Ruby 中充滿了一系列的隱藏變量，我們可以從這些預定義的全局變量中獲取一些有意思的信息。

全局進程變量

$$ 表示當前運行的 ruby 進程。
代碼如下:>> $$
=> 17170
我們可以從當前進程殺死它自己
代碼如下:
>> `kill -9 #{$$}`
[1]    17170 killed     irb
$? 表示最近一個子進程的狀態
代碼如下:
>> `echo hello`
=> "hello/n"
>> $?
=> #<Process::Status: pid 18048 exit 0>
>> $?.success?
=> true

異常和錯誤

$1 表示引起異常的信息。比如在這里 raise "there's no peanut butter"，它的值就是 there's no peanut butter。
代碼如下:
>> begin raise "this town ain't big enough for the both of us" rescue puts $! end
this town ain't big enough for the both of us
=> nil
$@ 可以給出完整的引起錯誤的棧調用信息，它是一個數組。
代碼如下:
>> begin raise 'no soup in kitchen' rescue $@.each { |trace| puts trace } end
(irb):13:in `irb_binding'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb/workspace.rb:80:in `eval'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb/workspace.rb:80:in `evaluate'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb/context.rb:254:in `evaluate'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:159:in `block (2 levels) in eval_input'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:273:in `signal_status'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:156:in `block in eval_input'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb/ruby-lex.rb:243:in `block (2 levels) in each_top_level_statement'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb/ruby-lex.rb:229:in `loop'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb/ruby-lex.rb:229:in `block in each_top_level_statement'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb/ruby-lex.rb:228:in `catch'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb/ruby-lex.rb:228:in `each_top_level_statement'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:155:in `eval_input'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:70:in `block in start'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:69:in `catch'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:69:in `start'
/home/meck/.rvm/rubies/ruby-1.9.3-p194/bin/>>'
=> ["(>>'"]

字符串和分隔符

$; 表示 String.split 里的分隔符，默認是空格。
代碼如下:
>> "One Spaceship, Two Tiny Tanks, Three Misplaced Socks".split
=> ["One Spaceship", " Two Tiny Tanks", " Three Misplaced Socks"]
>> $; = ","
=> ","
>> "One Spaceship, Two Tiny Tanks, Three Misplaced Socks".split
=> ["One Spaceship", " Two Tiny Tanks", " Three Misplaced Socks"]