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.

Returns the $stdout output from running command in a subshell; sets global variable $? to the process status.

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

Examples:

$ `date`                 # => "Wed Apr  9 08:56:30 CDT 2003\n"
$ `echo oops && exit 99` # => "oops\n"
$ $?                     # => #<Process::Status: pid 17088 exit 99>
$ $?.status              # => 99>

The built-in syntax %x{...} uses this method.

Terminate execution immediately, effectively by calling Kernel.exit(false). If msg is given, it is written to STDERR prior to terminating.

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 Kernel::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.

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

Returns a ::Binding object, describing the variable and method bindings at the point of call. This object can be used when calling .eval to execute the evaluated command in this environment. See also the description of class ::Binding.

def get_binding(param)
  binding
end
b = get_binding("hello")
eval("param", b)   #=> "hello"

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 x+i*y;

Complex(1, 2)    #=> (1+2i)
Complex('1+2i')  #=> (1+2i)
Complex(nil)     #=> TypeError
Complex(1, nil)  #=> TypeError

Complex(1, nil, exception: false)  #=> nil
Complex('1+2', exception: false)   #=> nil

Syntax of string form:

string form = extra spaces , complex , extra spaces ;
complex = real part | [ sign ] , imaginary part
        | real part , sign , imaginary part
        | rational , "@" , rational ;
real part = rational ;
imaginary part = imaginary unit | unsigned rational , imaginary unit ;
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 ] ;
imaginary unit = "i" | "I" | "j" | "J" ;
sign = "-" | "+" ;
digits = digit , { digit | "_" , digit };
digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
extra spaces = ? \s* ? ;

See String#to_c.

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 running the given external command, which can take one of the following forms:

exec(commandline)

command line string which is passed to the standard shell

exec(cmdname, arg1, ...)

command name and one or more arguments (no shell)

exec([cmdname, argv0], arg1, ...)

command name, argv[0] and zero or more arguments (no shell)

In the first form, the string is taken as a command line that is subject to shell expansion before being executed.

The standard shell always means "/bin/sh" on Unix-like systems, otherwise, ENV["RUBYSHELL"] or ENV["COMSPEC"] on Windows and similar. The command is passed as an argument to the "-c" switch to the shell, except in the case of COMSPEC.

If the string from the first form (exec("command")) follows these simple rules:

• no meta characters,

• not starting with shell reserved word or special built-in,

Ruby invokes the command directly without shell.

You can force shell invocation by adding “;” to the string (because “;” is a meta character).

Note that this behavior is observable by pid obtained (return value of spawn() and IO#pid for IO.popen) is the pid of the invoked command, not shell.

In the second form (exec("command1", "arg1", ...)), the first is taken as a command name and the rest are passed as parameters to command with no shell expansion.

In the third form (exec(["command", "argv0"], "arg1", ...)), starting a two-element array at the beginning of the command, the first element is the command to be executed, and the second argument is used as the argv[0] value, which may show up in process listings.

In order to execute the command, one of the exec(2) system calls are used, so the running command may inherit some of the environment of the original program (including open file descriptors).

This behavior is modified by the given env and options parameters. See .spawn for details.

If the command fails to execute (typically Errno::ENOENT when it was not found) a ::SystemCallError exception is raised.

This method modifies process attributes according to given options before exec(2) system call. See .spawn for more details about the given options.

The modified attributes may be retained when exec(2) system call fails.

For example, hard resource limits are not restorable.

Consider to create a child process using .spawn or .system if this is not acceptable.

exec "echo *"       # echoes list of files in current directory
# never get here

exec "echo", "*"    # echoes an asterisk
# never get here

Initiates the termination of the Ruby script by raising the ::SystemExit exception. This exception may be caught. The optional parameter is used to return a status code to the invoking environment. true and FALSE of status means success and failure respectively. The interpretation of other integer values are system dependent.

begin
  exit
  puts "never get here"
rescue SystemExit
  puts "rescued a SystemExit exception"
end
puts "after begin block"

produces:

rescued a SystemExit exception
after begin block

Just prior to termination, Ruby executes any .at_exit functions (see .at_exit) and runs any object finalizers (see ObjectSpace.define_finalizer).

at_exit { puts "at_exit function" }
ObjectSpace.define_finalizer("string",  proc { puts "in finalizer" })
exit

produces:

at_exit function
in finalizer

Exits the process immediately. No exit handlers are run. status is returned to the underlying system as the exit status.

Process.exit!(true)

With no arguments, raises the exception in $! or raises a ::RuntimeError if $! is nil. With a single .String argument, raises a ::RuntimeError with the string as a message. Otherwise, the first parameter should be an ::Exception class (or another object that returns an ::Exception object when sent an exception message). The optional second parameter sets the message associated with the exception (accessible via Exception#message), and the third parameter is an array of callback information (accessible via Exception#backtrace). The cause of the generated exception (accessible via Exception#cause) is automatically set to the “current” exception ($!), if any. An alternative value, either an ::Exception object or nil, can be specified via the :cause argument.

Exceptions are caught by the rescue clause of begin...end blocks.

raise "Failed to create socket"
raise ArgumentError, "No parameters", caller

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 a ::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 subprocess. If a block is specified, that block is run in the subprocess, and the subprocess terminates with a status of zero. Otherwise, the fork call returns twice, once in the parent, returning the process ID of the child, and once in the child, returning nil. The child process can exit using .exit! to avoid running any .at_exit functions. The parent process should use Process.wait to collect the termination statuses of its children or use Process.detach to register disinterest in their status; otherwise, the operating system may accumulate zombie processes.

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

If fork is not usable, Process.respond_to?(:fork) returns false.

Note that fork(2) is not available on some platforms like Windows and NetBSD 4. Therefore you should use spawn() instead of fork().

Returns the string resulting from formatting objects into format_string.

For details on format_string, see Format Specifications.

format is an alias for .sprintf.

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 integer argument object given, returns object:

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

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

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

string objects

With 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 an integer in the radix indicator if 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 equal 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 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.

• Raise 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, protecting the calling program’s global namespace. 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

::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 stream, file, or subprocess.

Required string argument path determines which of the following occurs:

• The file at the specified path is opened.

• The process forks.

• A subprocess is created.

Each of these is detailed below.

File Opened

If path does not start with a pipe character ('|'), a file stream is opened with File.open(path, mode, perm, **opts).

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.

Process Forked

If path is the 2-character string '|-', the process forks and the child process is connected to the parent.

With no block given:

io = open('|-')
if io
  $stderr.puts "In parent, child pid is #{io.pid}."
else
  $stderr.puts "In child, pid is #{$$}."
end

Output:

In parent, child pid is 27903.
In child, pid is 27903.

With a block given:

open('|-') do |io|
  if io
    $stderr.puts "In parent, child pid is #{io.pid}."
  else
    $stderr.puts "In child, pid is #{$$}."
  end
end

Output:

In parent, child pid is 28427.
In child, pid is 28427.

Subprocess Created

If path is '|command' ('command' != '-'), a new subprocess runs the command; its open stream is returned. Note that the command may be processed by shell if it contains shell metacharacters.

With no block given:

io = open('|echo "Hi!"') # => #<IO:fd 12>
print io.gets
io.close

Output:

"Hi!"

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

open('|echo "Hi!"') do |io|
  print io.gets
end

Output:

"Hi!"

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]

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 “.dll”, 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 any shared library extension. For example, on Linux the socket extension is “socket.so” and require 'socket.dll' 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 an integer 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
*   a filename
*   a line number
*   an object id
*   a binding
*   the name of a class

_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, classname|
       printf "%8s %s:%-2d %10s %8s\n", event, file, line, id, classname
    }
    t = Test.new
    t.test

      line prog.rb:11               false
    c-call prog.rb:11        new    Class
    c-call prog.rb:11 initialize   Object
  c-return prog.rb:11 initialize   Object
  c-return prog.rb:11        new    Class
      line prog.rb:12               false
      call prog.rb:2        test     Test
      line prog.rb:3        test     Test
      line prog.rb:4        test     Test
    return prog.rb:4        test     Test

Note that for c-call and c-return events, the binding returned is the binding of the nearest Ruby method calling the C method, since C methods themselves do not have bindings.

Suspends the current thread for duration seconds (which may be any number, including a .Float with fractional seconds). Returns the actual number of seconds slept (rounded), which may be less than that asked for if another thread calls Thread#run. Called without an argument, sleep() will sleep forever.

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

spawn executes specified command and return its pid.

pid = spawn("tar xf ruby-2.0.0-p195.tar.bz2")
Process.wait pid

pid = spawn(RbConfig.ruby, "-eputs'Hello, world!'")
Process.wait pid

This method is similar to .system but it doesn’t wait for the command to finish.

The parent process should use Process.wait to collect the termination status of its child or use Process.detach to register disinterest in their status; otherwise, the operating system may accumulate zombie processes.

spawn has bunch of options to specify process attributes:

env: hash
  name => val : set the environment variable
  name => nil : unset the environment variable

  the keys and the values except for nil must be strings.
command...:
  commandline                 : command line string which is passed to the standard shell
  cmdname, arg1, ...          : command name and one or more arguments (This form does not use the shell. See below for caveats.)
  [cmdname, argv0], arg1, ... : command name, argv[0] and zero or more arguments (no shell)
options: hash
  clearing environment variables:
    :unsetenv_others => true   : clear environment variables except specified by env
    :unsetenv_others => false  : don't clear (default)
  process group:
    :pgroup => true or 0 : make a new process group
    :pgroup => pgid      : join the specified process group
    :pgroup => nil       : don't change the process group (default)
  create new process group: Windows only
    :new_pgroup => true  : the new process is the root process of a new process group
    :new_pgroup => false : don't create a new process group (default)
  resource limit: resourcename is core, cpu, data, etc.  See Process.setrlimit.
    :rlimit_resourcename => limit
    :rlimit_resourcename => [cur_limit, max_limit]
  umask:
    :umask => int
  redirection:
    key:
      FD              : single file descriptor in child process
      [FD, FD, ...]   : multiple file descriptor in child process
    value:
      FD                        : redirect to the file descriptor in parent process
      string                    : redirect to file with open(string, "r" or "w")
      [string]                  : redirect to file with open(string, File::RDONLY)
      [string, open_mode]       : redirect to file with open(string, open_mode, 0644)
      [string, open_mode, perm] : redirect to file with open(string, open_mode, perm)
      [:child, FD]              : redirect to the redirected file descriptor
      :close                    : close the file descriptor in child process
    FD is one of follows
      :in     : the file descriptor 0 which is the standard input
      :out    : the file descriptor 1 which is the standard output
      :err    : the file descriptor 2 which is the standard error
      integer : the file descriptor of specified the integer
      io      : the file descriptor specified as io.fileno
  file descriptor inheritance: close non-redirected non-standard fds (3, 4, 5, ...) or not
    :close_others => false  : inherit
  current directory:
    :chdir => str

The cmdname, arg1, ... form does not use the shell. However, on different OSes, different things are provided as built-in commands. An example of this is ‘echo’, which is a built-in on Windows, but is a normal program on Linux and Mac OS X. This means that Process.spawn 'echo', '%Path%' will display the contents of the %Path% environment variable on Windows, but Process.spawn 'echo', '$PATH' prints the literal $PATH.

If a hash is given as env, the environment is updated by env before exec(2) in the child process. If a pair in env has nil as the value, the variable is deleted.

# set FOO as BAR and unset BAZ.
pid = spawn({"FOO"=>"BAR", "BAZ"=>nil}, command)

If a hash is given as options, it specifies process group, create new process group, resource limit, current directory, umask and redirects for the child process. Also, it can be specified to clear environment variables.

The :unsetenv_others key in options specifies to clear environment variables, other than specified by env.

pid = spawn(command, :unsetenv_others=>true) # no environment variable
pid = spawn({"FOO"=>"BAR"}, command, :unsetenv_others=>true) # FOO only

The :pgroup key in options specifies a process group. The corresponding value should be true, zero, a positive integer, or nil. true and zero cause the process to be a process leader of a new process group. A non-zero positive integer causes the process to join the provided process group. The default value, nil, causes the process to remain in the same process group.

pid = spawn(command, :pgroup=>true) # process leader
pid = spawn(command, :pgroup=>10) # belongs to the process group 10

The :new_pgroup key in options specifies to pass CREATE_NEW_PROCESS_GROUP flag to CreateProcessW() that is Windows API. This option is only for Windows. true means the new process is the root process of the new process group. The new process has CTRL+C disabled. This flag is necessary for Process.kill(:SIGINT, pid) on the subprocess. :new_pgroup is false by default.

pid = spawn(command, :new_pgroup=>true)  # new process group
pid = spawn(command, :new_pgroup=>false) # same process group

The :rlimit_foo key specifies a resource limit. foo should be one of resource types such as core. The corresponding value should be an integer or an array which have one or two integers: same as cur_limit and max_limit arguments for Process.setrlimit.

cur, max = Process.getrlimit(:CORE)
pid = spawn(command, :rlimit_core=>[0,max]) # disable core temporary.
pid = spawn(command, :rlimit_core=>max) # enable core dump
pid = spawn(command, :rlimit_core=>0) # never dump core.

The :umask key in options specifies the umask.

pid = spawn(command, :umask=>077)

The :in, :out, :err, an integer, an ::IO and an array key specifies a redirection. The redirection maps a file descriptor in the child process.

For example, stderr can be merged into stdout as follows:

pid = spawn(command, :err=>:out)
pid = spawn(command, 2=>1)
pid = spawn(command, STDERR=>:out)
pid = spawn(command, STDERR=>STDOUT)

The hash keys specifies a file descriptor in the child process started by #spawn. :err, 2 and STDERR specifies the standard error stream (stderr).

The hash values specifies a file descriptor in the parent process which invokes #spawn. :out, 1 and STDOUT specifies the standard output stream (stdout).

In the above example, the standard output in the child process is not specified. So it is inherited from the parent process.

The standard input stream (stdin) can be specified by :in, 0 and STDIN.

A filename can be specified as a hash value.

pid = spawn(command, :in=>"/dev/null") # read mode
pid = spawn(command, :out=>"/dev/null") # write mode
pid = spawn(command, :err=>"log") # write mode
pid = spawn(command, [:out, :err]=>"/dev/null") # write mode
pid = spawn(command, 3=>"/dev/null") # read mode

For stdout and stderr (and combination of them), it is opened in write mode. Otherwise read mode is used.

For specifying flags and permission of file creation explicitly, an array is used instead.

pid = spawn(command, :in=>["file"]) # read mode is assumed
pid = spawn(command, :in=>["file", "r"])
pid = spawn(command, :out=>["log", "w"]) # 0644 assumed
pid = spawn(command, :out=>["log", "w", 0600])
pid = spawn(command, :out=>["log", File::WRONLY|File::EXCL|File::CREAT, 0600])

The array specifies a filename, flags and permission. The flags can be a string or an integer. If the flags is omitted or nil, File::RDONLY is assumed. The permission should be an integer. If the permission is omitted or nil, 0644 is assumed.

If an array of IOs and integers are specified as a hash key, all the elements are redirected.

# stdout and stderr is redirected to log file.
# The file "log" is opened just once.
pid = spawn(command, [:out, :err]=>["log", "w"])

Another way to merge multiple file descriptors is [:child, fd]. [:child, fd] means the file descriptor in the child process. This is different from fd. For example, :err=>:out means redirecting child stderr to parent stdout. But :err=>[:child, :out] means redirecting child stderr to child stdout. They differ if stdout is redirected in the child process as follows.

# stdout and stderr is redirected to log file.
# The file "log" is opened just once.
pid = spawn(command, :out=>["log", "w"], :err=>[:child, :out])

[:child, :out] can be used to merge stderr into stdout in IO.popen. In this case, IO.popen redirects stdout to a pipe in the child process and [:child, :out] refers the redirected stdout.

io = IO.popen(["sh", "-c", "echo out; echo err >&2", :err=>[:child, :out]])
p io.read #=> "out\nerr\n"

The :chdir key in options specifies the current directory.

pid = spawn(command, :chdir=>"/var/tmp")

spawn closes all non-standard unspecified descriptors by default. The “standard” descriptors are 0, 1 and 2. This behavior is specified by :close_others option. :close_others doesn’t affect the standard descriptors which are closed only if :close is specified explicitly.

pid = spawn(command, :close_others=>true)  # close 3,4,5,... (default)
pid = spawn(command, :close_others=>false) # don't close 3,4,5,...

:close_others is false by default for spawn and IO.popen.

Note that fds which close-on-exec flag is already set are closed regardless of :close_others option.

So IO.pipe and spawn can be used as IO.popen.

# similar to r = IO.popen(command)
r, w = IO.pipe
pid = spawn(command, :out=>w)   # r, w is closed in the child process.
w.close

:close is specified as a hash value to close a fd individually.

f = open(foo)
system(command, f=>:close)        # don't inherit f.

If a file descriptor need to be inherited, io=>io can be used.

# valgrind has --log-fd option for log destination.
# log_w=>log_w indicates log_w.fileno inherits to child process.
log_r, log_w = IO.pipe
pid = spawn("valgrind", "--log-fd=#{log_w.fileno}", "echo", "a", log_w=>log_w)
log_w.close
p log_r.read

It is also possible to exchange file descriptors.

pid = spawn(command, :out=>:err, :err=>:out)

The hash keys specify file descriptors in the child process. The hash values specifies file descriptors in the parent process. So the above specifies exchanging stdout and stderr. Internally, spawn uses an extra file descriptor to resolve such cyclic file descriptor mapping.

See .exec for the standard shell.

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.

Executes command… in a subshell. command… is one of following forms.

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

commandline

command line string which is passed to the standard shell

cmdname, arg1, ...

command name and one or more arguments (no shell)

[cmdname, argv0], arg1, ...

command name, argv[0] and zero or more arguments (no shell)

system returns true if the command gives zero exit status, false for non zero exit status. Returns nil if command execution fails. An error status is available in $?.

If the exception: true argument is passed, the method raises an exception instead of returning false or nil.

The arguments are processed in the same way as for .spawn.

The hash arguments, env and options, are same as #exec and #spawn. See .spawn for details.

system("echo *")
system("echo", "*")

produces:

config.h main.rb
*

Error handling:

system("cat nonexistent.txt")
# => false
system("catt nonexistent.txt")
# => nil

system("cat nonexistent.txt", exception: true)
# RuntimeError (Command failed with exit 1: cat)
system("catt nonexistent.txt", exception: true)
# Errno::ENOENT (No such file or directory - catt)

See .exec for the standard shell.

Uses the character cmd to perform various tests on file1 (first table below) or on file1 and file2 (second table).

::File tests on a single file:

Cmd    Returns   Meaning
"A"  | Time    | Last access time for file1
"b"  | boolean | True if file1 is a block device
"c"  | boolean | True if file1 is a character device
"C"  | Time    | Last change time for file1
"d"  | boolean | True if file1 exists and is a directory
"e"  | boolean | True if file1 exists
"f"  | boolean | True if file1 exists and is a regular file
"g"  | boolean | True if file1 has the setgid bit set
"G"  | boolean | True if file1 exists and has a group
     |         | ownership equal to the caller's group
"k"  | boolean | True if file1 exists and has the sticky bit set
"l"  | boolean | True if file1 exists and is a symbolic link
"M"  | Time    | Last modification time for file1
"o"  | boolean | True if file1 exists and is owned by
     |         | the caller's effective uid
"O"  | boolean | True if file1 exists and is owned by
     |         | the caller's real uid
"p"  | boolean | True if file1 exists and is a fifo
"r"  | boolean | True if file1 is readable by the effective
     |         | uid/gid of the caller
"R"  | boolean | True if file is readable by the real
     |         | uid/gid of the caller
"s"  | int/nil | If file1 has nonzero size, return the size,
     |         | otherwise return nil
"S"  | boolean | True if file1 exists and is a socket
"u"  | boolean | True if file1 has the setuid bit set
"w"  | boolean | True if file1 exists and is writable by
     |         | the effective uid/gid
"W"  | boolean | True if file1 exists and is writable by
     |         | the real uid/gid
"x"  | boolean | True if file1 exists and is executable by
     |         | the effective uid/gid
"X"  | boolean | True if file1 exists and is executable by
     |         | the real uid/gid
"z"  | boolean | True if file1 exists and has a zero length

Tests that take two files:

"-"  | boolean | True if file1 and file2 are identical
"="  | boolean | True if the modification times of file1
     |         | and file2 are equal
"<"  | boolean | True if the modification time of file1
     |         | is prior to that of file2
">"  | boolean | True if the modification time of file1
     |         | is after that of file2

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")

<em>produces:</em>

  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

<em>produces:</em>

  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 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.

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"

Good usage for 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 block, the method returns ::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

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

"my string".yield_self {|s| s.upcase }   #=> "MY STRING"

Good usage for #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) }