System::Command(3)

NAME
System::Command - Object for running system commands

SYNOPSIS
    use System::Command;

    # invoke an external command, and return an object
    $cmd = System::Command->new( @cmd );

    # options can be passed as a hashref
    $cmd = System::Command->new( @cmd, \%option );

    # $cmd is basically a hash, with keys / accessors
    $cmd->stdin();     # filehandle to the process' stdin (write)
    $cmd->stdout();    # filehandle to the process' stdout (read)
    $cmd->stderr();    # filehandle to the process' stdout (read)
    $cmd->pid();       # pid of the child process

    # find out if the child process died
    if ( $cmd->is_terminated() ) {
        # the handles are not closed yet
        # but $cmd->exit() et al. are available
    }

    # done!
    $cmd->close();

    # exit information
    $cmd->exit();      # exit status
    $cmd->signal();    # signal
    $cmd->core();      # core dumped? (boolean)

    # cut to the chase
    my ( $pid, $in, $out, $err ) = System::Command->spawn(@cmd);

DESCRIPTION
"System::Command" is a class that launches external system commands and return an object representing them, allowing to interact with them through their "STDIN", "STDOUT" and "STDERR" handles.

METHODS
"System::Command" supports the following methods:

  new( @cmd )
Runs an external command using the list in @cmd.

If @cmd contains a hash reference, it is taken as an option hash.

The recognized keys are:

"cwd"
    The current working directory in which the command will be run.

"env"
    A hashref containing key / values to add to the command environment.

    If a value is "undef", the variable corresponding to the key will be removed from the environment.

"input"
    A string that is send to the command's standard input, which is then closed.

    Using the empty string as "input" will close the command's standard input without writing to it.

    Using "undef" as "input" will not do anything. This behaviour provides a way to modify previous options populated by some other part of the program.

    On some systems, some commands may close standard input on startup, which will cause a SIGPIPE when trying to write to it. This will raise an exception.

If several option hashes are passed to "new()", they will be merged together with individual values being overridden by those (with the same key) from hashes that appear later in the list.

The "System::Command" object returned by "new()" has a number of attributes defined (see below).

  close()
Close all pipes to the child process, collects exit status, etc. and defines a number of attributes (see below).

  is_terminated()
Returns a true value if the underlying process was terminated.

If the process was indeed terminated, collects exit status, etc. and defines the same attributes as "close()", but does not close all pipes to the child process,

  spawn( @cmd )
This shortcut method calls "new()" (and so accepts options in the same manner) and directly returns the "pid", "stdin", "stdout" and "stderr" attributes, in that order.

  Accessors
The attributes of a "System::Command" object are also accessible through a number of accessors.

The object returned by "new()" will have the following attributes defined:

cmdline()
    Return the command-line actually executed, as a list of strings.

options()
    The merged list of options used to run the command.

pid()
    The PID of the underlying command.

stdin()
    A filehandle opened in write mode to the child process' standard input.

stdout()
    A filehandle opened in read mode to the child process' standard output.

stderr()
    A filehandle opened in read mode to the child process' standard error output.

Regarding the handles to the child process, note that in the following code:

    my $fh = System::Command->new( @cmd )->stdout;

$fh is opened and points to the output handle of the child process, while the anonymous "System::Command" object has been destroyed. Once $fh is destroyed, the subprocess will be reaped, thus avoiding zombies.

After the call to "close()" or after "is_terminated()" returns true, the following attributes will be defined:

exit()
    The exit status of the underlying command.

core()
    A boolean value indicating if the command dumped core.

signal()
    The signal, if any, that killed the command.

CAVEAT EMPTOR
Note that "System::Command" uses "waitpid()" to catch the status information of the child processes it starts. This means that if your code (or any module you "use") does something like the following:

    local $SIG{CHLD} = 'IGNORE';    # reap child processes

"System::Command" will not be able to capture the "exit", "core" and "signal" attributes. It will instead set all of them to the impossible value "-1", and display the warning "Child process already reaped, check for a SIGCHLD handler".

To silence this warning (and accept the impossible status information), load "System::Command" with:

    use System::Command -quiet;

It is also possible to more finely control the warning by setting the $System::Command::QUIET variable (the warning is not emitted if the variable is set to a true value).

If the subprocess started by "System::Command" has a short life expectancy, and no other child process is expected to die during that time, you could even disable the handler locally (use at your own risks):

    {
        local $SIG{CHLD};
        my $cmd = System::Command->new(@cmd);
        ...
    }

AUTHOR
Philippe Bruhat (BooK), "<book at cpan.org>"

ACKNOWLEDGEMENTS
Thanks to Alexis Sukrieh who, when he saw the description of "Git::Repository::Command" during my talk at OSDC.fr 2010, asked why it was not an independent module. This module was started by taking out of "Git::Repository::Command" 1.08 the parts that weren't related to Git.

BUGS
Please report any bugs or feature requests to "bug-system-command at rt.cpan.org", or through the web interface at <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=System-Command>. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.

SUPPORT
You can find documentation for this module with the perldoc command.

    perldoc System::Command

You can also look for information at:

•   RT: CPAN's request tracker

    <http://rt.cpan.org/NoAuth/Bugs.html?Dist=System-Command>

•   AnnoCPAN: Annotated CPAN documentation

    <http://annocpan.org/dist/System-Command>

•   CPAN Ratings

    <http://cpanratings.perl.org/d/System-Command>

•   Search CPAN

    <http://search.cpan.org/dist/System-Command/>

COPYRIGHT
Copyright 2010-2011 Philippe Bruhat (BooK).

LICENSE
This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See http://dev.perl.org/licenses/ for more information.