From: Hans Fugal <fugalh@...>
Date: 2007-08-03T01:40:00+09:00
Subject: Piping two shell commands together

I want to pipe two system commands together from within ruby. cmd1 and 
cmd2 are arrays, e.g.

cmd1 = ['oggdec','-o','-',oldpath]
cmd2 = ['lame','-',newpath]

I think I can do the following,

IO.popen('-','r') do |p1|
   exec cmd1 unless p1
   IO.popen('-','w') do |p2|
     exec cmd2 unless p2
     p2.write(p1.read)
   end
end

but I don't like the line "p2.write(p1.read)", because I think it will 
read into memory from p1 before turning around and writing it to p2. 
This may work for the above example, but it's not very pipelike and I 
worry that read might return early (I'm not sure what the semantics of 
that are in Ruby, but in C I'd have to account for that).

Plus I don't like the popen '-'/exec combination. I'd love a more 
elgant, efficient, and/or safer idiom if anyone can think of one.