From: Robert Klemme <shortcutter@...>
Date: 2009-11-06T19:15:52+09:00
Subject: Re: reading from an external process with IO.popen

2009/11/5 Robert Citek <robert.citek@gmail.com>:
> Hello all,
>
> I'm trying to wrap my head around IO.popen with some simple examples
> that send data to and read data from an
> external process.  I've create a sample case in the shell like this:
>
> $ { echo hello ; sleep 2 ; echo world; } | cat
> hello
> world
>
> I've written the same in ruby like so, which works:
>
> $ cat foo.rb
> #!/usr/bin/env ruby
> if $0 == __FILE__
>  cat = IO.popen("cat", "w+") ;
>  cat.puts("hello, ") ;
>  puts(cat.gets) ;
>  sleep 2 ;
>  cat.puts("world") ;
>  puts(cat.gets) ;
> end
>
> $ ./foo.rb
> hello
> world
>
> However, if I change the cat command to a sed command, the ruby
> version no longer works.  The command-line equivalent does work, but
> the ruby version waits forever and has to be interrupted:

That's probably because you do not close the write end of the pipe in
Ruby code.  Also, it's better to place the reading portion in a
separate thread in order to prevent deadlocks.  And, please use the
block form of IO.popen which is more robust.

Try this pattern:

IO.popen("cat", "w+") do |cat|
  # background output
  t = Thread.new { cat.each {|l| puts l} }

  # main work
  cat.puts "hello, "
  sleep 2
  cat.puts "world"

  # terminate processing:
  cat.close_write
  t.join
end

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/