Ruby 中$開頭的全局變數、內部變數、隱藏變數介紹

ruby

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

全局進程變數

$$ 表示當前運行的 ruby 進程。

>> $$=> 17170

我們可以從當前進程殺死它自己

>> `kill -9 #{$$}`[1]    17170 killed     irb

$? 表示最近一個子進程的狀態

>> `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。比如在這裡 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 $! endthis 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。

>> 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/>>'=> ["(>>'"]

字元串和分隔符

$; 表示 String.split 里的分隔符，默認是空格。

>> "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，默認是 nil。>> ['one', 'two', 'three', 'green'].join=> "onetwothreegreen">> $, = "-"=> "-">> ['one', 'two', 'three', 'green'].join=> "one-two-three-green"

$/ 表述讀取輸入的行分隔符。它被用在 Kernel.gets 里。它通常表示新行，但可以被修改。這個很難展示，因為 irb 依賴 \n 作為讀取分隔符，如果把 $/ 設置成 nil，gets 就會讀取整個文件。

$\ 正好相反，它是作為輸出的行分隔符。

>> $\ = "mooooooo "=> "mooooooo ">> puts aNameError-: -undefined local variable or method `a' for main:Object-mooooooo        from (irb):25from /home/meck/.rvm/rubies/ruby-1.9.3-p194/bin/>>'-mooooooo >> a

文件

假設有個叫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

$. 表示文件當前被讀取的行號。

>> 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>

$_ 表示最後讀取的行。

>> open('letter.txt').each { |line| puts $_.nil? }truetruetruetruetrue=> #<File:letter.txt>

匹配和正則表達式

$~ 表示最近一次正則匹配到的信息，如果有的話它就返回 MatchData 的示例，否則就是 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元串。

>> "the robots are coming, the robots are coming, the robots are coming" =~ /ro/=> 4>> $&

$' 表示匹配不分後面的字元串。

>> "There were once ten tin robots standing in a row." =~ /robot/=> 24>> $'=> "s standing in a row."=> "ro"=> nil

其他

$> 表示ruby 默認的輸出對象，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>'

$* 可能是最常用全局變數，它表示包含傳給 ruby 文件的所有變數的數組，假設有個叫 argument_echoer.rb 文件：

$*.each { |arg| puts arg }

運行它：

$ ruby argument_echoer.rb Who What When Where and WhyWhoWhatWhenWhereandWhy