Pre-Defined Global Variables

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

To use the module:

require 'English'

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.

`$~` (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.

`$;` (Input Field Separator)

An input field separator, initially nil.

English - $FIELD_SEPARATOR, $FS.

Aliased as $-F.

`$\` (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 - <tt>$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

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__

Output:

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

Pre-Defined Global Variables

Exceptions

$! (Exception)

$@ (Backtrace)

Pattern Matching

$~ (MatchData)

$& (Matched Substring)

$` (Pre-Match Substring)

$' (Post-Match Substring)

$+ (Last Matched Group)

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

Separators

$/ (Input Record Separator)

$; (Input Field Separator)

$\ (Output Record Separator)

Streams

$stdin (Standard Input)

$stdout (Standard Output)

$stderr (Standard Error)

$< (ARGF or $stdin)

$> (Default Standard Output)

$. (Input Position)

$_ (Last Read Line)

Processes

$0

$* (ARGV)

$$ (Process ID)

$? (Child Status)

$LOAD_PATH (Load Path)

$LOADED_FEATURES

Debugging

$FILENAME

$DEBUG

$VERBOSE

Other Variables

$-a

$-i

$-l

$-p

Deprecated

$=

$,

Pre-Defined Global Constants

Streams

STDIN

STDOUT

STDERR

Environment

ENV

ARGF

ARGV

TOPLEVEL_BINDING

RUBY_VERSION

RUBY_RELEASE_DATE

RUBY_PLATFORM

RUBY_PATCHLEVEL

RUBY_REVISION

RUBY_COPYRIGHT

RUBY_ENGINE

RUBY_ENGINE_VERSION

RUBY_DESCRIPTION

Embedded Data

DATA

`$!` (Exception)

`$@` (Backtrace)

`$~` (MatchData)

`$&` (Matched Substring)

`$'` (Post-Match Substring)

`$+` (Last Matched Group)

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

`$/` (Input Record Separator)

`$;` (Input Field Separator)

`$\` (Output Record Separator)

`$stdin` (Standard Input)

`$stdout` (Standard Output)

`$stderr` (Standard Error)

`$<` (ARGF or $stdin)

`$>` (Default Standard Output)

`$.` (Input Position)

`$_` (Last Read Line)

`$0`

`$*` (ARGV)

`$$` (Process ID)

`$?` (Child Status)

`$LOAD_PATH` (Load Path)

`$LOADED_FEATURES`

`$FILENAME`

`$DEBUG`

`$VERBOSE`

`$-a`

`$-i`

`$-l`

`$-p`

`$=`

`$,`

`STDIN`

`STDOUT`

`STDERR`

`ARGV`

`TOPLEVEL_BINDING`

`RUBY_VERSION`

`RUBY_RELEASE_DATE`

`RUBY_PLATFORM`

`RUBY_PATCHLEVEL`

`RUBY_REVISION`

`RUBY_COPYRIGHT`

`RUBY_ENGINE`

`RUBY_ENGINE_VERSION`

`RUBY_DESCRIPTION`

`DATA`