ruby/doc/globals.md

Pre-Defined Global Variables

Some of the pre-defined global variables have synonyms that are available via module English. For each of those, the \English synonym is given.

To use the module:

require 'English'

Summary

Exceptions

Variable	English	Contains
$!	$ERROR_INFO	Exception object; set by Kernel#raise.
$@	$ERROR_POSITION	Array of backtrace positions; set by Kernel#raise.

Pattern Matching

Variable	English	Contains
$~	$LAST_MATCH_INFO	MatchData object; set by matcher method.
$&	$MATCH	Matched substring; set by matcher method.
$`	$PRE_MATCH	Substring left of match; set by matcher method.
$'	$POST_MATCH	Substring right of match; set by matcher method.
$+	$LAST_PAREN_MATCH	Last group matched; set by matcher method.
$1		First group matched; set by matcher method.
$2		Second group matched; set by matcher method.
$n		nth group matched; set by matcher method.

Separators

Variable	English	Contains
$/	$INPUT_RECORD_SEPARATOR	Input record separator; initially newline.
$\\\\	$OUTPUT_RECORD_SEPARATOR	Output record separator; initially nil.

Streams

Variable	English	Contains
$stdin		Standard input stream; initially STDIN.
$stdout		Standard input stream; initially STDIOUT.
$stderr		Standard input stream; initially STDERR.
$<	$DEFAULT_INPUT	Default standard input; ARGF or $stdin.
$>	$DEFAULT_OUTPUT	Default standard output; initially $stdout.
$.	$INPUT_LINE_NUMBER, $NR	Input position of most recently read stream.
$_	$LAST_READ_LINE	String from most recently read stream.

Processes

Variable	English	Contains
$0		Initially, the name of the executing program.
$*	$ARGV	Points to the ARGV array.
$$	$PROCESS_ID, $PID	Process ID of the current process.
$?	$CHILD_STATUS	Process::Status of most recently exited child process.
LOAD_PATH</tt>, <tt>:, $-I		Array of paths to be searched.
$LOADED_FEATURES, $"		Array of paths to loaded files.

Debugging

Variable	English	Contains
$FILENAME		The value returned by method ARGF.filename.
$DEBUG		Initially, whether option -d or --debug was given.
$VERBOSE		Initially, whether option -V or -W was given.

Other Variables

Variable	English	Contains
$-a		Whether option -a was given.
$-i		Extension given with command-line option -i.
$-l		Whether option -l was given.
$-p		Whether option -p was given.

Exceptions

$! (\Exception)

Contains the Exception object set by Kernel#raise:

begin
  raise RuntimeError.new('Boo!')
rescue RuntimeError
  p $!
end

Output:

#<RuntimeError: Boo!>

English - $ERROR_INFO

$@ (Backtrace)

Same as $!.backtrace; returns an array of backtrace positions:

begin
  raise RuntimeError.new('Boo!')
rescue RuntimeError
  pp $@.take(4)
end

Output:

["(irb):338:in `<top (required)>'",
 "/snap/ruby/317/lib/ruby/3.2.0/irb/workspace.rb:119:in `eval'",
 "/snap/ruby/317/lib/ruby/3.2.0/irb/workspace.rb:119:in `evaluate'",
 "/snap/ruby/317/lib/ruby/3.2.0/irb/context.rb:502:in `evaluate'"]

English - $ERROR_POSITION.

Pattern Matching

These global variables store information about the most recent successful match in the current scope.

For details and examples, see {Regexp Global Variables}[rdoc-ref:Regexp@Global+Variables].

$~ (\MatchData)

MatchData object created from the match; thread-local and frame-local.

English - $LAST_MATCH_INFO.

$& (Matched Substring)

The matched string.

English - $MATCH.

$` (Pre-Match Substring)

The string to the left of the match.

English - $PREMATCH.

$' (Post-Match Substring)

The string to the right of the match.

English - $POSTMATCH.

$+ (Last Matched Group)

The last group matched.

English - $LAST_PAREN_MATCH.

$1, $2, \Etc. (Matched Group)

For $_n_ the nth group of the match.

No \English.

Separators

$/ (Input Record Separator)

An input record separator, initially newline.

English - $INPUT_RECORD_SEPARATOR, $RS.

Aliased as $-0.

$\\ (Output Record Separator)

An output record separator, initially +nil+.

English - $OUTPUT_RECORD_SEPARATOR, $ORS.

Streams

$stdin (Standard Input)

The current standard input stream; initially:

$stdin # => #<IO:<STDIN>>

$stdout (Standard Output)

The current standard output stream; initially:

$stdout # => #<IO:<STDOUT>>

$stderr (Standard Error)

The current standard error stream; initially:

$stderr # => #<IO:<STDERR>>

$< (\ARGF or $stdin)

Points to stream ARGF if not empty, else to stream $stdin; read-only.

English - $DEFAULT_INPUT.

$> (Default Standard Output)

An output stream, initially $stdout.

English - $DEFAULT_OUTPUT

$. (Input Position)

The input position (line number) in the most recently read stream.

English - $INPUT_LINE_NUMBER, $NR

$_ (Last Read Line)

The line (string) from the most recently read stream.

English - $LAST_READ_LINE.

Processes

$0

Initially, contains the name of the script being executed; may be reassigned.

$* (\ARGV)

Points to ARGV.

English - $ARGV.

$$ (Process ID)

The process ID of the current process. Same as Process.pid.

English - $PROCESS_ID, $PID.

$? (Child Status)

Initially +nil+, otherwise the Process::Status object created for the most-recently exited child process; thread-local.

English - $CHILD_STATUS.

$LOAD_PATH (Load Path)

Contains the array of paths to be searched by Kernel#load and Kernel#require.

Singleton method $LOAD_PATH.resolve_feature_path(feature) returns:

• [:rb, _path_], where +path+ is the path to the Ruby file to be loaded for the given +feature+.
• [:so+ _path_], where +path+ is the path to the shared object file to be loaded for the given +feature+.
• +nil+ if there is no such +feature+ and +path+.

Examples:

$LOAD_PATH.resolve_feature_path('timeout')
# => [:rb, "/snap/ruby/317/lib/ruby/3.2.0/timeout.rb"]
$LOAD_PATH.resolve_feature_path('date_core')
# => [:so, "/snap/ruby/317/lib/ruby/3.2.0/x86_64-linux/date_core.so"]
$LOAD_PATH.resolve_feature_path('foo')
# => nil

Aliased as $: and $-I.

$LOADED_FEATURES

Contains an array of the paths to the loaded files:

$LOADED_FEATURES.take(10)
# =>
["enumerator.so",
 "thread.rb",
 "fiber.so",
 "rational.so",
 "complex.so",
 "ruby2_keywords.rb",
 "/snap/ruby/317/lib/ruby/3.2.0/x86_64-linux/enc/encdb.so",
 "/snap/ruby/317/lib/ruby/3.2.0/x86_64-linux/enc/trans/transdb.so",
 "/snap/ruby/317/lib/ruby/3.2.0/x86_64-linux/rbconfig.rb",
 "/snap/ruby/317/lib/ruby/3.2.0/rubygems/compatibility.rb"]

Aliased as $".

Debugging

$FILENAME

The value returned by method ARGF.filename.

$DEBUG

Initially +true+ if command-line option -d or --debug is given, otherwise initially +false+; may be set to either value in the running program.

When +true+, prints each raised exception to $stderr.

Aliased as $-d.

$VERBOSE

Initially +true+ if command-line option -v or -w is given, otherwise initially +false+; may be set to either value, or to +nil+, in the running program.

When +true+, enables Ruby warnings.

When +nil+, disables warnings, including those from Kernel#warn.

Aliased as $-v and $-w.

Other Variables

$-a

Whether command-line option -a was given; read-only.

$-i

Contains the extension given with command-line option -i, or +nil+ if none.

An alias of ARGF.inplace_mode.

$-l

Whether command-line option -l was set; read-only.

$-p

Whether command-line option -p was given; read-only.

Deprecated

$=

$,

$;

Pre-Defined Global Constants

Summary

Streams

Constant	Contains
STDIN	Standard input stream.
STDOUT	Standard output stream.
STDERR	Standard error stream.

Environment

Constant	Contains
ENV	Hash of current environment variable names and values.
ARGF	String concatenation of files given on the command line, or $stdin if none.
ARGV	Array of the given command-line arguments.
TOPLEVEL_BINDING	Binding of the top level scope.
RUBY_VERSION	String Ruby version.
RUBY_RELEASE_DATE	String Ruby release date.
RUBY_PLATFORM	String Ruby platform.
RUBY_PATCH_LEVEL	String Ruby patch level.
RUBY_REVISION	String Ruby revision.
RUBY_COPYRIGHT	String Ruby copyright.
RUBY_ENGINE	String Ruby engine.
RUBY_ENGINE_VERSION	String Ruby engine version.
RUBY_DESCRIPTION	String Ruby description.

Embedded Data

Constant	Contains
DATA	File containing embedded data (lines following END, if any).

Streams

STDIN

The standard input stream (the default value for $stdin):

STDIN # => #<IO:<STDIN>>

STDOUT

The standard output stream (the default value for $stdout):

STDOUT # => #<IO:<STDOUT>>

STDERR

The standard error stream (the default value for $stderr):

STDERR # => #<IO:<STDERR>>

Environment

ENV

A hash of the contains current environment variables names and values:

ENV.take(5)
# =>
[["COLORTERM", "truecolor"],
 ["DBUS_SESSION_BUS_ADDRESS", "unix:path=/run/user/1000/bus"],
 ["DESKTOP_SESSION", "ubuntu"],
 ["DISPLAY", ":0"],
 ["GDMSESSION", "ubuntu"]]

ARGF

The virtual concatenation of the files given on the command line, or from $stdin if no files were given, "-" is given, or after all files have been read.

ARGV

An array of the given command-line arguments.

TOPLEVEL_BINDING

The Binding of the top level scope:

TOPLEVEL_BINDING # => #<Binding:0x00007f58da0da7c0>

RUBY_VERSION

The Ruby version:

RUBY_VERSION # => "3.2.2"

RUBY_RELEASE_DATE

The release date string:

RUBY_RELEASE_DATE # => "2023-03-30"

RUBY_PLATFORM

The platform identifier:

RUBY_PLATFORM # => "x86_64-linux"

RUBY_PATCHLEVEL

The integer patch level for this Ruby:

RUBY_PATCHLEVEL # => 53

For a development build the patch level will be -1.

RUBY_REVISION

The git commit hash for this Ruby:

RUBY_REVISION # => "e51014f9c05aa65cbf203442d37fef7c12390015"

RUBY_COPYRIGHT

The copyright string:

RUBY_COPYRIGHT
# => "ruby - Copyright (C) 1993-2023 Yukihiro Matsumoto"

RUBY_ENGINE

The name of the Ruby implementation:

RUBY_ENGINE # => "ruby"

RUBY_ENGINE_VERSION

The version of the Ruby implementation:

RUBY_ENGINE_VERSION # => "3.2.2"

RUBY_DESCRIPTION

The description of the Ruby implementation:

RUBY_DESCRIPTION
# => "ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-linux]"

Embedded \Data

DATA

Defined if and only if the program has this line:

__END__

When defined, DATA is a File object containing the lines following the __END__, positioned at the first of those lines:

p DATA
DATA.each_line { |line| p line }
__END__
Foo
Bar
Baz

Output:

#<File:t.rb>
"Foo\n"
"Bar\n"
"Baz\n"