Ruby on Rails(RoR) 3 Variables

Variable of Ruby on Rails is as follows

• local variables
• constants 
• instance variable(object variables) .
• class variables
• global variables

Local Variables

• A variable whose name begins with a lowercase letter (a-z) or underscore ( _ ) .

Example

distance

Instance Variables

• A variable whose name begins with '@' is an instance variable of self.
• Uninitialized instance variables have a value of nil

Example

@distance

Class Variables

• A variable whose name begins with '@@' 
• A class variable is shared by all instances of a class
• class variable is shared by all the descendants of the class

Example

@@distance

Global Variables

• A variable whose name begins with '$'
• can be accessed from anywhere within the program during runtime

Example

$distance

Constants

• A variable whose name begins with an uppercase letter (A-Z)
• A constant can be reassigned a value after its initialization, but doing so will generate a warning
• Every class is a constant
• Trying to access an uninitialized constant raises the NameError exception.

Example

DISTANCE

Pseudo Variables

1. self
2. nil
3. true
4. false

Pre-defined Variables

$!	The exception information message set by the last 'raise' (last exception thrown).
$@	Array of the backtrace of the last exception thrown.
$&	The string matched by the last successful pattern match in this scope.
$`	The string to the left of the last successful match.
$'	The string to the right of the last successful match.
$+	The last bracket matched by the last successful match.
$1 to $9	The Nth group of the last successful regexp match.
$~	The information about the last match in the current scope.
$=	The flag for case insensitive, nil by default (deprecated).
$/	The input record separator, newline by default.
$\	The output record separator for the print and IO#write. Default is nil.
$,	The output field separator for the print and Array#join.
$;	The default separator for String#split.
$.	The current input line number of the last file that was read.
$<	The virtual concatenation file of the files given on command line.
$>	The default output for print, printf. $stdout by default.
$_	The last input line of string by gets or readline.
$0	Contains the name of the script being executed. May be assignable.
$*	Command line arguments given for the script. Same as ARGV.
$$	The process number of the Ruby running this script. Same as Process.pid.
$?	The status of the last executed child process.
$:	Load path for scripts and binary modules by load or require.
$"	The array contains the module names loaded by require.
$LOADED_FEATURES	An english friendlier alias to $"
$DEBUG	The status of the -d switch. Assignable.
$FILENAME	Current input file from $<. Same as $<.filename.
$KCODE	Character encoding of the source code.
$LOAD_PATH	An alias to $:
$stderr	The current standard error output.
$stdin	The current standard input.
$stdout	The current standard output.
$VERBOSE	The verbose flag, which is set by the -v switch.
$-0	The alias to $/
$-a	True if option -a ("autosplit" mode) is set. Read-only variable.
$-d	The alias to $DEBUG.
$-F	The alias to $;
$-i	If in-place-edit mode is set, this variable holds the extension, otherwise nil.
$-I	The alias to $:
$-K	The alias to $KCODE.
$-l	True if option -l is set ("line-ending processing" is on). Read-only variable.
$-p	True if option -p is set ("loop" mode is on). Read-only variable.
$-v	The alias to $VERBOSE.
$-w	True if option -w is set.

default the values of the pre-defined variables will be nil
Pre-defined constants

__FILE__ (current file)
__LINE__ (current line)