Ruby: List of Predefined Global Variables

By Xah Lee. Date: 2013-01-02. Last updated: 2013-02-28.

`require "English"`	Variable	Meaning
◇	`$0`	name of the Ruby script file currently executing
`$ARGV`	`$*`	An array of strings containing the command-line options from the invocation of the program. Options used by the Ruby interpreter will have been removed. Read only. Also known simply as +ARGV+.
`$LOADED_FEATURES`	◇	◇
◇	`$:`	Load path for scripts and binary modules by load or require.

`require "English"`	Variable	Meaning
`$ERROR_INFO`	`$!`	The exception object passed to +raise+
`$ERROR_POSITION`	`$@`	The stack backtrace generated by the last exception. `See Kernel.caller` for details. Thread local.
`$FS` or `$FIELD_SEPARATOR`	`$;`	The default separator pattern used by `String.split`. May be set from the command line using the `-F` flag.
`$OFS` or `$OUTPUT_FIELD_SEPARATOR`	`$,`	The separator string output between the parameters to methods such as `Kernel.print` and `Array.join`. Defaults to +nil+, which adds no text.
`$RS` or `$INPUT_RECORD_SEPARATOR`	`$/`	The input record separator (newline by default). This is the value that routines such as `Kernel.gets` use to determine record boundaries. If set to +nil+, +gets+ will read the entire file.
`$ORS` or `$OUTPUT_RECORD_SEPARATOR`	`$\`	The string appended to the output of every call to methods such as `Kernel.print` and `IO.write`. The default value is +nil+.
`$NR` or `$INPUT_LINE_NUMBER`	`$.`	The number of the last line read from the current input file.
`$LAST_READ_LINE`	`$_`	The last line read by `Kernel.gets` or `Kernel.readline`. Many string-related functions in the +Kernel+ module operate on `$_` by default. The variable is local to the current scope. Thread local.
`$DEFAULT_OUTPUT`	`$>`	The destination of output for `Kernel.print` and `Kernel.printf`. The default value is `$stdout`.
`$DEFAULT_INPUT`	`$<`	An object that provides access to the concatenation of the contents of all the files given as command-line arguments, or `$stdin`(in the case where there are no arguments). `$<` supports methods similar to a +File+ object: +inmode+, +close+,`closed?`, +each+,`each_byte`, `each_line`, +eof+, `eof?`, +file+, +filename+, +fileno+, +getc+, +gets+, +lineno+,`lineno=`, +path+, +pos+, `pos=`, +read+, +readchar+, +readline+, +readlines+, +rewind+, +seek+, +skip+, +tell+, `to_a`, `to_i`,`to_io`, `to_s`, along with the methods in +Enumerable+. The method +file+ returns a +File+ object for the file currently being read. This may change as `$<` reads through the files on the command line. Read only.
`$PID` or `$PROCESS_ID`	`$$`	The process number of the program being executed. Read only.
`$CHILD_STATUS`	`$?`	The exit status of the last child process to terminate. Read only. Thread local.

Regex Match

`require "English"`	Variable	Meaning
`$IGNORECASE`	`$=`	If set to any value apart from +nil+ or +false+, all pattern matches will be case insensitive, string comparisons will ignore case, and string hash values will be case insensitive. Deprecated
`$LAST_MATCH_INFO`	`$~`	A +MatchData+ object that encapsulates the results of a successful pattern match. The variables `$&`, $`, `$'`, and `$1` to `$9` are all derived from `$~`. Assigning to `$~` changes the values of these derived variables. This variable is local to the current scope. Thread local.
`$MATCH`	`$&`	The string matched by the last successful pattern match. This variable is local to the current scope. Read only. Thread local.
`$PREMATCH`	$`	The string preceding the match in the last successful pattern match. This variable is local to the current scope. Read only. Thread local.
`$POSTMATCH`	`$'`	The string following the match in the last successful pattern match. This variable is local to the current scope. Read only. Thread local.
`$LAST_PAREN_MATCH`	`$+`	The contents of the highest-numbered group matched in the last successful pattern match. Thus, in `"cat" =~ /(c\|a)(t\|z)/`, `$+` will be set to "t". This variable is local to the current scope. Read only. Thread local.

misc

Where in the doc are those predefined vars?

in Ruby 2, it's documented at http://ruby-doc.org/core-2.0/doc/globals_rdoc.html .

for Ruby 1.9.3, it's not really documented. The closest is http://www.ruby-doc.org/stdlib-1.9.3/libdoc/English/rdoc/Object.html but the doc is empty.

You might read the file. Do print $: to locate the search path. Then, search for file English.rb.

If you read the file English.rb, you see the use of $LOADED_FEATURES, but that isn't defined in the file.
Also, the file doesn't contain $:, which is supposedly the lib search path.

Reference: /usr/lib/ruby/1.9.1/English.rb