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