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.

STDERR and $stderr are pointing to the same thing initially; you can reassign the global variable but you shouldn't mess with the constant. $stdin and STDIN, $stdout and STDOUT pairs are likewise.

I had to change STDERR a couple of times as an alternative to monkey-patching some gems outputting error messages with STDERR.puts. If you reassign with STDERR = $stdout you get a warning while STDERR.reopen('nul', 'w') goes without saying.