Scope defines where in a program a variable is accessible. Ruby has four types of variable scope, local,global, instance and class. In addition, Ruby has one constant type. Each variable type is declared by using a special character at the start of the variable name as outlined in the following table.

Name Begins With Variable Scope
$ A global variable
@ An instance variable
[a-z] or _ A local variable
[A-Z] A constant
@@ A class variable

It is useful to know, however, that a number of pre-defined global variables are available to you as a Ruby developer to obtain information about the Ruby environment. A brief summary of each of these variables is contained in the following table.

Variable Name Variable Value
$@ The location of latest error
$_ The string last read by gets
$. The line number last read by interpreter
$& The string last matched by regexp
$~ The last regexp match, as an array of subexpressions
$n The nth subexpression in the last match (same as $~[n])
$= The case-insensitivity flag
$/ The input record separator
$\ The output record separator
$0 The name of the ruby script file currently executing
$* The command line arguments used to invoke the script
$$ The Ruby interpreter's process ID
$? The exit status of last executed child process