Ruby 中充满了一系列的隐藏变量，我们可以从这些预定义的全局变量中获取一些有意思的信息。
全局进程变量
$$ 表示当前运行的 ruby 进程。
>> $$
=> 17170
我们可以从当前进程杀死它自己
>> `kill -9 #{$$}`
[1] 17170 killed irb
$? 表示最近一个子进程的状态
>> `echo hello`
=> "hello\n"
>> $?
=> #<Process::Status: pid 18048 exit 0>
>> $?.success?
=> true
异常和错误
$1 表示引起异常的信息。比如在这里 raise "there's no peanut butter"，它的值就是 there's no peanut butter。
>> 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
$@ 可以给出完整的引起错误的栈调用信息，它是一个数组。
>> 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，默认是空格。
>> "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"]
$, 用在 Array.join 和 Kernel.print 里，默认是 nil。
>> ['one', 'two', 'three', 'green'].join
=> "onetwothreegreen"
>> $, = "-"
=> "-"
>> ['one', 'two', 'three', 'green'].join
=> "one-two-three-green"
$/ 表述读取输入的行分隔符。它被用在 Kernel.gets 里。它通常表示新行，但可以被修改。这个很难展示，因为 irb 依赖 \n 作为读取分隔符，如果把 $/ 设置成 nil，gets 就会读取整个文件。
$\ 正好相反，它是作为输出的行分隔符。
>> $\ = "mooooooo "
=> "mooooooo "
>> puts a
NameError-: -undefined local variable or method `a' for main:Object-
mooooooo from (irb):25
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/bin/>>'-
mooooooo >> a
文件
假设有个叫letter.text的文件：
Dear Caroline,
I think we need some honey for tea.
I also think that I may have misplaced my red tie, have you seen it?
-Nick
$. 表示文件当前被读取的行号。
>> open('letter.txt').each { |line| puts "#{$.}: #{line}" }
1: Dear Caroline,
2: I think we need some honey for tea.
3: I also think that I may have misplaced my red tie, have you seen it?
4:
5: -Nick
=> #<File:letter.txt>
$_ 表示最后读取的行。
>> open('letter.txt').each { |line| puts $_.nil? }
true
true
true
true
true
=> #<File:letter.txt>
匹配和正则表达式
$~ 表示最近一次正则匹配到的信息，如果有的话它就返回 MatchData 的示例，否则就是 nil。
>> "the robots are coming, the robots are coming, the robots are coming" =~ /ro/
=> 4
>> $~
=> #<MatchData "ro">
>> $~.to_s
=> "ro"
>> "the robots are coming, the robots are coming, the robots are coming" =~ /cats/
=> nil
>> $~
$& 跟 $~ 非常相似，它返回最近一次匹配到的字符串。
>> "the robots are coming, the robots are coming, the robots are coming" =~ /ro/
=> 4
>> $&
$' 表示匹配不分后面的字符串。
>> "There were once ten tin robots standing in a row." =~ /robot/
=> 24
>> $'
=> "s standing in a row."
=> "ro"
=> nil
其他
$> 表示ruby 默认的输出对象，用在 Kernel.print 里。
>> $> = $> = $stderr
=> #<IO:<STDERR>>
>> puts 'no no no'
no no no
=> nil
>> $> = $stdin
/home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:168:in `write': not opened for writing (IOError)
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:168:in `print'
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:168:in `block (2 levels) in eval_input'
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:273:in `signal_status'
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:156:in `block in eval_input'
from /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'
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb/ruby-lex.rb:229:in `loop'
from /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'
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb/ruby-lex.rb:228:in `catch'
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb/ruby-lex.rb:228:in `each_top_level_statement'
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:155:in `eval_input'
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:70:in `block in start'
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:69:in `catch'
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/irb.rb:69:in `start'
from /home/meck/.rvm/rubies/ruby-1.9.3-p194/bin/irb:12:in `<main>'
$* 可能是最常用全局变量，它表示包含传给 ruby 文件的所有变量的数组，假设有个叫 argument_echoer.rb 文件：
$*.each { |arg| puts arg }
运行它：
$ ruby argument_echoer.rb Who What When Where and Why
Who
What
When
Where
and
Why
