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

Question

Ruby has two ways of referring to the standard input: The STDIN constant , and the $stdin global variable.

Aside from the fact that I can assign a different IO object to $stdin because it's not a constant (e.g. before forking to redirect IO in my children), what's the difference between STDIN and $stdin? When should I use each in my code?

If I reassign $stdin, does it affect STDIN?

And does this also apply to STDOUT/$stdout and STDER/$stderr?

Answer 1

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.