From: Brian Marick <marick@...>
Date: 2001-09-22T20:09:50+09:00
Subject: [ruby-talk:21521] Re: Interaction system() / $stdout ?

Hmm. Perhaps the correct answer to "How do I redirect $stdout" is "Don't."

It seems that if you redirect $stdout, output to $defout is also 
redirected. However, before the redirection, mixtures of messages to 
$stdout and $defout are printed in the order they're executed. After, 
they're not (NT, ruby 1.6.3). Not surprising, as $stdout and $defout are 
now different objects (because of having to clone the original value of 
$stdout.)

When redirecting $defout, it seems you don't have to clone it to restore 
it. So everything can be put back together so that $stdout, $defout, and $> 
are once again the same object.

Moreover, you don't need the hack I mentioned previously, where you can't 
close the file until after you restore $stdout:
     $save_for_close = $stdout
     $stdout = $original_stdout
     $save_for_close.close
You can do it the simple way:
     $defout.close
     $defout = $saved_defout

When redirecting $defout, output to $stdout isn't redirected. But that 
might be OK. What uses $stdout? (I was surprised to see that 'p' doesn't 
use $stdout - it appears to use $defout.)

(Perhaps 'ruby -w' should warn about assignments to $stdout?)

Code that demonstrates all this below.

=========================================

puts "This is an example of redirecting $stdout to log-file."

def inspector(tag)
   puts "============="
   puts "#{tag}, $stdout = #{$stdout.inspect}, $defout=#{$defout.inspect}, 
$> = #{$>.inspect}"
end

inspector("Originally")
$stdout.puts "A message to $stdout - shows ordering preserved."
$defout.puts "A message to $defout - shows ordering preserved."
$stdout.puts "Another message to $stdout - shows ordering preserved."
$defout.puts "Another message to $defout - shows ordering preserved."

$saved_original = $stdout.dup
$stdout = File.open("log-file", "w")

inspector("After redirect")
$stdout.puts "After redirect, $stdout goes to log-file."
$defout.puts "After redirect, $defout goes to log-file."
$>.puts "After redirect, $> goes to log-file."
p "Note that 'p' is redirected as well."

$save_for_close = $stdout
$stdout = $saved_original
$save_for_close.close

inspector("After restoration")
$stdout.puts "After restoration, $stdout goes to standard output."
$defout.puts "After restoration, $defout goes to standard output."
$>.puts "After restoration, $> goes to standard output."

$stdout.puts "Another message to $stdout - shows ordering no longer preserved."
$defout.puts "Another message to $defout - shows ordering no longer preserved."

==================

puts "This is an example of redirecting $defout to log-file."

def inspector(tag)
   puts "============="
   puts "#{tag}, $stdout = #{$stdout.inspect}, $defout=#{$defout.inspect}, 
$> = #{$>.inspect}"
end

inspector("Originally")
$stdout.puts "A message to $stdout - shows ordering preserved."
$defout.puts "A message to $defout - shows ordering preserved."
$stdout.puts "Another message to $stdout - shows ordering preserved."
$defout.puts "Another message to $defout - shows ordering preserved."

$saved_original = $defout
$defout = File.open("log-file", "w")

inspector("After redirect")
$stdout.puts "After redirect, $stdout goes to standard output."
$defout.puts "After redirect, $defout goes to log-file."
$>.puts "After redirect, $> goes to log-file."
p "Note that 'p' is redirected as well."

$defout.close
$defout = $saved_original

inspector("After restoration")
$stdout.puts "After restoration, $stdout goes to standard output."
$defout.puts "After restoration, $defout goes to standard output."
$>.puts "After restoration, $> goes to standard output."

$stdout.puts "Another message to $stdout - shows ordering preserved."
$defout.puts "Another message to $defout - shows ordering preserved."

--
Brian Marick, marick@testing.com
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