From: Joel VanderWerf <vjoel@...>
Date: 2005-02-25T08:22:54+09:00
Subject: Re: How to extend $stdout#write for system calls?

Jim Freeze wrote:
> * Joel VanderWerf <vjoel@PATH.Berkeley.EDU> [2005-02-25 07:19:12 +0900]:
> 
> 
>>How could this work if echo is not even a ruby program? Maybe if you run 
>>echo with popen, and print the output of that... But if you want it to 
>>work with arb. ruby code, this won't help.
> 
> 
> It may not be possible. My knowledge of $stdout doesn't run too deep.
> So far, I have wrapped the write call, so puts is captured and I 
> have reopened stdout to write to a file. So, I figured there should
> be a way to capture a system call (since it inherits $stdout from the
> current process).

A child process inherits the file handle that $stdout refers to, but not 
the ruby object itself, even if the child process is ruby.

> The idea is to do the following:
> 
> IO::Tee(file) { 
>   # all output is to go to file and screen
>   puts "from ruby puts"
>   system("echo", "from system")
>   exec("echo", "from system")
> }
> 
> and have the contents of file and the screen be identical.

You'll be disappointed by the following if you are using windows, but 
what else is new.

------
class IO
   def tee
     IO.popen("-") do |pipe| # Won't work on mswin.
       if pipe # parent
         while (str = pipe.gets)
           puts str
           $stdout.puts str
         end
       else # child
         yield
       end
     end
   end
end

f=File.open("/tmp/tee-example-output", "w+")

f.tee do
   puts "something"
   puts "something else"
end

f.rewind
puts "The contents of file is:\n#{f.read.inspect}"
-------
output:

something
something else
The contents of file is:
"something\nsomething else\n"

Note that the "tee" block is executed in a child process, so state (e.g. 
local vars) will not be changed when you get back to the main process.