Ruby Predefined Globals · YokerHood

Below is the summary table for the predefined globals in ruby.

$!	$+	$-W	$-v	$2	$8	$>
$"	$,	$-a	$-w	$3	$9	$?
$$	$-0	$-d	$.	$4	$:	$@
$&	$-F	$-i	$/	$5	$;	$\
$'	$-I	$-l	$0	$6	$<	$_
$*	$-K	$-p	$1	$7	$~	$`

Variable	Description
`$!` Exception === $! unless $!.nil?	read-only thread-local *The last exception, that was thrown and rescued* in the current context.** Locals to the `rescue` clause. YOKERHOODCODEPH0ENDPH
`$"` Array === $"	read-only Array of strings, containing absolute paths to loaded files. Files, loaded within both `Kernel’s` `load`, `require`, `require_relative` directives *and* platform-specific DL/Fiddle, are shown in the list. YOKERHOODCODEPH1ENDPH
`$$` Fixnum === $$	read-only Current process ID. YOKERHOODCODEPH2ENDPH
`$&` String === $& unless $&.nil?	read-only thread-local The matched string from the previous successful pattern match. Locals to the pattern match scope. YOKERHOODCODEPH3ENDPH
`$'` String === $' unless $'.nil?	read-only thread-local *The rest of the string after the matched* substring in the previous successful pattern match.** Locals to the pattern match scope. YOKERHOODCODEPH4ENDPH
`$` Array === $	read-only The command line arguments passed to the currently executed ruby script. An alias for `ARGV`. YOKERHOODCODEPH5ENDPH
`$+` String === $+ unless $+.nil?	read-only thread-local *The last captured* match from the previous successful pattern match.** Locals to the pattern match scope. Contains `nil` if there was no capture (even while match was successful.) YOKERHOODCODEPH6ENDPH
`$,` String === $, unless $,.nil?	read-write Separator for both `Kernel#print` and `Array#join`. Defaults to nil. YOKERHOODCODEPH7ENDPH
`$-0` `$/` String === $-0 unless $-0.nil? String === $/ unless $/.nil?	read-write Input record separator. Defaults to `\n`. May be set with the `-0` command line parameter (as octal value.) YOKERHOODCODEPH8ENDPH
`$-F` `$;` String === $-F unless $-F.nil? String === $: unless $:.nil?	read-write Default field separator for `String#split`. Defaults to `nil`. May be set with the `-F` command line parameter. YOKERHOODCODEPH9ENDPH
`$-I` `$:` Array === $-I Array === $:	read-only Array of include paths. Absolute paths to be searched by `Kernel.load` and/or `Kernel.require`. YOKERHOODCODEPH10ENDPH
`$-K` String === $-K unless $-K.nil?	obsolete read-write Determines the encoding to be used whilst parsing `.rb` files. One should avoid using that variable since `ruby-1.9`. YOKERHOODCODEPH11ENDPH
`$-W` Fixnum === $-W	read-only Current verbosity level. Defaults to `1`. May be set to `0` with the `-W0` command line arg and to `2` with one of `-w`, `-v`, or `--verbose` switches. YOKERHOODCODEPH12ENDPH
`$-a` ∈ [true, false]	read-only Denotes whether the auto-split mode was enabled with `-a` command line argument. `-a` switch makes sense when used together with either `-p` or `-n` args. In auto-split mode, Ruby executes `$F = $_.split` at the beginning of each loop. YOKERHOODCODEPH13ENDPH
`$-d` ∈ [true, false]	read-only Denotes whether the debug mode was enabled with `-d` switch. YOKERHOODCODEPH14ENDPH
`$-i` ∈ [true, false]	read-only Value of the `-i` command line argument, if given. Defaults to `nil`. When provided, this argument enables inplace-edit mode; the parameter specifies an extension of backup file to be created. YOKERHOODCODEPH15ENDPH
`$-l` ∈ [true, false]	read-only Denotes whether the automatic line-ending processing mode was enabled with `-l` switch. Automatic line-endings sets `$\` to the value of `$/`, and (when used with either `-p` or `-n` args) chops every line read using `chop!`. YOKERHOODCODEPH16ENDPH
`$-p` ∈ [true, false]	read-only Denotes whether the “gets loop around” mode was enabled with `-p` switch. `-p` switch acts mostly like `-n` sibling with one exception: it prints the value of variable `$_` at the each end of the loop. For some unknown reason there is no internal global to get aknowledged whether the `-n` command line switch was given. In case it were, Ruby is to assume the following loop around your script, which provides an iteration over filename arguments somewhat like `sed -n` and `awk` do. YOKERHOODCODEPH17ENDPH An example of usage follows: YOKERHOODCODEPH18ENDPH
`$-v` `$-w` ∈ [true, false]	read-only Denotes whether the verbose mode was enabled with either `-v` or `-w` switch. YOKERHOODCODEPH19ENDPH
`$.` Fixnum === $.	read-only Number of the last line read from the current input file `ARGF`. ARGF is a stream designed to use in scripts that process files given as command-line arguments or passed in via `STDIN`. YOKERHOODCODEPH20ENDPH
`$0…$9` String === $0 unless $0.nil? String === $1 unless $1.nil? String === $2 unless $2.nil? String === $3 unless $3.nil? String === $4 unless $4.nil? String === $5 unless $5.nil? String === $6 unless $6.nil? String === $7 unless $7.nil? String === $8 unless $8.nil? String === $9 unless $9.nil?	read-only The `N-th` capture of the previous successful pattern match. Defaults to `nil` if the match failed or if `N` is greater than an amount of captured groups. YOKERHOODCODEPH21ENDPH
`$<` ARGF === $<	read-only Read‐only alias for ARGF. YOKERHOODCODEPH22ENDPH
`$=` ∈ [true, false]	obsolete YOKERHOODCODEPH23ENDPH
`$>` IO === $>	read-write Standard output stream. YOKERHOODCODEPH24ENDPH
`$?` Process::Status === $? unless $?.nil?	read-only Exit status of the last terminated process within current context. YOKERHOODCODEPH25ENDPH
`$@` Array === $@ unless $@.nil?	read-only thread-local Shorthand to `$!.backtrace` Locals to the `rescue` clause. YOKERHOODCODEPH26ENDPH
`$\` String === $\ unless $\.nil?	read-write Appended to `Kernel.print` output. Defaults to `nil`, or to `$/` if the `-l` switch was given. YOKERHOODCODEPH27ENDPH
`$_` String === $_ unless $_.nil?	read-only thread-local Last `String` read from `IO` by one of `Kernel.gets`, `Kernel.readline` or siblings. Widely used with `-p` and `-n` switches. YOKERHOODCODEPH28ENDPH
$` String === $YOKERHOODCODEPH29ENDPH.nil?	read-only thread-local *The rest of the string before* the matched substring in the previous successful pattern match.** Locals to the pattern match scope. YOKERHOODCODEPH30ENDPH
`$~` MatchData === $~ unless $~.nil?	read-only thread-local The `MatchData` from the previous successful pattern match. YOKERHOODCODEPH31ENDPH

Credits to Jim Neath and runpaint , some examples came from these posts.