Tinkering in irb earlier, I ran this line of code.
global_variables.each{ |var| puts var + ": " + eval(var).inspect }
Ruby’s global variables - I have long been aware of their existence in the background, but I hadn’t realised quite how many are initialized by default. What are they all for - and what cool metaprogramming tricks can I use them for?
I actually use one of these variables very often, and it deserves a special mention. $! points to the most recently raised exception, and is a useful shortcut in many contexts. A lot of the others promise to be most useful in a shell/CGI script environment - but it’s good to know they’re there, and a little peek under the hood of Ruby usually proves instructional.
Here’s the global variables initialized by default on my system (Ruby 1.8.6 on OSX 10.5) - where I have some idea what they refer to, I’ve added that too.
| Variable name | Value (on my machine) | Purpose? |
|---|---|---|
| $0 | “irb” | Name of current script executing |
| $” | [”e2mmap.rb”, “irb/init.rb”, “irb/workspace.rb”, “irb/context.rb”, “irb/extend-command.rb”, “irb/output-method.rb”, “irb/notifier.rb”, “irb/slex.rb”, “irb/ruby-token.rb”, “irb/ruby-lex.rb”, “readline.bundle”, “irb/input-method.rb”, “irb/locale.rb”, “irb.rb”] | Alias for $LOADED_FEATURES |
| $$ | 6100 | Ruby interpreter process ID |
| $< | ARGF | Points to ARGF (user input) |
| $_ | nil | Last string read by ‘gets’ |
| $-K | “NONE” | $KCODE option passed via command line |
| $DEBUG | false | Can be set to true with ruby –debug myscript.rb. Just for program use I think? |
| $-i | nil | See ruby command line options: in-place-edit mode |
| $deferr | # | Usually points to $stderr |
| $/ | “\n” | Input record separator |
| $’ | nil | Text after regexp match |
| $stdout | # | Standard output (eg, the console usually) |
| $-l | false | See ruby command line options: automatic line-ending processing |
| $LOAD_PATH | [”/usr/local/lib/ruby/site_ruby/1.8″, “/usr/local/lib/ruby/site_ruby/1.8/i686-darwin9.0.0″, “/usr/local/lib/ruby/site_ruby”, “/usr/local/lib/ruby/1.8″, “/usr/local/lib/ruby/1.8/i686-darwin9.0.0″, “.”] | |
| $. | 184 | Last line of script read by interpreter |
| $KCODE | “NONE” | KanjiCODE - determines how Ruby handles characters |
| $-w | false | See Ruby intepreter command line options - sets $VERBOSE to true |
| $FILENAME | “-” | ? |
| $defout | # | Usually points to $stdout |
| $, | nil | ? |
| $` | nil | Text before regexp match |
| $* | [] | Command line arguments passed to current script |
| $stdin | # | Standard user input (usually, console) |
| $-F | nil | Field separator passed via command line |
| $-p | false | Used for command line file processing/looping (See also: -n) |
| $-I | [”/usr/local/lib/ruby/site_ruby/1.8″, “/usr/local/lib/ruby/site_ruby/1.8/i686-darwin9.0.0″, “/usr/local/lib/ruby/site_ruby”, “/usr/local/lib/ruby/1.8″, “/usr/local/lib/ruby/1.8/i686-darwin9.0.0″, “.”] | Library script path (passed via command line option -I) |
| $\ | nil | Output record separator |
| $= | false | Was last regular expression match case-sensitive? |
| $binding | nil | ? |
| $-v | false | Alias for –verbose command line option |
| $> | # | Points to standard output |
| $& | nil | Last matched regular expression (also $1, $2, etc) |
| $! | nil | Last raised exception |
| $PROGRAM_NAME | “irb” | Alias for $0 |
| $LOADED_FEATURES | [”e2mmap.rb”, “irb/init.rb”, “irb/workspace.rb”, “irb/context.rb”, “irb/extend-command.rb”, “irb/output-method.rb”, “irb/notifier.rb”, “irb/slex.rb”, “irb/ruby-token.rb”, “irb/ruby-lex.rb”, “readline.bundle”, “irb/input-method.rb”, “irb/locale.rb”, “irb.rb”] | Alias for $” |
| $? | nil | Status of last executed child process (useful with Kernel#system and similar) |
| $; | nil | Input field separator |
| $SAFE | 0 | Ruby safe mode |
| $-d | false | Command line option –debug |
| $: | [”/usr/local/lib/ruby/site_ruby/1.8″, “/usr/local/lib/ruby/site_ruby/1.8/i686-darwin9.0.0″, “/usr/local/lib/ruby/site_ruby”, “/usr/local/lib/ruby/1.8″, “/usr/local/lib/ruby/1.8/i686-darwin9.0.0″, “.”] | Alias for library path(?) |
| $-0 | “\n” | Separator character(?) |
| $+ | nil | ? |
| $-a | false | Ruby command line options: auto-split mode |
| $VERBOSE | false | Ruby command line options: –verbose (some methods print extra information if this is on) |
| $stderr | # | Standard error destination |
| $~ | nil | Last matched regular expression data (instance of MatchData) |
| $@ | nil | Position of an error occurence |
References
http://www.ruby-doc.org/docs/UsersGuide/rg/globalvars.html
http://cbcg.net/talks/rubyidioms/index.html
http://www.math.hokudai.ac.jp/~gotoken/ruby/ruby-uguide/uguide20.html
Hal Fulton: The Ruby Way (2nd Edition), Addison Wesley
3 Comments
Nice writeup.
$` and $’ are the text before regexp match and text after regexp match, respectively.
Cheers, updated.
My version of the Pickaxe shows $FILENAME is the name of the current input file, $, is the separator string output between parameters such as print and join. $+ is for pattern matching, and returns the contents of the highest-numbered group matched following a successful pattern match (last match in a group of successful matches I guess?). Dunno about $binding though….