Difference Between $Stdout and Stdout in Ruby

Difference between $stdout and STDOUT in Ruby

$stdout is a global variable that represents the current standard output. STDOUT is a constant representing standard output and is typically the default value of $stdout.

With STDOUT being a constant, you shouldn't re-define it, however, you can re-define $stdout without errors/warnings (re-defining STDOUT will raise a warning). for example, you can do:

$stdout = STDERR

Same goes for $stderr and STDERR

So, to answer the other part of your question, use the global variables to redirect output, not the constants. Just be careful to change it back further on in your code, re-defining global variables can impact other parts of your application.

What is the difference between STDIN and $stdin in Ruby?

If $stdin is reassigned, STDIN is not affected. Likewise $stdin is not affected when STDIN is reassigned (which is perfectly possible (though pointless), but will produce a warning). However if neither variable has been reassigned, they both point to the same IO object, so calling reopen¹ on one will affect the other.

All the built-in ruby methods use $< (a.k.a. ARGF) to read input. If ARGV is empty, ARGF reads from $stdin, so if you reassign $stdin, that will affect all built-in methods. If you reassign STDIN it will have no effect unless some 3rd party method uses STDIN.

In your own code you should use $stdin to be consistent with the built-in methods².

¹ reopen is a method which can redirect an IO object to another stream or file. However you can't use it to redirect an IO to a StringIO, so it does not eliminate all uses cases of reassigning $stdin.

² You may of course also use $</ARGF to be even more consistent with the built-in methods, but most of the time you don't want the ARGF behavior if you're explicitly using the stdin stream.

Understanding how ruby method $stdout works

$stdout is a global variable. By default it stores an object of type IO associated with the standard output of the program (which is, by default, the console).

puts is a method of the Kernel module that actually calls $stdout.send() and pass it the list of arguments it receives. As the documentation explains, puts(obj, ...) is equivalent to $stdout.puts(obj, ...).

Your code replaces $stdout with an object of type File that extends class IO. When it is created, your object opens the file out.txt for writing and together with its inheritance from IO it is fully compatible with the default behaviour of $stdout.

Since by default, all the output goes to $stdout, your new definition of $stdout ensures the output is written to the file out.txt without other changes in the code.

Why do Global Variables exist in Ruby?

Global variables are not bad. They're not evil. They're just incredibly, incredibly powerful. Which is why you shouldn't use them.

Global variables are global- they can be accessed and modified anywhere in the code. A single global variable has the potential to affect all of your classes, all of your functions, all of the classes and functions of every single library or dependency you load into your project, and all of the classes and functions of every single project which loads your project as a dependency, as well as the projects that load those projects, and so and and so forth, for ever and always, for the rest of time.

The second people start feeling comfortable using global variables, the namespace gets insanely cluttered and we get conflicts left and right and the stability of the programming language itself is threatened. Which is why the use of global variables is emphatically and repeatedly discouraged.

But global variables are not bad. They're like the highway lanes labeled "for emergency vehicles only," or like those fire-axes behind glass labeled "break glass in case of emergency."

It's entirely possible that at some point, in the distant future, you will have an incredibly unusual situation which merits the use of a single global variable. But that day is not today. And it is probably not tomorrow, or a month from now, or a year from now. Daily life, daily code- it just doesn't call for the unbridled power of a global variable.

$stdout is a great example of why global variables are sometimes important. $stdout is the default stream in ruby- the one where things will print if no other stream is specified. $stdout should be accessible from every class and every function in every library because it acts like a giant funnel, shoveling all output to a single location. The whole world knows and agrees that $stdout exists in ruby, and its uses are well-documented, so its power is well-managed.

This isn't to be confused with STDOUT which is a constant representing the actual pipe which sets up a stream between ruby and its parent program (usually a terminal). $stdout = STDOUT by default, but $stdout can be changed to anything. If you want your program to print to a file, you can change $stdout to a file stream.

I don't think this name choice is confusing for a seasoned rubyist. A variable is designed to be modified and a constant is designed to be constant. The difference between $stdout and STDOUT is that the former can be modified to change the standard output location of your program, and the latter is a constant, always pointing to the stdout stream. The capitalization makes a world of difference and conveys very different meanings.

As for why global constants are uninitialized and global variables are nil, that actually has nothing to do with globals. Ruby automatically initializes all variables as nil. You can easily see this with instance variables such as @foo or @@foo. In almost every situation, an undefined local variable will throw a NameError because ruby cannot tell whether it is a variable or a method. But in strange situations, they too are initialized as nil:

puts foo # => NameError: undefined local variable or method 'foo'
foo = 42 if false
puts foo # => nil

puts bar # => NameError
bar = bar
puts bar # => nil

It was a conscious design choice in Ruby not to automatically initialize constants. Because a constant is, by definition, something which is initialized once and then never changed, it would break the definition for a constant to be nil at first and then a different value later in the code.

I should also mention that global constants are considered acceptable, even among people who tout global variables as bad. The difference is that constants can only be assigned once and generally throw a warning or error if they get assigned again. This protects programmers from situations where conflicting global constants might cause problems.

What happens to the object I assign to $stdout in Ruby?

Are you sure there is no other manipulation of $stdout or capturehappening in between?

For me, output looks different. Both capture and $stdout are the same object and subsequently answer to string with the same response (ruby 1.9.2):

require 'stringio'                                                                                                                             
$stdout.to_s              # => #<IO:0x2584b30>                                                                                                 

capture = StringIO.new                                                                                                                         
$stdout = capture                                                                                                                              

puts $stdout.to_s              # => #<StringIO:0x89a38c0>                                                                                      
puts capture.to_s              # => #<StringIO:0x89a38c0>                                                                                      
puts "redirected"

$stderr.puts $stdout.string # => '#<StringIO:0x89a38c0>\n#<StringIO:0x89a38c0>\nredirected'                                                                           
$stderr.puts capture.string # => '#<StringIO:0x89a38c0>\n#<StringIO:0x89a38c0>\nredirected'

Difference Between $Stdout and Stdout in Ruby