });

    Please note that if a new environment is specified, the new binary's environment will be altered before the call to exec() (but after the fork(), so the caller's environment will be unchanged), so the new environment will take effect before the new binary is launched. This means that if you alter a part of the environment needed to launch the binary (eg, by changing PATH, LD_LIBRARY_PATH, etc), these new variables will need to be set such that the binary can be executed successfully.

  • stdin

    A string to be passed to the target binary's standard input stream. The string will be written into the stream and then the stream will be closed.

     my $r = spawn_safe({ argv => [ '/usr/bin/tr', 'a', 'b' ], stdin => 'aaa' });

Return value

A hash reference will be returned containing one of the following sets of values:

  • If the binary could not be spawned, the single key, 'error' will be set, which is a text description of the reason the binary could not be spawned.

  • If the binary was executed successfully, but terminated due to a timeout, the keys 'error', 'stdout', and 'stderr', will be set. The value for 'error' will be set to 'timed out'. Any data collected from the executed binary's stdout or stderr will also be made available, but since the binary was forcefully terminated, the data may be incomplete.

  • If the binary was executed successfully and ran to completion, the keys 'exit_code', 'stdout, and 'stderr', will all be available.

The key "exit_zero" will always be present, which is true if the binary is executed successfully and exited with a code of zero.


The current PATH will be searched for the binary, if available. Open filehandles are subject to Perl's standard close-on-exec behavior. A shell will not be invoked unless explicitly defined as the target binary, as such output redirection and other shell features are unavailable.

If passed invalid parameters, spawn_safe will croak.

Please note that when specifying a timeout, alarm() is no longer used. If the clock is stepped significantly backwards during a timeout, a possibly false timeout error may be thrown. Timeout accuracy should be within one second.

If a timeout does occur, the spawned program will be sent a SIGKILL before spawn_safe returns.


This module attempts to work on MSWin32 but I've been unable to get it working due to strange issues with IO::Select. I haven't been able to track down the exact cause, so for now I don't believe this module functions on MSWin32.

Linux and BSD are tested and supported platforms.


This module is licensed under the same terms as Perl itself.


Version 2.006 - 2013-11-12, jeagle

Modify PIPE_BUF_SIZE to be more conservative to ensure non-blocking writes on all OSs.

Version 2.005 - 2013-11-11, jeagle

Add stdin option, clarify docs, add exit_zero return flag.

Version 2.004 - 2012-08-13, jeagle

Include license. Oops.

Version 2.003 - 2012-04-01, jeagle

Untie any tied filehandles before we re-open them to ourselves to work around any weird tie behavior (should fix issues running under FCGI). Thanks Charly.

Version 2.002 - 2012-01-04, jeagle

Correct documentation (RT#72831, thanks Stas)

Update unit tests to specify number of tests instead of using no_plan, otherwse CPAN Testers reports tests fail.

Version 2.001 - 2011-06-13, jeagle

Give the spawned program its own STDIN.

Version 2.000 - 2011-05-12, jeagle

Correct timeout handling. Attempt to correct unit tests for MSWin32, but there seems to be an issue with IO::Select preventing it from working properly. Update docs for MSWin32.

Version 1.9 - 2011-05-10, jeagle

Don't use clock_gettime(), use time() and return a timeout if time steps backwards.

Version 1.8 - 2011-05-09, jeagle

Clean up docs, stop using SIGALARM for timeouts.

Version 1.7 - 2010-07-09, jeagle

Clean up for release to CPAN.

Version 0.4 - 2009-05-13, jeagle

Correct a warning issued when using spawn_safe without a timeout.

Fix compatibility with perl < 5.8.

Version 0.3 - 2009-04-21, jeagle

Clarify documentation regarding use of SIGALRM and for passing of a new environment.

Correct a warning thrown by exec().

Correct an issue with incorrectly handled timeouts.

Version 0.2 - 2009-04-20, jeagle

Modify API, breaking compatibility, for clarity and expandability.

Add the ability to specify the target program's environment.

Return the (partial) stdout and stderr on a timeout.

Update and clarify documentation.

Version 0.1 - 2009-04-11, jeagle

Inital release.