# NAME IPC::Pipeline - Create a shell-like pipeline of many running commands # SYNOPSIS use IPC::Pipeline; my @pids = pipeline( my $first_child_in, my $last_child_out, my $err, [qw(filter1 args)], sub { filter2(); return 0 }, [qw(filter3 args)], ... [qw(commandN args)] ); ... do stuff ... my %statuses = map { waitpid($_, 0); $_ => ($? >> 8); } @pids; # DESCRIPTION __IPC::Pipeline__ exports a single function `pipeline()`. Similar in calling convention to [IPC::Open3](https://metacpan.org/pod/IPC::Open3), `pipeline()` spawns N children, connecting the first child to the `$first_child_in` handle, the final child to `$last_child_out`, and each child to a shared standard error handle, `$err`. Each subsequent filter specified causes a new process to be `fork()`ed. Each process is linked to the last with a file descriptor pair created by `pipe()`, using `dup2()` to chain each process' standard input to the last standard output. __IPC::Pipeline__ does not work on __MSWin32__, but it works on __cygwin__. ## FEATURES __IPC::Pipeline__ accepts external commands to be executed in the form of ARRAY references containing the command name and each argument, as well as CODE references that are executed within their own processes as well, each as independent parts of a pipeline. ### ARRAY REFS When a filter is passed in the form of an ARRAY containing an external system command, each such item is executed in its own subprocess in the following manner. exec(@$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](https://metacpan.org/pod/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, an ARRAY reference of the process IDs will be returned. Also like [IPC::Open3](https://metacpan.org/pod/IPC::Open3), one may use `select()` to multiplex reading and writing to each of the handles returned by `pipeline()`, preferably with non-buffered [sysread()](https://metacpan.org/pod/perlfunc#sysread) and [syswrite()](https://metacpan.org/pod/perlfunc#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. Unlike [IPC::Open3](https://metacpan.org/pod/IPC::Open3), __IPC::Pipeline__ will NOT redirect child process stderr to stdout if no file handle for stderr is specified. As of version 0.6, the caller will always need to handle standard error, to prevent any children from blocking; it would make little sense to pass one process' standard error as an input to the next process. # 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); open(my $err, '<', '/dev/null'); my @pids = pipeline(my ($in, $out), $err, [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; open(my $err, '<', '/dev/null'); my @pids = pipeline(my ($in, $out), $err, [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; open(my $err, '<', '/dev/null'); my @pids = pipeline(my ($in, $out), $err, 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](https://metacpan.org/pod/IPC::Open3) - [IPC::Run](https://metacpan.org/pod/IPC::Run), for a Swiss Army knife of Unix I/O gizmos It should be mentioned that mst's [IO::Pipeline](https://metacpan.org/pod/IO::Pipeline) has very little in common with __IPC::Pipeline__. # AUTHOR Written by Xan Tronix # COPYRIGHT Copyright (c) 2014, cPanel, Inc. All rights reserved. http://cpanel.net/ This is free software; you can redistribute it and/or modify it under the same terms as Perl itself. See the LICENSE file for further details.