From: Robert Citek <robert.citek@...>
Date: 2009-11-06T03:49:15+09:00
Subject: reading from an external process with IO.popen

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:

$ { echo hello ; sleep 2 ; echo world; } | sed -ne p
hello
world

$ cat foo.rb
#!/usr/bin/env ruby
if $0 == __FILE__
 cat = IO.popen("sed -ne p", "w+") ;
 cat.puts("hello, ") ;
 puts(cat.gets) ;
 sleep 2 ;
 cat.puts("world") ;
 puts(cat.gets) ;
end

$ ./foo.rb
./foo.rb:6:in `gets': Interrupt
       from ./foo.rb:6

Why does ruby work in the first case but wait forever in the second?

Using this version of ruby:

$ ruby -v
ruby 1.8.6 (2007-09-24 patchlevel 111) [i486-linux]

Thanks in advance for any pointers to references.

Regards,
- Robert