Hold open a fifo from both ends, allowing input from external processes

pipe-exec

#!/usr/bin/env ruby
begin
  require 'optparse'

  options = {
    :direction => "in"
  }
  OptionParser.new do |opts|
    opts.banner = %{
usage: pipe-exec [options] FIFO_FILE COMMAND....

  Executes the command with stdin reading from the fifo and FD 3 holding open
  the fifo on the write side. As a result other processes can write to the
  fifo, and thereby to the command, without the fifo closing. It's a
  persistent pipe.

  Use the --direction option to do the same but flipping stdin with stdout.

options:

}.lstrip

    opts.on("-d", "--direction IN_OR_OUT", "the direction of the pipe (#{options[:direction]})") do |value|
      options[:direction] = value
    end

    opts.on("-h", "--help", "print this help") do
      puts opts
      exit
    end
  end.parse!

  case ARGV.length
  when 0
    $stderr.puts "no fifo specified"
    exit
  when 1
    $stderr.puts "no command specified"
    exit
  end
  fifo = ARGV.shift

  # The trick here is using NONBLOCK and opening the fifo for read first such
  # that the process will not deadlock with a fifo open for write, waiting for
  # the reader that will never open.
  #
  # See http://man7.org/linux/man-pages/man7/fifo.7.html
  # Or better 'The Linux Programming Interface' p917

  case options[:direction].downcase
  when "in"  then exec(*ARGV, 0 => [fifo, File::RDONLY | File::NONBLOCK], 3 => [fifo, File::WRONLY])
  when "out" then exec(*ARGV, 3 => [fifo, File::RDONLY | File::NONBLOCK], 1 => [fifo, File::WRONLY])
  else raise "invalid direction: #{options[:direction]} (should be 'in' or 'out')"
  end
end