Module: Kernel

Deprecated. Use block_given? instead.

Returns the canonicalized absolute path of the directory of the file from which this method is called. It means symlinks in the path is resolved. If __FILE__ is nil, it returns nil. The return value equals to File.dirname(File.realpath(__FILE__)).

Returns the name at the definition of the current method as a ::Symbol. If called outside of a method, it returns nil.

Terminates execution immediately, effectively by calling Kernel.exit(false).

If string argument msg is given, it is written to STDERR prior to termination; otherwise, if an exception was raised, prints its message and backtrace.

Returns an array converted from object.

Tries to convert object to an array using to_ary first and to_a second:

Array([0, 1, 2])        # => [0, 1, 2]
Array({foo: 0, bar: 1}) # => [[:foo, 0], [:bar, 1]]
Array(0..4)             # => [0, 1, 2, 3, 4]

Returns object in an array, [object], if object cannot be converted:

Array(:foo)             # => [:foo]

Converts block to a ::Proc object (and therefore binds it at the point of call) and registers it for execution when the program exits. If multiple handlers are registered, they are executed in reverse order of registration.

def do_at_exit(str1)
  at_exit { print str1 }
end
at_exit { puts "cruel world" }
do_at_exit("goodbye ")
exit

produces:

goodbye cruel world

Registers filename to be loaded (using .require) the first time that const (which may be a .String or a symbol) is accessed.

autoload(:MyModule, "/usr/local/lib/modules/my_module.rb")

If const is defined as autoload, the file name to be loaded is replaced with filename. If const is defined but not as autoload, does nothing.

Returns filename to be loaded if name is registered as .autoload in the current namespace or one of its ancestors.

autoload(:B, "b")
autoload?(:B)            #=> "b"

module C
  autoload(:D, "d")
  autoload?(:D)          #=> "d"
  autoload?(:B)          #=> nil
end

class E
  autoload(:F, "f")
  autoload?(:F)          #=> "f"
  autoload?(:B)          #=> "b"
end

Returns a ::Binding object, describing the variable and method bindings at the point of call. This object can be used when calling Binding#eval to execute the evaluated command in this environment, or extracting its local variables.

class User
  def initialize(name, position)
    @name = name
    @position = position
  end

  def get_binding
    binding
  end
end

user = User.new('Joan', 'manager')
template = '{name: @name, position: @position}'

# evaluate template in context of the object
eval(template, user.get_binding)
#=> {:name=>"Joan", :position=>"manager"}

Binding#local_variable_get can be used to access the variables whose names are reserved Ruby keywords:

# This is valid parameter declaration, but {if} parameter can't
# be accessed by name, because it is a reserved word.
def validate(field, validation, if: nil)
  condition = binding.local_variable_get('if')
  return unless condition

  # ...Some implementation ...
end

validate(:name, :empty?, if: false) # skips validation
validate(:name, :empty?, if: true) # performs validation

Generates a ::Continuation object, which it passes to the associated block. You need to require 'continuation' before using this method. Performing a cont.call will cause the #callcc to return (as will falling through the end of the block). The value returned by the #callcc is the value of the block, or the value passed to cont.call. See class ::Continuation for more details. Also see .throw for an alternative mechanism for unwinding a call stack.

Returns the current execution stack—an array containing strings in the form file:line or file:line: in `method'.

The optional start parameter determines the number of initial stack entries to omit from the top of the stack.

A second optional length parameter can be used to limit how many entries are returned from the stack.

Returns nil if start is greater than the size of current execution stack.

Optionally you can pass a range, which will return an array containing the entries within the specified range.

def a(skip)
  caller(skip)
end
def b(skip)
  a(skip)
end
def c(skip)
  b(skip)
end
c(0)   #=> ["prog:2:in `a'", "prog:5:in `b'", "prog:8:in `c'", "prog:10:in `<main>'"]
c(1)   #=> ["prog:5:in `b'", "prog:8:in `c'", "prog:11:in `<main>'"]
c(2)   #=> ["prog:8:in `c'", "prog:12:in `<main>'"]
c(3)   #=> ["prog:13:in `<main>'"]
c(4)   #=> []
c(5)   #=> nil

Returns the current execution stack—an array containing backtrace location objects.

See ::Thread::Backtrace::Location for more information.

The optional start parameter determines the number of initial stack entries to omit from the top of the stack.

A second optional length parameter can be used to limit how many entries are returned from the stack.

Returns nil if start is greater than the size of current execution stack.

Optionally you can pass a range, which will return an array containing the entries within the specified range.

catch executes its block. If .throw is not called, the block executes normally, and catch returns the value of the last expression evaluated.

catch(1) { 123 }            # => 123

If throw(tag2, val) is called, Ruby searches up its stack for a catch block whose tag has the same object_id as tag2. When found, the block stops executing and returns val (or nil if no second argument was given to .throw).

catch(1) { throw(1, 456) }  # => 456
catch(1) { throw(1) }       # => nil

When tag is passed as the first argument, catch yields it as the parameter of the block.

catch(1) {|x| x + 2 }       # => 3

When no tag is given, catch yields a new unique object (as from Object.new) as the block parameter. This object can then be used as the argument to .throw, and will match the correct catch block.

catch do |obj_A|
  catch do |obj_B|
    throw(obj_B, 123)
    puts "This puts is not reached"
  end

  puts "This puts is displayed"
  456
end

# => 456

catch do |obj_A|
  catch do |obj_B|
    throw(obj_A, 123)
    puts "This puts is still not reached"
  end

  puts "Now this puts is also not reached"
  456
end

# => 123

Equivalent to $_ = $_.chomp(string). See String#chomp. Available only when -p/-n command line option specified.

Equivalent to ($_.dup).chop!, except nil is never returned. See String#chop!. Available only when -p/-n command line option specified.

Returns a new Complex object if the arguments are valid; otherwise raises an exception if exception is true; otherwise returns nil.

With Numeric arguments real and imag, returns Complex.rect(real, imag) if the arguments are valid.

With string argument s, returns a new Complex object if the argument is valid; the string may have:

• One or two numeric substrings, each of which specifies a Complex, Float, Integer, Numeric, or Rational value, specifying rectangular coordinates:

  • Sign-separated real and imaginary numeric substrings (with trailing character 'i'):

    Complex('1+2i')  # => (1+2i)
    Complex('12i') # => (1+2i)
    Complex('+1-2i') # => (1-2i)
    Complex('-1+2i') # => (-1+2i)
    Complex('-1-2i') # => (-1-2i)

  • Real-only numeric string (without trailing character 'i'):

    Complex('1')  # => (1+0i)
    Complex('+1') # => (1+0i)
    Complex('-1') # => (-1+0i)

  • Imaginary-only numeric string (with trailing character 'i'):

    Complex('1i')  # => (0+1i)
    Complex('+1i') # => (0+1i)
    Complex('-1i') # => (0-1i)

• At-sign separated real and imaginary rational substrings, each of which specifies a Rational value, specifying polar coordinates:

  Complex('1/2@3/4')   # => (0.36584443443691045+0.34081938001166706i)
  Complex('1/2@3/4') # => (0.36584443443691045+0.34081938001166706i)
  Complex('+1/2@-3/4') # => (0.36584443443691045-0.34081938001166706i)
  Complex('-1/2@+3/4') # => (-0.36584443443691045-0.34081938001166706i)
  Complex('-1/2@-3/4') # => (-0.36584443443691045+0.34081938001166706i)

Evaluates the Ruby expression(s) in string. If binding is given, which must be a ::Binding object, the evaluation is performed in its context. If the optional filename and lineno parameters are present, they will be used when reporting syntax errors.

def get_binding(str)
  return binding
end
str = "hello"
eval "str + ' Fred'"                      #=> "hello Fred"
eval "str + ' Fred'", get_binding("bye")  #=> "bye Fred"

Replaces the current process by doing one of the following:

• Passing string command_line to the shell.

• Invoking the executable at exe_path.

This method has potential security vulnerabilities if called with untrusted input; see Command Injection.

The new process is created using the exec system call; it may inherit some of its environment from the calling program (possibly including open file descriptors).

Argument env, if given, is a hash that affects ::ENV for the new process; see Execution Environment.

Argument options is a hash of options for the new process; see Execution Options.

The first required argument is one of the following:

• command_line if it is a string, and if it begins with a shell reserved word or special built-in, or if it contains one or more meta characters.

• exe_path otherwise.

Argument command_line

String argument command_line is a command line to be passed to a shell; it must begin with a shell reserved word, begin with a special built-in, or contain meta characters:

exec('if true; then echo "Foo"; fi') # Shell reserved word.
exec('exit')                         # Built-in.
exec('date > date.tmp')              # Contains meta character.

The command line may also contain arguments and options for the command:

exec('echo "Foo"')

Output:

Foo

See Execution Shell for details about the shell.

Raises an exception if the new process could not execute.

Argument exe_path

Argument exe_path is one of the following:

• The string path to an executable to be called.

• A 2-element array containing the path to an executable and the string to be used as the name of the executing process.

Example:

exec('/usr/bin/date')

Output:

Sat Aug 26 09:38:00 AM CDT 2023

Ruby invokes the executable directly. This form does not use the shell; see Arguments args for caveats.

exec('doesnt_exist') # Raises Errno::ENOENT

If one or more args is given, each is an argument or option to be passed to the executable:

exec('echo', 'C*')
exec('echo', 'hello', 'world')

Output:

C*
hello world

Raises an exception if the new process could not execute.

Initiates termination of the Ruby script by raising SystemExit; the exception may be caught. Returns exit status status to the underlying operating system.

Values true and false for argument status indicate, respectively, success and failure; The meanings of integer values are system-dependent.

Example:

begin
  exit
  puts 'Never get here.'
rescue SystemExit
  puts 'Rescued a SystemExit exception.'
end
puts 'After begin block.'

Output:

Rescued a SystemExit exception.
After begin block.

Just prior to final termination, Ruby executes any at-exit procedures (see .at_exit) and any object finalizers (see ObjectSpace.define_finalizer).

Example:

at_exit { puts 'In at_exit function.' }
ObjectSpace.define_finalizer('string', proc { puts 'In finalizer.' })
exit

Output:

In at_exit function.
In finalizer.

Exits the process immediately; no exit handlers are called. Returns exit status status to the underlying operating system.

Process.exit!(true)

Values true and false for argument status indicate, respectively, success and failure; The meanings of integer values are system-dependent.

Raises an exception; see Exceptions.

Argument exception sets the class of the new exception; it should be class ::Exception or one of its subclasses (most commonly, ::RuntimeError or ::StandardError), or an instance of one of those classes:

begin
  raise(StandardError)
rescue => x
  p x.class
end
# => StandardError

Argument message sets the stored message in the new exception, which may be retrieved by method Exception#message; the message must be a string-convertible object or nil:

begin
  raise(StandardError, 'Boom')
rescue => x
  p x.message
end
# => "Boom"

If argument message is not given, the message is the exception class name.

See Messages.

Argument backtrace sets the stored backtrace in the new exception, which may be retrieved by method Exception#backtrace; the backtrace must be an array of strings or nil:

begin
  raise(StandardError, 'Boom', %w[foo bar baz])
rescue => x
  p x.backtrace
end
# => ["foo", "bar", "baz"]

If argument backtrace is not given, the backtrace is set according to an array of ::Thread::Backtrace::Location objects, as derived from the call stack.

See Backtraces.

Keyword argument cause sets the stored cause in the new exception, which may be retrieved by method Exception#cause; the cause must be an exception object (Exception or one of its subclasses), or nil:

begin
  raise(StandardError, cause: RuntimeError.new)
rescue => x
  p x.cause
end
# => #<RuntimeError: RuntimeError>

If keyword argument cause is not given, the cause is the value of $!.

See Cause.

In the alternate calling sequence, where argument exception not given, raises a new exception of the class given by $!, or of class ::RuntimeError if $! is nil:

begin
  raise
rescue => x
  p x
end
# => RuntimeError

With argument exception not given, argument message and keyword argument cause may be given, but argument backtrace may not be given.

Returns arg converted to a float. ::Numeric types are converted directly, and with exception to .String and nil, the rest are converted using arg.to_f. Converting a .String with invalid characters will result in an ::ArgumentError. Converting nil generates a ::TypeError. Exceptions can be suppressed by passing exception: false.

Float(1)                 #=> 1.0
Float("123.456")         #=> 123.456
Float("123.0_badstring") #=> ArgumentError: invalid value for Float(): "123.0_badstring"
Float(nil)               #=> TypeError: can't convert nil into Float
Float("123.0_badstring", exception: false)  #=> nil

Creates a child process.

With a block given, runs the block in the child process; on block exit, the child terminates with a status of zero:

puts "Before the fork: #{Process.pid}"
fork do
  puts "In the child process: #{Process.pid}"
end                   # => 382141
puts "After the fork: #{Process.pid}"

Output:

Before the fork: 420496
After the fork: 420496
In the child process: 420520

With no block given, the fork call returns twice:

• Once in the parent process, returning the pid of the child process.

• Once in the child process, returning nil.

Example:

puts "This is the first line before the fork (pid #{Process.pid})"
puts fork
puts "This is the second line after the fork (pid #{Process.pid})"

Output:

This is the first line before the fork (pid 420199)
420223
This is the second line after the fork (pid 420199)

This is the second line after the fork (pid 420223)

In either case, the child process may exit using .exit! to avoid the call to .at_exit.

To avoid zombie processes, the parent process should call either:

• Process.wait, to collect the termination statuses of its children.

• Process.detach, to register disinterest in their status.

The thread calling fork is the only thread in the created child process; fork doesn’t copy other threads.

Note that method fork is available on some platforms, but not on others:

Process.respond_to?(:fork) # => true # Would be false on some.

If not, you may use .spawn instead of fork.

Returns the string resulting from formatting objects into format_string.

For details on format_string, see Format Specifications.

Returns (and assigns to $_) the next line from the list of files in ARGV (or $*), or from standard input if no files are present on the command line. Returns nil at end of file. The optional argument specifies the record separator. The separator is included with the contents of each record. A separator of nil reads the entire contents, and a zero-length separator reads the input one paragraph at a time, where paragraphs are divided by two consecutive newlines. If the first argument is an integer, or optional second argument is given, the returning string would not be longer than the given value in bytes. If multiple filenames are present in ARGV, gets(nil) will read the contents one file at a time.

ARGV << "testfile"
print while gets

produces:

This is line one
This is line two
This is line three
And so on...

The style of programming using $_ as an implicit parameter is gradually losing favor in the Ruby community.

Returns an array of the names of global variables. This includes special regexp global variables such as $~ and $+, but does not include the numbered regexp global variables ($1, $2, etc.).

global_variables.grep /std/   #=> [:$stdin, :$stdout, :$stderr]

Equivalent to $_.gsub..., except that $_ will be updated if substitution occurs. Available only when -p/-n command line option specified.

Returns a hash converted from object.

• If object is:

  • A hash, returns object.

  • An empty array or nil, returns an empty hash.

• Otherwise, if object.to_hash returns a hash, returns that hash.

• Otherwise, returns ::TypeError.

Examples:

Hash({foo: 0, bar: 1}) # => {:foo=>0, :bar=>1}
Hash(nil)              # => {}
Hash([])               # => {}

Returns an integer converted from object.

Tries to convert object to an integer using to_int first and to_i second; see below for exceptions.

With a non-zero base, object must be a string or convertible to a string.

Numeric objects

With an integer argument object given, returns object:

Integer(1)                # => 1
Integer(-1)               # => -1

With a floating-point argument object given, returns object truncated to an integer:

Integer(1.9)              # => 1  # Rounds toward zero.
Integer(-1.9)             # => -1 # Rounds toward zero.

String objects

With a string argument object and zero base given, returns object converted to an integer in base 10:

Integer('100')    # => 100
Integer('-100')   # => -100

With base zero, string object may contain leading characters to specify the actual base (radix indicator):

Integer('0100')  # => 64  # Leading '0' specifies base 8.
Integer('0b100') # => 4   # Leading '0b' specifies base 2.
Integer('0x100') # => 256 # Leading '0x' specifies base 16.

With a positive base (in range 2..36) given, returns object converted to an integer in the given base:

Integer('100', 2)   # => 4
Integer('100', 8)   # => 64
Integer('-100', 16) # => -256

With a negative base (in range -36..-2) given, returns object converted to the radix indicator if it exists or base:

Integer('0x100', -2)   # => 256
Integer('100', -2)     # => 4
Integer('0b100', -8)   # => 4
Integer('100', -8)     # => 64
Integer('0o100', -10)  # => 64
Integer('100', -10)    # => 100

base -1 is equivalent to the -10 case.

When converting strings, surrounding whitespace and embedded underscores are allowed and ignored:

Integer(' 100 ')      # => 100
Integer('-1_0_0', 16) # => -256

Other classes

Examples with object of various other classes:

Integer(Rational(9, 10)) # => 0  # Rounds toward zero.
Integer(Complex(2, 0))   # => 2  # Imaginary part must be zero.
Integer(Time.now)        # => 1650974042

Keywords

With the optional keyword argument exception given as true (the default):

• Raises TypeError if object does not respond to to_int or to_i.

• Raises TypeError if object is nil.

• Raises ArgumentError if object is an invalid string.

With exception given as false, an exception of any kind is suppressed and nil is returned.

Equivalent to Proc.new, except the resulting ::Proc objects check the number of parameters passed when called.

Loads and executes the Ruby program in the file filename.

If the filename is an absolute path (e.g. starts with ‘/’), the file will be loaded directly using the absolute path.

If the filename is an explicit relative path (e.g. starts with ‘./’ or ‘../’), the file will be loaded using the relative path from the current directory.

Otherwise, the file will be searched for in the library directories listed in $LOAD_PATH ($:). If the file is found in a directory, it will attempt to load the file relative to that directory. If the file is not found in any of the directories in $LOAD_PATH, the file will be loaded using the relative path from the current directory.

If the file doesn’t exist when there is an attempt to load it, a ::LoadError will be raised.

If the optional wrap parameter is true, the loaded script will be executed under an anonymous module. If the optional wrap parameter is a module, the loaded script will be executed under the given module. In no circumstance will any local variables in the loaded file be propagated to the loading environment.

Returns the names of the current local variables.

fred = 1
for i in 1..10
   # ...
end
local_variables   #=> [:fred, :i]

Repeatedly executes the block.

If no block is given, an enumerator is returned instead.

loop do
  print "Input: "
  line = gets
  break if !line or line =~ /^q/i
  # ...
end

A StopIteration raised in the block breaks the loop. In this case, loop returns the “result” value stored in the exception.

enum = Enumerator.new { |y|
  y << "one"
  y << "two"
  :ok
}

result = loop {
  puts enum.next
} #=> :ok

Creates an ::IO object connected to the given file.

This method has potential security vulnerabilities if called with untrusted input; see Command Injection.

With no block given, file stream is returned:

open('t.txt') # => #<File:t.txt>

With a block given, calls the block with the open file stream, then closes the stream:

open('t.txt') {|f| p f } # => #<File:t.txt (closed)>

Output:

#<File:t.txt>

See File.open for details.

For each object obj, executes:

$stdout.write(obj.inspect, "\n")

With one object given, returns the object; with multiple objects given, returns an array containing the objects; with no object given, returns nil.

Examples:

r = Range.new(0, 4)
p r                 # => 0..4
p [r, r, r]         # => [0..4, 0..4, 0..4]
p                   # => nil

Output:

0..4
[0..4, 0..4, 0..4]

p is designed for debugging purposes. Ruby implementations may define p to be uninterruptible in whole or in part. On CRuby, Kernel#p’s writing of data is uninterruptible.

Equivalent to $stdout.print(*objects), this method is the straightforward way to write to $stdout.

Writes the given objects to $stdout; returns nil. Appends the output record separator $OUTPUT_RECORD_SEPARATOR $\), if it is not nil.

With argument objects given, for each object:

• Converts via its method to_s if not a string.

• Writes to stdout.

• If not the last object, writes the output field separator $OUTPUT_FIELD_SEPARATOR ($, if it is not nil.

With default separators:

objects = [0, 0.0, Rational(0, 1), Complex(0, 0), :zero, 'zero']
$OUTPUT_RECORD_SEPARATOR
$OUTPUT_FIELD_SEPARATOR
print(*objects)

Output:

nil
nil
00/10+0izerozero

With specified separators:

$OUTPUT_RECORD_SEPARATOR = "\n"
$OUTPUT_FIELD_SEPARATOR = ','
print(*objects)

Output:

0,0.0,0/1,0+0i,zero,zero

With no argument given, writes the content of $_ (which is usually the most recent user input):

gets  # Sets $_ to the most recent user input.
print # Prints $_.

Equivalent to:

io.write(sprintf(format_string, *objects))

For details on format_string, see Format Specifications.

With the single argument format_string, formats objects into the string, then writes the formatted string to $stdout:

printf('%4.4d %10s %2.2f', 24, 24, 24.0)

Output (on $stdout):

0024         24 24.00#

With arguments io and format_string, formats objects into the string, then writes the formatted string to io:

printf($stderr, '%4.4d %10s %2.2f', 24, 24, 24.0)

Output (on $stderr):

0024         24 24.00# => nil

With no arguments, does nothing.

Equivalent to Proc.new.

Equivalent to:

$stdout.putc(int)

See IO#putc for important information regarding multi-byte characters.

Equivalent to

$stdout.puts(objects)

Alias for .fail.

If called without an argument, or if max.to_i.abs == 0, rand returns a pseudo-random floating point number between 0.0 and 1.0, including 0.0 and excluding 1.0.

rand        #=> 0.2725926052826416

When max.abs is greater than or equal to 1, rand returns a pseudo-random integer greater than or equal to 0 and less than max.to_i.abs.

rand(100)   #=> 12

When max is a ::Range, rand returns a random number where range.member?(number) == true.

Negative or floating point values for max are allowed, but may give surprising results.

rand(-100) # => 87
rand(-0.5) # => 0.8130921818028143
rand(1.9)  # equivalent to rand(1), which is always 0

.srand may be used to ensure that sequences of random numbers are reproducible between different runs of a program.

See also Random.rand.

Returns x/y or arg as a Rational.

Rational(2, 3)   #=> (2/3)
Rational(5)      #=> (5/1)
Rational(0.5)    #=> (1/2)
Rational(0.3)    #=> (5404319552844595/18014398509481984)

Rational("2/3")  #=> (2/3)
Rational("0.3")  #=> (3/10)

Rational("10 cents")  #=> ArgumentError
Rational(nil)         #=> TypeError
Rational(1, nil)      #=> TypeError

Rational("10 cents", exception: false)  #=> nil

Syntax of the string form:

string form = extra spaces , rational , extra spaces ;
rational = [ sign ] , unsigned rational ;
unsigned rational = numerator | numerator , "/" , denominator ;
numerator = integer part | fractional part | integer part , fractional part ;
denominator = digits ;
integer part = digits ;
fractional part = "." , digits , [ ( "e" | "E" ) , [ sign ] , digits ] ;
sign = "-" | "+" ;
digits = digit , { digit | "_" , digit } ;
digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
extra spaces = ? \s* ? ;

See also String#to_r.

Equivalent to method .gets, except that it raises an exception if called at end-of-stream:

$ cat t.txt | ruby -e "p readlines; readline"
["First line\n", "Second line\n", "\n", "Fourth line\n", "Fifth line\n"]
in `readline': end of file reached (EOFError)

Optional keyword argument .chomp specifies whether line separators are to be omitted.

Returns an array containing the lines returned by calling .gets until the end-of-stream is reached; (see Line IO).

With only string argument sep given, returns the remaining lines as determined by line separator sep, or nil if none; see Line Separator:

# Default separator.
$ cat t.txt | ruby -e "p readlines"
["First line\n", "Second line\n", "\n", "Fourth line\n", "Fifth line\n"]

# Specified separator.
$ cat t.txt | ruby -e "p readlines 'li'"
["First li", "ne\nSecond li", "ne\n\nFourth li", "ne\nFifth li", "ne\n"]

# Get-all separator.
$ cat t.txt | ruby -e "p readlines nil"
["First line\nSecond line\n\nFourth line\nFifth line\n"]

# Get-paragraph separator.
$ cat t.txt | ruby -e "p readlines ''"
["First line\nSecond line\n\n", "Fourth line\nFifth line\n"]

With only integer argument limit given, limits the number of bytes in the line; see Line Limit:

$cat t.txt | ruby -e "p readlines 10"
["First line", "\n", "Second lin", "e\n", "\n", "Fourth lin", "e\n", "Fifth line", "\n"]

$cat t.txt | ruby -e "p readlines 11"
["First line\n", "Second line", "\n", "\n", "Fourth line", "\n", "Fifth line\n"]

$cat t.txt | ruby -e "p readlines 12"
["First line\n", "Second line\n", "\n", "Fourth line\n", "Fifth line\n"]

With arguments sep and limit given, combines the two behaviors (see Line Separator and Line Limit).

Optional keyword argument .chomp specifies whether line separators are to be omitted:

$ cat t.txt | ruby -e "p readlines(chomp: true)"
["First line", "Second line", "", "Fourth line", "Fifth line"]

Optional keyword arguments enc_opts specify encoding options; see {Encoding options}.

Loads the given name, returning true if successful and false if the feature is already loaded.

If the filename neither resolves to an absolute path nor starts with ‘./’ or ‘../’, the file will be searched for in the library directories listed in $LOAD_PATH ($:). If the filename starts with ‘./’ or ‘../’, resolution is based on Dir.pwd.

If the filename has the extension “.rb”, it is loaded as a source file; if the extension is “.so”, “.o”, or the default shared library extension on the current platform, Ruby loads the shared library as a Ruby extension. Otherwise, Ruby tries adding “.rb”, “.so”, and so on to the name until found. If the file named cannot be found, a ::LoadError will be raised.

For Ruby extensions the filename given may use “.so” or “.o”. For example, on macOS the socket extension is “socket.bundle” and require 'socket.so' will load the socket extension.

The absolute path of the loaded file is added to $LOADED_FEATURES ($"). A file will not be loaded again if its path already appears in $". For example, require 'a'; require './a' will not load a.rb again.

require "my-library.rb"
require "db-driver"

Any constants or globals within the loaded source file will be available in the calling program’s global namespace. However, local variables will not be propagated to the loading environment.

Ruby tries to load the library named string relative to the directory containing the requiring file. If the file does not exist a ::LoadError is raised. Returns true if the file was loaded and false if the file was already loaded before.

Invokes system call select(2), which monitors multiple file descriptors, waiting until one or more of the file descriptors becomes ready for some class of I/O operation.

Not implemented on all platforms.

Each of the arguments read_ios, write_ios, and error_ios is an array of ::IO objects.

Argument timeout is a numeric value (such as integer or float) timeout interval in seconds.

The method monitors the IO objects given in all three arrays, waiting for some to be ready; returns a 3-element array whose elements are:

• An array of the objects in read_ios that are ready for reading.

• An array of the objects in write_ios that are ready for writing.

• An array of the objects in error_ios have pending exceptions.

If no object becomes ready within the given timeout, nil is returned.

IO.select peeks the buffer of IO objects for testing readability. If the IO buffer is not empty, IO.select immediately notifies readability. This “peek” only happens for IO objects. It does not happen for IO-like objects such as OpenSSL::SSL::SSLSocket.

The best way to use IO.select is invoking it after non-blocking methods such as #read_nonblock, #write_nonblock, etc. The methods raise an exception which is extended by ::IO::WaitReadable or ::IO::WaitWritable. The modules notify how the caller should wait with IO.select. If ::IO::WaitReadable is raised, the caller should wait for reading. If ::IO::WaitWritable is raised, the caller should wait for writing.

So, blocking read (#readpartial) can be emulated using #read_nonblock and IO.select as follows:

begin
  result = io_like.read_nonblock(maxlen)
rescue IO::WaitReadable
  IO.select([io_like])
  retry
rescue IO::WaitWritable
  IO.select(nil, [io_like])
  retry
end

Especially, the combination of non-blocking methods and IO.select is preferred for ::IO like objects such as OpenSSL::SSL::SSLSocket. It has #to_io method to return underlying ::IO object. IO.select calls #to_io to obtain the file descriptor to wait.

This means that readability notified by IO.select doesn’t mean readability from OpenSSL::SSL::SSLSocket object.

The most likely situation is that OpenSSL::SSL::SSLSocket buffers some data. IO.select doesn’t see the buffer. So IO.select can block when OpenSSL::SSL::SSLSocket#readpartial doesn’t block.

However, several more complicated situations exist.

SSL is a protocol which is sequence of records. The record consists of multiple bytes. So, the remote side of SSL sends a partial record, IO.select notifies readability but OpenSSL::SSL::SSLSocket cannot decrypt a byte and OpenSSL::SSL::SSLSocket#readpartial will block.

Also, the remote side can request SSL renegotiation which forces the local SSL engine to write some data. This means OpenSSL::SSL::SSLSocket#readpartial may invoke #write system call and it can block. In such a situation, OpenSSL::SSL::SSLSocket#read_nonblock raises ::IO::WaitWritable instead of blocking. So, the caller should wait for ready for writability as above example.

The combination of non-blocking methods and IO.select is also useful for streams such as tty, pipe socket socket when multiple processes read from a stream.

Finally, Linux kernel developers don’t guarantee that readability of select(2) means readability of following read(2) even for a single process; see select(2)

Invoking IO.select before IO#readpartial works well as usual. However it is not the best way to use IO.select.

The writability notified by select(2) doesn’t show how many bytes are writable. IO#write method blocks until given whole string is written. So, IO#write(two or more bytes) can block after writability is notified by IO.select. IO#write_nonblock is required to avoid the blocking.

Blocking write (#write) can be emulated using #write_nonblock and IO.select as follows: ::IO::WaitReadable should also be rescued for SSL renegotiation in OpenSSL::SSL::SSLSocket.

while 0 < string.bytesize
  begin
    written = io_like.write_nonblock(string)
  rescue IO::WaitReadable
    IO.select([io_like])
    retry
  rescue IO::WaitWritable
    IO.select(nil, [io_like])
    retry
  end
  string = string.byteslice(written..-1)
end

Example:

rp, wp = IO.pipe
mesg = "ping "
100.times {
  # IO.select follows IO#read.  Not the best way to use IO.select.
  rs, ws, = IO.select([rp], [wp])
  if r = rs[0]
    ret = r.read(5)
    print ret
    case ret
    when /ping/
      mesg = "pong\n"
    when /pong/
      mesg = "ping "
    end
  end
  if w = ws[0]
    w.write(mesg)
  end
}

Output:

ping pong
ping pong
ping pong
(snipped)
ping

Establishes proc as the handler for tracing, or disables tracing if the parameter is nil.

Note: this method is obsolete, please use ::TracePoint instead.

proc takes up to six parameters:

• an event name string

• a filename string

• a line number

• a method name symbol, or nil

• a binding, or nil

• the class, module, or nil

proc is invoked whenever an event occurs.

Events are:

"c-call"

call a C-language routine

"c-return"

return from a C-language routine

"call"

call a Ruby method

"class"

start a class or module definition

"end"

finish a class or module definition

"line"

execute code on a new line

"raise"

raise an exception

"return"

return from a Ruby method

Tracing is disabled within the context of proc.

class Test
  def test
    a = 1
    b = 2
  end
end

set_trace_func proc { |event, file, line, id, binding, class_or_module|
  printf "%8s %s:%-2d %16p %14p\n", event, file, line, id, class_or_module
}
t = Test.new
t.test

Produces:

c-return prog.rb:8   :set_trace_func         Kernel
    line prog.rb:11              nil            nil
  c-call prog.rb:11             :new          Class
  c-call prog.rb:11      :initialize    BasicObject
c-return prog.rb:11      :initialize    BasicObject
c-return prog.rb:11             :new          Class
    line prog.rb:12              nil            nil
    call prog.rb:2             :test           Test
    line prog.rb:3             :test           Test
    line prog.rb:4             :test           Test
  return prog.rb:5             :test           Test

Suspends execution of the current thread for the number of seconds specified by numeric argument secs, or forever if secs is nil; returns the integer number of seconds suspended (rounded).

Time.new  # => 2008-03-08 19:56:19 +0900
sleep 1.2 # => 1
Time.new  # => 2008-03-08 19:56:20 +0900
sleep 1.9 # => 2
Time.new  # => 2008-03-08 19:56:22 +0900

Creates a new child process by doing one of the following in that process:

• Passing string command_line to the shell.

• Invoking the executable at exe_path.

This method has potential security vulnerabilities if called with untrusted input; see Command Injection.

Returns the process ID (pid) of the new process, without waiting for it to complete.

To avoid zombie processes, the parent process should call either:

• Process.wait, to collect the termination statuses of its children.

• Process.detach, to register disinterest in their status.

The new process is created using the exec system call; it may inherit some of its environment from the calling program (possibly including open file descriptors).

Argument env, if given, is a hash that affects ::ENV for the new process; see Execution Environment.

Argument options is a hash of options for the new process; see Execution Options.

The first required argument is one of the following:

• command_line if it is a string, and if it begins with a shell reserved word or special built-in, or if it contains one or more meta characters.

• exe_path otherwise.

Argument command_line

String argument command_line is a command line to be passed to a shell; it must begin with a shell reserved word, begin with a special built-in, or contain meta characters:

spawn('if true; then echo "Foo"; fi') # => 798847 # Shell reserved word.
Process.wait                          # => 798847
spawn('exit')                         # => 798848 # Built-in.
Process.wait                          # => 798848
spawn('date > /tmp/date.tmp')         # => 798879 # Contains meta character.
Process.wait                          # => 798849
spawn('date > /nop/date.tmp')         # => 798882 # Issues error message.
Process.wait                          # => 798882

The command line may also contain arguments and options for the command:

spawn('echo "Foo"') # => 799031
Process.wait        # => 799031

Output:

Foo

See Execution Shell for details about the shell.

Raises an exception if the new process could not execute.

Argument exe_path

Argument exe_path is one of the following:

• The string path to an executable to be called.

• A 2-element array containing the path to an executable to be called, and the string to be used as the name of the executing process.

  spawn('/usr/bin/date') # Path to date on Unix-style system.
  Process.wait

  Output:

  Mon Aug 28 11:43:10 AM CDT 2023

Ruby invokes the executable directly. This form does not use the shell; see Arguments args for caveats.

If one or more args is given, each is an argument or option to be passed to the executable:

spawn('echo', 'C*')             # => 799392
Process.wait                    # => 799392
spawn('echo', 'hello', 'world') # => 799393
Process.wait                    # => 799393

Output:

C*
hello world

Raises an exception if the new process could not execute.

Alias for .format.

Seeds the system pseudo-random number generator, with number. The previous seed value is returned.

If number is omitted, seeds the generator using a source of entropy provided by the operating system, if available (/dev/urandom on Unix systems or the RSA cryptographic provider on Windows), which is then combined with the time, the process id, and a sequence number.

srand may be used to ensure repeatable sequences of pseudo-random numbers between different runs of the program. By setting the seed to a known value, programs can be made deterministic during testing.

srand 1234               # => 268519324636777531569100071560086917274
[ rand, rand ]           # => [0.1915194503788923, 0.6221087710398319]
[ rand(10), rand(1000) ] # => [4, 664]
srand 1234               # => 1234
[ rand, rand ]           # => [0.1915194503788923, 0.6221087710398319]

Returns a string converted from object.

Tries to convert object to a string using to_str first and to_s second:

String([0, 1, 2])        # => "[0, 1, 2]"
String(0..5)             # => "0..5"
String({foo: 0, bar: 1}) # => "{:foo=>0, :bar=>1}"

Raises ::TypeError if object cannot be converted to a string.

Equivalent to $_.sub(args), except that $_ will be updated if substitution occurs. Available only when -p/-n command line option specified.

Invokes Posix system call syscall(2), which calls a specified function.

Calls the operating system function identified by integer_callno; returns the result of the function or raises ::SystemCallError if it failed. The effect of the call is platform-dependent. The arguments and returned value are platform-dependent.

For each of arguments: if it is an integer, it is passed directly; if it is a string, it is interpreted as a binary sequence of bytes. There may be as many as nine such arguments.

Arguments integer_callno and argument, as well as the returned value, are platform-dependent.

Note: ::Method syscall is essentially unsafe and unportable. The DL (Fiddle) library is preferred for safer and a bit more portable programming.

Not implemented on all platforms.

Creates a new child process by doing one of the following in that process:

• Passing string command_line to the shell.

• Invoking the executable at exe_path.

This method has potential security vulnerabilities if called with untrusted input; see Command Injection.

Returns:

• true if the command exits with status zero.

• false if the exit status is a non-zero integer.

• nil if the command could not execute.

Raises an exception (instead of returning false or nil) if keyword argument exception is set to true.

Assigns the command’s error status to $?.

The new process is created using the system system call; it may inherit some of its environment from the calling program (possibly including open file descriptors).

Argument env, if given, is a hash that affects ::ENV for the new process; see Execution Environment.

Argument options is a hash of options for the new process; see Execution Options.

The first required argument is one of the following:

• command_line if it is a string, and if it begins with a shell reserved word or special built-in, or if it contains one or more meta characters.

• exe_path otherwise.

Argument command_line

String argument command_line is a command line to be passed to a shell; it must begin with a shell reserved word, begin with a special built-in, or contain meta characters:

system('if true; then echo "Foo"; fi')          # => true  # Shell reserved word.
system('exit')                                  # => true  # Built-in.
system('date > /tmp/date.tmp')                  # => true  # Contains meta character.
system('date > /nop/date.tmp')                  # => false
system('date > /nop/date.tmp', exception: true) # Raises RuntimeError.

Assigns the command’s error status to $?:

system('exit')                             # => true  # Built-in.
$?                                         # => #<Process::Status: pid 640610 exit 0>
system('date > /nop/date.tmp')             # => false
$?                                         # => #<Process::Status: pid 640742 exit 2>

The command line may also contain arguments and options for the command:

system('echo "Foo"') # => true

Output:

Foo

See Execution Shell for details about the shell.

Raises an exception if the new process could not execute.

Argument exe_path

Argument exe_path is one of the following:

• The string path to an executable to be called.

• A 2-element array containing the path to an executable and the string to be used as the name of the executing process.

Example:

system('/usr/bin/date') # => true # Path to date on Unix-style system.
system('foo')           # => nil  # Command failed.

Output:

Mon Aug 28 11:43:10 AM CDT 2023

Assigns the command’s error status to $?:

system('/usr/bin/date') # => true
$?                      # => #<Process::Status: pid 645605 exit 0>
system('foo')           # => nil
$?                      # => #<Process::Status: pid 645608 exit 127>

Ruby invokes the executable directly. This form does not use the shell; see Arguments args for caveats.

system('doesnt_exist') # => nil

If one or more args is given, each is an argument or option to be passed to the executable:

system('echo', 'C*')             # => true
system('echo', 'hello', 'world') # => true

Output:

C*
hello world

Raises an exception if the new process could not execute.

:markup: markdown

Performs a test on one or both of the filesystem entities at the given paths path0 and path1:

• Each path path0 or path1 points to a file, directory, device, pipe, etc.

• Character char selects a specific test.

The tests:

• Each of these tests operates only on the entity at path0, and returns true or ‘false`; for a non-existent entity, returns false (does not raise exception):

  | Character    | Test                                                                      |
  |:------------:|:--------------------------------------------------------------------------|
  | <tt>'b'</tt> | Whether the entity is a block device.                                     |
  | <tt>'c'</tt> | Whether the entity is a character device.                                 |
  | <tt>'d'</tt> | Whether the entity is a directory.                                        |
  | <tt>'e'</tt> | Whether the entity is an existing entity.                                 |
  | <tt>'f'</tt> | Whether the entity is an existing regular file.                           |
  | <tt>'g'</tt> | Whether the entity's setgid bit is set.                                   |
  | <tt>'G'</tt> | Whether the entity's group ownership is equal to the caller's.            |
  | <tt>'k'</tt> | Whether the entity's sticky bit is set.                                   |
  | <tt>'l'</tt> | Whether the entity is a symbolic link.                                    |
  | <tt>'o'</tt> | Whether the entity is owned by the caller's effective uid.                |
  | <tt>'O'</tt> | Like <tt>'o'</tt>, but uses the real uid (not the effective uid).         |
  | <tt>'p'</tt> | Whether the entity is a FIFO device (named pipe).                         |
  | <tt>'r'</tt> | Whether the entity is readable by the caller's effective uid/gid.         |
  | <tt>'R'</tt> | Like <tt>'r'</tt>, but uses the real uid/gid (not the effective uid/gid). |
  | <tt>'S'</tt> | Whether the entity is a socket.                                           |
  | <tt>'u'</tt> | Whether the entity's setuid bit is set.                                   |
  | <tt>'w'</tt> | Whether the entity is writable by the caller's effective uid/gid.         |
  | <tt>'W'</tt> | Like <tt>'w'</tt>, but uses the real uid/gid (not the effective uid/gid). |
  | <tt>'x'</tt> | Whether the entity is executable by the caller's effective uid/gid.       |
  | <tt>'X'</tt> | Like <tt>'x'</tt>, but uses the real uid/gid (not the effective uid/git). |
  | <tt>'z'</tt> | Whether the entity exists and is of length zero.                          |

• This test operates only on the entity at path0, and returns an integer size or nil:

  | Character    | Test                                                                                         |
  |:------------:|:---------------------------------------------------------------------------------------------|
  | <tt>'s'</tt> | Returns positive integer size if the entity exists and has non-zero length, {nil} otherwise. |

• Each of these tests operates only on the entity at path0, and returns a Time object; raises an exception if the entity does not exist:

  | Character    | Test                                   |
  |:------------:|:---------------------------------------|
  | <tt>'A'</tt> | Last access time for the entity.       |
  | <tt>'C'</tt> | Last change time for the entity.       |
  | <tt>'M'</tt> | Last modification time for the entity. |

• Each of these tests operates on the modification time (mtime) of each of the entities at path0 and path1, and returns a true or ‘false`; returns false if either entity does not exist:

  | Character    | Test                                                            |
  |:------------:|:----------------------------------------------------------------|
  | <tt>'<'</tt> | Whether the {mtime} at {path0} is less than that at {path1}.    |
  | <tt>'='</tt> | Whether the {mtime} at {path0} is equal to that at {path1}.     |
  | <tt>'>'</tt> | Whether the {mtime} at {path0} is greater than that at {path1}. |

• This test operates on the content of each of the entities at path0 and path1, and returns a true or ‘false`; returns false if either entity does not exist:

  | Character    | Test                                          |
  |:------------:|:----------------------------------------------|
  | <tt>'-'</tt> | Whether the entities exist and are identical. |

Transfers control to the end of the active .catch block waiting for tag. Raises ::UncaughtThrowError if there is no .catch block for the tag. The optional second parameter supplies a return value for the .catch block, which otherwise defaults to nil. For examples, see .catch.

Controls tracing of assignments to global variables. The parameter symbol identifies the variable (as either a string name or a symbol identifier). cmd (which may be a string or a ::Proc object) or block is executed whenever the variable is assigned. The block or ::Proc object receives the variable’s new value as a parameter. Also see #untrace_var.

trace_var :$_, proc {|v| puts "$_ is now '#{v}'" }
$_ = "hello"
$_ = ' there'

produces:

$_ is now 'hello'
$_ is now ' there'

Specifies the handling of signals. The first parameter is a signal name (a string such as “SIGALRM”, “SIGUSR1”, and so on) or a signal number. The characters “SIG” may be omitted from the signal name. The command or block specifies code to be run when the signal is raised. If the command is the string “IGNORE” or “SIG_IGN”, the signal will be ignored. If the command is “DEFAULT” or “SIG_DFL”, the Ruby’s default handler will be invoked. If the command is “EXIT”, the script will be terminated by the signal. If the command is “SYSTEM_DEFAULT”, the operating system’s default handler will be invoked. Otherwise, the given command or block will be run. The special signal name “EXIT” or signal number zero will be invoked just prior to program termination. trap returns the previous handler for the given signal.

Signal.trap(0, proc { puts "Terminating: #{$$}" })
Signal.trap("CLD")  { puts "Child died" }
fork && Process.wait

produces:

Terminating: 27461
Child died
Terminating: 27460

Removes tracing for the specified command on the given global variable and returns nil. If no command is specified, removes all tracing for that variable and returns an array containing the commands actually removed.

If warnings have been disabled (for example with the -W0 flag), does nothing. Otherwise, converts each of the messages to strings, appends a newline character to the string if the string does not end in a newline, and calls Warning#warn with the string.

warn("warning 1", "warning 2")

produces:

warning 1
warning 2

If the uplevel keyword argument is given, the string will be prepended with information for the given caller frame in the same format used by the rb_warn C function.

# In baz.rb
def foo
  warn("invalid call to foo", uplevel: 1)
end

def bar
  foo
end

bar

produces:

baz.rb:6: warning: invalid call to foo

If category keyword argument is given, passes the category to Warning#warn. The category given must be one of the following categories:

:deprecated

Used for warning for deprecated functionality that may

be removed in the future.

:experimental

Used for experimental features that may change in

future releases.

:performance

Used for warning about APIs or pattern that have

negative performance impact

Internal helper for built-in initializations to define methods only when YJIT is enabled. This method is removed in yjit_hook.rb.

Produces a shallow copy of obj—the instance variables of obj are copied, but not the objects they reference. #clone copies the frozen value state of obj, unless the :freeze keyword argument is given with a false or true value. See also the discussion under Object#dup.

class Klass
   attr_accessor :str
end
s1 = Klass.new      #=> #<Klass:0x401b3a38>
s1.str = "Hello"    #=> "Hello"
s2 = s1.clone       #=> #<Klass:0x401b3998 @str="Hello">
s2.str[1,4] = "i"   #=> "i"
s1.inspect          #=> "#<Klass:0x401b3a38 @str=\"Hi\">"
s2.inspect          #=> "#<Klass:0x401b3998 @str=\"Hi\">"

This method may have class-specific behavior. If so, that behavior will be documented under the #initialize_copy method of the class.

Alias for pp.

Yields self to the block and then returns self. The primary purpose of this method is to “tap into” a method chain, in order to perform operations on intermediate results within the chain.

(1..10)                  .tap {|x| puts "original: #{x}" }
  .to_a                  .tap {|x| puts "array:    #{x}" }
  .select {|x| x.even? } .tap {|x| puts "evens:    #{x}" }
  .map {|x| x*x }        .tap {|x| puts "squares:  #{x}" }

Yields self to the block and returns the result of the block.

3.next.then {|x| x**x }.to_s             #=> "256"

A good use of then is value piping in method chains:

require 'open-uri'
require 'json'

construct_url(arguments)
  .then {|url| URI(url).read }
  .then {|response| JSON.parse(response) }

When called without a block, the method returns an ::Enumerator, which can be used, for example, for conditional circuit-breaking:

# Meets condition, no-op
1.then.detect(&:odd?)            # => 1
# Does not meet condition, drop value
2.then.detect(&:odd?)            # => nil

Alias for #then.