Cryptic Ruby Global Variables and Their Meanings

[briangonzalez]

http://jimneath.org/2010/01/04/cryptic-ruby-global-variables-and-their-meanings.html

[telemachos]

For a longer list (though the explanations are briefer), see Zenspider's Ruby QuickRef[1]. It also includes a handy listing of predefined global constants[2] (and a lot more).[3]

[1]: http://zenspider.com/Languages/Ruby/QuickRef.html#pre-define...

[2]: http://zenspider.com/Languages/Ruby/QuickRef.html#pre-define...

[3]: http://zenspider.com/Languages/Ruby/QuickRef.html

[dlisboa → telemachos]

$YOUR_RUBY_PATH/lib/ruby/1.9.1/English.rb has the full list, with pretty decent explanation.

[twelvechairs → dlisboa]

also here http://stdlib.rubyonrails.org/libdoc/English/rdoc/files/Engl...

[Empact → twelvechairs]

Yep, if you must use the variables, much much better to use "require 'english'" and these more meaningful names than the cryptic originals.

[zenspider → dlisboa]

I have a TODO in omnifocus to fold those in. It's a shame I didn't do that earlier.

[draegtun → dlisboa]

I didn't realise Ruby also had this. Here's the Perl corollary:

  perldoc English
  perldoc perlvar

links: http://perldoc.perl.org/English.html | http://perldoc.perl.org/perlvar.html

[ef4]

This is one area where Matz kept too much of Perl. Thankfully, it's rare to see Ruby programmers making heavy use of these.

[wonnage → ef4]

The former ubiquity of Perl made most of these common knowledge. For example, $? works in both Perl and bash (actually, I don't know which did this first...). So in the context of shell scripts, where Ruby started, having these magic globals makes a lot of sense.

IMO they can make code easier to understand, once you've learned what they mean. $2 conveys the same idea as matches[2] without having to think about array indices, and why the matches aren't zero-indexed like a normal array. This is obviously totally subjective.

[draegtun → ef4]

Especially when Ruby has no way of scoping these variables... unlike Perl:

  {
      # level 1
      local $\ = "[1]\n";
      some_print_operation();
  
      {
          # level 2
          local $\ = "[2]\n";
          some_print_operation();
      }

      # level 1 again
      some_print_operation();
  }

  # $\ back to what it was before all this :)

[briangonzalez → ef4]

True, but I have seen -- $:.unshift File.dirname(__FILE__) -- used quite a bit in the wild, especially in MacRuby projects.

[vidarh → briangonzalez]

$: and $1 .. $9 are relatively frequently used. Most of the other ones are rare - several of them I've never seen used at all.

[rjbond3rd → ef4]

It's also rare to see them heavily used in Perl nowadays. Lots of recent code doesn't use them at all (e.g., with autodie, no need for $!)

[ZoFreX → ef4]

If you write your shell scripts in Ruby they do come in handy. I certainly wouldn't use them outside of that context.

[haileys → ef4]

I use $1, $2, $3, etc. quite often. $' and $` are also useful for throwaway scripts.

[riffraff → haileys]

for throwaway irb sessions, I often use stuff like

    foo.map { |x| x.something rescue $! }

[damncabbage]

When using the debugger you can sometimes end up in an exception handler that doesn't provide a way to get access to said exception. $! can be a life-saver to help figure out what's going on, eg.

  (rdb:1) list
     11    do_something_stupid
     12  rescue
  => 13    log("Something went wrong")
     14  end
 
  (rdb:2) e $!
  #<Errno::ENOENT: No such file or directory - /tmp/durr>

[Xylakant]

I rarely see those used except when you look at codegolf contests. There you can find all of those variables put to good and creative use :)