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ruby/doc/globals.md
Nobuyoshi Nakada c5c894c6e4
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2025-08-12 23:43:46 +09:00

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# 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:
```ruby
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. |
| `$2` | | Second group matched; set by matcher method. |
| <tt>$_n_</tt> | | <i>n</i>th 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`, `$:`, `$-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:
```ruby
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:
```ruby
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 <tt>$_n_</tt> the <i>n</i>th 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:
```ruby
$stdin # => #<IO:<STDIN>>
```
### `$stdout` (Standard Output)
The current standard output stream; initially:
```ruby
$stdout # => #<IO:<STDOUT>>
```
### `$stderr` (Standard Error)
The current standard error stream; initially:
```ruby
$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:
- <tt>[:rb, _path_]</tt>, where `path` is the path to the Ruby file to be
loaded for the given `feature`.
- <tt>[:so, _path_]</tt>, 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:
```ruby
$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:
```ruby
$LOADED_FEATURES.take(10)
# =>
["enumerator.so",
"thread.rb",
"fiber.so",
"rational.so",
"complex.so",
"ruby2_keywords.rb",
"/snap/ruby/317/lib/ruby/3.2.0/x86_64-linux/enc/encdb.so",
"/snap/ruby/317/lib/ruby/3.2.0/x86_64-linux/enc/trans/transdb.so",
"/snap/ruby/317/lib/ruby/3.2.0/x86_64-linux/rbconfig.rb",
"/snap/ruby/317/lib/ruby/3.2.0/rubygems/compatibility.rb"]
```
Aliased as `$"`.
## 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`):
```ruby
STDIN # => #<IO:<STDIN>>
```
### `STDOUT`
The standard output stream (the default value for `$stdout`):
```ruby
STDOUT # => #<IO:<STDOUT>>
```
### `STDERR`
The standard error stream (the default value for `$stderr`):
```ruby
STDERR # => #<IO:<STDERR>>
```
## Environment
### `ENV`
A hash of the contains current environment variables names and values:
```ruby
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:
```ruby
TOPLEVEL_BINDING # => #<Binding:0x00007f58da0da7c0>
```
### `RUBY_VERSION`
The Ruby version:
```ruby
RUBY_VERSION # => "3.2.2"
```
### `RUBY_RELEASE_DATE`
The release date string:
```ruby
RUBY_RELEASE_DATE # => "2023-03-30"
```
### `RUBY_PLATFORM`
The platform identifier:
```ruby
RUBY_PLATFORM # => "x86_64-linux"
```
### `RUBY_PATCHLEVEL`
The integer patch level for this Ruby:
```ruby
RUBY_PATCHLEVEL # => 53
```
For a development build the patch level will be -1.
### `RUBY_REVISION`
The git commit hash for this Ruby:
```ruby
RUBY_REVISION # => "e51014f9c05aa65cbf203442d37fef7c12390015"
```
### `RUBY_COPYRIGHT`
The copyright string:
```ruby
RUBY_COPYRIGHT
# => "ruby - Copyright (C) 1993-2023 Yukihiro Matsumoto"
```
### `RUBY_ENGINE`
The name of the Ruby implementation:
```ruby
RUBY_ENGINE # => "ruby"
```
### `RUBY_ENGINE_VERSION`
The version of the Ruby implementation:
```ruby
RUBY_ENGINE_VERSION # => "3.2.2"
```
### `RUBY_DESCRIPTION`
The description of the Ruby implementation:
```ruby
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:
```ruby
__END__
```
When defined, `DATA` is a File object
containing the lines following the `__END__`,
positioned at the first of those lines:
```ruby
p DATA
DATA.each_line { |line| p line }
__END__
Foo
Bar
Baz
```
Output:
```
#<File:t.rb>
"Foo\n"
"Bar\n"
"Baz\n"
```
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