From: Brian Candler <b.candler@...>
Date: 2009-12-14T18:58:01+09:00
Subject: Re: Redirecting standard output

Robert Gleeson wrote:
> I don't see how reassignment or #reopen would make a difference. They 
> reference the same object after all - for this particular case, their 
> behavior is the same.
> 
> So what gives?

When you assign to $stdout, you are only changing the Ruby global 
variable called '$stdout'

As it happens, Kernel#puts is just a shorthand for $stdout.puts, so it 
will do what you expect. But the *real* stdout of the process (that is, 
file descriptor 1 in the process' FD table) is untouched. You can still 
get at it using the STDOUT constant, or IO.new(1).

When you do $stdout.reopen(f), you are actually re-opening FD 1 with a 
different file - underneath, ruby is doing a dup2() call to copy 
f.fileno to FD 1. This means that the process' actual stdout has 
changed. This is what gets inherited by children you fork off.

So:

$ ruby -e '$stdout = File.open("/dev/null","w"); system("echo hello")'
hello
$ ruby -e '$stdout.reopen(File.open("/dev/null","w")); system("echo 
hello")'
$

As for deprecation: I have a suspicion it's $defout not $stdout which is 
deprecated.
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