Runs shell command with similar IO stream behavior as a shell script: Stdout and stderr of command are captured and piped to caller's stdout and stderr.

shell_command.rb

require "open4"
require 'stringio'

class ShellCommand
  class CommandError < StandardError
    attr_reader :stderr_string

    def initialize(message, stderr_string)
      super(message)
      @stderr_string = stderr_string
    end
  end

  def self.call(command)
    new.call(command)
  end

  def thread_for_output_stream(stream, dest_stream, tee_buffer=nil)
    Thread.new do
      begin
        while !stream.eof?
          str = stream.read
          dest_stream.puts(str)
          tee_buffer.puts(str) if tee_buffer
        end
      ensure
        stream.close
      end
    end
  end

  def call(command)
    pid, p_i, p_o, p_e = Open4.popen4(command)
    p_i.close
    error_output = StringIO.new

    threads = [
      thread_for_output_stream(p_o, $stdout),
      thread_for_output_stream(p_e, $stderr, error_output)
    ]

    _, status = Process::waitpid2(pid)
    threads.each { |t| t.join }
    raise CommandError.new("Command #{command} failed", error_output.string) if status.exitstatus != 0
  end
end