global_variables

list of predefined variables

        $!         The exception information message set by 'raise'.
        $@         Array of 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.
        $/         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 sans args.
        $$         The process number of the Ruby running this script.
        $?         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.
        $DEBUG     The status of the -d switch.
        $FILENAME  Current input file from $<. Same as $<.filename.
        $LOAD_PATH The alias to the $:.
        $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 $:.
        $-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.

Source: http://en.wikibooks.org/wiki/Ruby_Programming/Syntax/Variables_and_Constants#Pre-defined_Variables