Globalization

This is an overview of all the special, two-letter (and other) global variables in Ruby, which Ruby inherited from Perl.

For the purpose of improving code readability, Ruby comes with English.rb in its standard library (or Deutsch.rb as gem), which provides non-cryptic aliases and some documentation.

Ruby also defines some three-letter global variables that mirror CLI options ($-…)

Types of Special Global Variables

Idiosyncratically, not all special global variables are global:

Type Scope
global Real global variable
thread Thread-global variable: Can have different values in different threads
pseudo Looks like a global variable, but is a local variable
defunct Not working, anymore (removed from Ruby)
read-only Variable cannot be set to a new value
(read-only) Variable cannot be set to a new value, but is an array that is mutable

List of All Special Global Variables

The first table contains all two-letter variables and their aliases:

Perlish Type English Short Info
$! thread $ERROR_INFO - RDoc
$@ thread $ERROR_POSITION - RDoc
$; global $FIELD_SEPARATOR $FS, $-F¹ RDoc
$, global $OUTPUT_FIELD_SEPARATOR $OFS RDoc
$/ global $INPUT_RECORD_SEPARATOR $RS, $-0¹ RDoc
$\ global $OUTPUT_RECORD_SEPARATOR $ORS RDoc
$. global $INPUT_LINE_NUMBER $NR RDoc
$_ pseudo $LAST_READ_LINE - RDoc
$> global $DEFAULT_OUTPUT $stdout¹ IO in Ruby
$< global $DEFAULT_INPUT - (ARGF) RDoc
$$ read-only $PROCESS_ID $PID Unix Processes
$? thread / read-only $CHILD_STATUS - RDoc
$~ pseudo $LAST_MATCH_INFO - RDoc
$= defunct $IGNORECASE - -
$* (read-only) $ARGV - Explanation
$& pseudo / read-only $MATCH - RDoc
$` pseudo / read-only $PREMATCH - RDoc
$' pseudo / read-only $POSTMATCH - RDoc
$+ pseudo / read-only $LAST_PAREN_MATCH² - RDoc
$: (read-only) $LOAD_PATH¹ $-I¹ RHG: Loading
$" (read-only) $LOADED_FEATURES¹ - RHG: Loading
$0 global $PROGRAM_NAME¹ - RDoc
$1 - $9³ pseudo - - RDoc
$F⁴ global - - Auto-Splitting Lines

¹ Available without requiring English.rb
² No T, because it stands for PARENTHESES, not PARENT
³ Starting with Ruby 2.4, $1 - $9 only get defined when they are currently set
⁴ With command-line option -a

Other Special Global Variables

Name Type Info
$stdin global IO in Ruby
$stderr global IO in Ruby
$FILENAME read-only The current file being read via ARGF. Same as ARGF.filename.
$DEBUG, $-d global Global Debug States
$VERBOSE, $-v, $-w global Global Debug States
$-W global Global Debug States
$-p read-only Stream Editing
$-l read-only Specify Line Format
$-a read-only Auto-Splitting Lines
$-i global Inplace-Editing Files

More Idiosyncratic Ruby