Difference between $stdout and STDOUT in Ruby

Question

In Ruby, what is the difference between $stdout (preceded by a dollar sign) and STDOUT (in all caps)? When doing output redirection, which should be used and why? The same goes for $stderr and STDERR.

Edit: Just found a related question.

Answer 1

$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

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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.

Answer 2

Both $stdout and STDOUT have different meanings. Ruby's documentation is pretty clear on this topic:

• $stdout – The current standard output.
• STDOUT – The standard output. The default value for $stdout.

When you want to write to the standard output, then you actually mean the current standard output, thus you should write to $stdout.

STDOUT isn't useless too. It stores the default value for $stdout. If you ever reassign $stdout, then you can restore it to the previous value with $stdout = STDOUT.

Furthermore, there's one more predefined variable:

• $> – The default output for print, printf, which is $stdout by default.

However it looks like in Ruby 2.3 it simply behaves as an alias for $stdout. Reassigning $stdout changes the value of $> and vice versa.