From: Joel VanderWerf <vjoel@...>
Date: 2006-03-23T02:55:05+09:00
Subject: Re: Reading from standard error

Thiago Arrais wrote:
> Maybe this one has already been answered somewhere. If so, my
> searching skills really suck.
> 
> How can I start a sub-process from Ruby and read its standard error
> and standard output stream separately? I have tried to do it with
> IO.popen, but it only reads the output stream. The Process  module
> doesn't help either.
> 
> Cheers,
> 
> Thiago Arrais

require 'open3'

stdin, stdout, stderr = Open3.popen3(
  'ruby -e "\$stdout.sync = true; while (s=gets); 5.times
{\$stderr.puts(s.reverse); puts s}; end"')

t1 = Thread.new do
  while (s=stderr.gets)
    puts "STDERR: #{s.inspect}"
  end
  puts "STDERR: DONE"
end

t2 = Thread.new do
  while (s=stdout.gets)
    puts "STDOUT: #{s.inspect}"
  end
  puts "STDOUT: DONE"
end

stdin.puts "foo"
stdin.close
[t1, t2].each do |t| t.join end



The purpose of the $stdout.sync = true in the subprocess is so that you
can read the subprocess's stdout as soon as it is written to. If all you
care about is getting all the stdout _eventually_, you don't need it.

-- 
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407