NAME

IPC::Pipeline - Create a shell-like pipeline of many running commands

SYNOPSIS

use IPC::Pipeline;

my @pids = pipeline(\*FIRST_CHLD_IN, \*LAST_CHLD_OUT, \*CHILDREN_ERR,
    [qw(filter1 args)],
    sub { filter2(); return 0 },
    [qw(filter3 args)],
    ...
    [qw(commandN args)]
);

... do stuff ...

my %statuses = map {
    waitpid($_, 0);
    $_ => ($? >> 8);
} @pids;

DESCRIPTION

Similar in calling convention to IPC::Open3, pipeline() spawns N children, connecting the first child to the FIRST_CHLD_IN handle, the final child to LAST_CHLD_OUT, and each child to a shared standard error handle, CHILDREN_ERR. Each subsequent filter specified causes a new process to be fork()ed. Each process is linked to the last with a file descriptor pair created by pipe(), using dup2() to chain each process' standard input to the last standard output.

FEATURES

IPC::Pipeline accepts external commands to be executed in the form of ARRAY references containing the command name and each argument, as well as CODE references that are executed within their own processes as well, each as independent parts of a pipeline.

ARRAY REFS

When a filter is passed in the form of an ARRAY containing an external system command, each such item is executed in its own subprocess in the following manner.

exec(@$filter) or die("Cannot exec(): $!");

CODE REFS

When a filter is passed in the form of a CODE ref, each such item is executed in its own subprocess in the following way.

exit $filter->();

BEHAVIOR

If fileglobs or numeric file descriptors are passed in any of the three positional parameters, then they will be duplicated onto the file handles allocated as a result of the process pipelining. Otherwise, simple scalar assignment will be performed.

Like IPC::Open3, pipeline() returns immediately after spawning the process chain, though differing slightly in that the IDs of each process is returned in order of specification in a list when called in array context. When called in scalar context, only the ID of the first child process spawned is returned.

Also like IPC::Open3, one may use select() to multiplex reading and writing to each of the handles returned by pipeline(), preferably with non-buffered sysread() and syswrite() calls. Using this to handle reading standard output and error from the children is ideal, as blocking and buffering considerations are alleviated.

CAVEATS

If any child process dies prematurely, or any of the piped file handles are closed for any reason, the calling process inherits the kernel behavior of receiving a SIGPIPE, which requires the installation of a signal handler for appropriate recovery.

Please be advised that any usage of numeric file descriptors will result in an implicit import of POSIX::dup2() at runtime.

EXAMPLE ONE - OUTPUT ONLY

The following example implements a quick and dirty, but relatively sane tar and gzip solution. For proper error handling from any of the children, use select() to multiplex the output and error streams.

use IPC::Pipeline;

my @paths = qw(/some /random /locations);

my @pids = pipeline(my ($in, $out), undef,
    [qw(tar pcf -), @paths],
    ['gzip']
);

open(my $fh, '>', 'file.tar.gz');
close $in;

while (my $len = sysread($out, my $buf, 512)) {
    syswrite($fh, $buf, $len);
}

close $fh;
close $out;

#
# We may need to wait for the children to die in some extraordinary
# circumstances.
#
foreach my $pid (@pids) {
    waitpid($pid, 1);
}

EXAMPLE TWO - INPUT AND OUTPUT

The following solution implements a true I/O stream filter as provided by any Unix-style shell.

use IPC::Pipeline;

my @pids = pipeline(my ($in, $out), undef,
    [qw(tr A-Ma-mN-Zn-z N-Zn-zA-Ma-m)],
    [qw(cut -d), ':', qw(-f 2)]
);

my @records = qw(
    foo:bar:baz
    eins:zwei:drei
    cats:dogs:rabbits
);

foreach my $record (@records) {
    print $in $record ."\n";
}

close $in;

while (my $len = sysread($out, my $buf, 512)) {
    syswrite(STDOUT, $buf, $len);
}

close $out;

foreach my $pid (@pids) {
    waitpid($pid, 1);
}

EXAMPLE THREE - MIXING COMMANDS AND CODEREFS

The following solution demonstrates the ability of IPC::Pipeline to execute CODE references in the midst of a pipeline.

use IPC::Pipeline;

my @pids = pipeline(my ($in, $out), undef,
    sub { print 'cats'; return 0 },
    [qw(tr acst lbhe)]
);

close $in;

while (my $line = readline($out)) {
    chomp $line;
    print "Got '$line'\n";
}

close $out;

SEE ALSO

• IPC::Open3

• IPC::Run, for a Swiss Army knife of Unix I/O gizmos

  It should be mentioned that mst's IO::Pipeline has very little in common with IPC::Pipeline.

COPYRIGHT

Copyright 2011, Erin Schönhals <wrath@cpan.org>. Released under the terms of the MIT license.

cpanm IPC::Pipeline

perl -MCPAN -e shell
install IPC::Pipeline