Programming Ruby

The Pragmatic Programmer's Guide
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module Kernel

Index:

Array Float Integer String ` (backquote) abort at_exit autoload binding block_given? callcc caller catch chomp chomp! chop chop! eval exec exit exit! fail fork format gets global_variables gsub gsub! iterator? lambda load local_variables loop open p print printf proc putc puts raise rand readline readlines require scan select set_trace_func singleton_method_added sleep split sprintf srand sub sub! syscall system test throw trace_var trap untrace_var

The Kernel module is included by class Object, so its methods are available in every Ruby object. The Kernel instance methods are documented in class Object beginning on page 351. This section documents the module methods. These methods are called without a receiver and thus can be called in functional form.

class methods
Array Array( arg ) -> anArray

Returns arg .to_a.

Array(1..5) » [1, 2, 3, 4, 5]

Float Float( arg ) -> aFloat

Returns arg converted to a float. Numeric types are converted directly, nil is converted to 0.0, and the rest are converted using arg.to_f.

Float(1) » 1.0
Float(nil) » 0.0
Float("123.456") » 123.456

Integer Integer( arg ) -> anInteger

Converts arg to a Fixnum or Bignum. Numeric types are converted directly (with floating point numbers being truncated). If arg is a String, leading radix indicators (0, 0b, and 0x) are honored. This behavior is different from that of String#to_i .

Integer(123.999) » 123
Integer("0x1a") » 26
Integer(Time.new) » 1023599977

String String( arg ) -> aString

Converts arg to a String by calling its to_s method.

String(self) » "main"
String(self.type) » "Object"
String(123456) » "123456"

` (backquote) `cmd` -> aString

Returns the standard output of running cmd in a subshell. The built-in syntax %x{...} described on page 73 uses this method.

`date` » "Sun Jun  9 00:19:37 CDT 2002\n"
`ls testdir`.split[1] » "main.rb"

abort abort

Terminate execution immediately, effectively by calling Kernel.exit(1).

at_exit at_exit { block } -> aProc

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

autoload autoload( aModule, aFile ) -> nil

Registers aFile to be loaded (using Kernel::require ) the first time that aModule (which may be a String or a symbol) is accessed. 

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

binding binding -> aBinding

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. Also see the description of Binding beginning on page 291.

def getBinding(param)
  return binding
end
b = getBinding("hello")
eval "param", b » "hello"

block_given? block_given? -> true or false

Returns true if yield would execute a block in the current context.

def try
  if block_given?
    yield
  else
    "no block"
  end
end
try » "no block"
try { "hello" } » "hello"
try do
  "hello"
end

callcc callcc {| cont | block } -> anObject

Generates a Continuation object, which it passes to the associated block. 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 Continuation on page 294 for more details. Also see Kernel::throw for an alternative mechanism for unwinding a call stack.

caller caller( [ anInteger ] ) -> anArray

Returns the current execution stack---an array containing strings in the form ``file:line'' or ``file:line: in `method'''. The optional anInteger parameter determines the number of initial stack entries to omit from the result.

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"]
c(1) » ["prog:5:in `b'", "prog:8:in `c'", "prog:11"]
c(2) » ["prog:8:in `c'", "prog:12"]
c(3) » ["prog:13"]

catch catch( symbol ) {| | block }

-> anObject

catch executes its block. If a throw is executed, Ruby searches up its stack for a catch block with a tag corresponding to the throw's symbol. If found, that block is terminated, and catch returns the value given to throw. If throw is not called, the block terminates normally, and the value of catch is the value of the last expression evaluated. catch expressions may be nested, and the throw call need not be in lexical scope. 

def routine(n)
  puts n
  throw :done if n <= 0
  routine(n-1)
end


catch(:done) { routine(3) }
produces: 
3
2
1
0

chomp chomp( [ aString ] ) -> $_ or aString
Equivalent to $_ = $_.chomp(aString). See String#chomp on page 367.

$_ = "now\n"
chomp » "now"
$_ » "now"
chomp "ow" » "n"
$_ » "n"
chomp "xxx" » "n"
$_ » "n"

chomp! chomp!( [ aString ] ) -> $_ or nil

Equivalent to $_.chomp!(aString). See String#chomp!

$_ = "now\n"
chomp! » "now"
$_ » "now"
chomp! "x" » nil
$_ » "now"

chop chop -> aString

Equivalent to ($_.dup).chop!, except nil is never returned. See String#chop! on page 367.

a  =  "now\r\n"
$_ = a
chop » "now"
$_ » "now"
chop » "no"
chop » "n"
chop » ""
chop » ""
a » "now\r\n"

chop! chop! -> $_ or nil

Equivalent to $_.chop!.

a  = "now\r\n"
$_ = a
chop! » "now"
chop! » "no"
chop! » "n"
chop! » ""
chop! » nil
$_ » ""
a » ""

eval eval( aString [, aBinding [ file [ line ] ] ]) -> anObject

Evaluates the Ruby expression(s) in aString. If aBinding is given, the evaluation is performed in its context. The binding may be a Binding object or a Proc object. If the optional file and line parameters are present, they will be used when reporting syntax errors.

def getBinding(str)
  return binding
end
str = "hello"
eval "str + ' Fred'" » "hello Fred"
eval "str + ' Fred'", getBinding("bye") » "bye Fred"

exec exec( command [, args ])

Replaces the current process by running the given external command. If exec is given a single argument, that argument is taken as a line that is subject to shell expansion before being executed. If multiple arguments are given, the second and subsequent arguments are passed as parameters to command with no shell expansion. If the first argument is a two-element array, 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 MSDOS environments, the command is executed in a subshell; otherwise, one of the exec(2) system calls is used, so the running command may inherit some of the environment of the original program (including open file descriptors). 

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


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

exit exit( anInteger=0 )

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. 

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 and runs any object finalizers (see ObjectSpace beginning on page 430). 

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

exit! exit!( anInteger=-1 )

Similar to Kernel::exit , but exception handling, at_exit functions, and finalizers are bypassed.

fail fail
fail( aString )
fail( anException [, aString [ anArray ] ] )

Synonym for Kernel::raise .

fork fork [{ block } ] -> aFixnum or nil
Creates a subshell. If a block is specified, that block is run in the subshell, and the subshell 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 Kernel::exit! to avoid running any at_exit functions. The parent process should use Process::wait to collect the termination statuses of its children; otherwise, the operating system may accumulate zombie processes. 

fork do
  3.times {|i| puts "Child: #{i}" }
end
3.times {|i| puts "Parent: #{i}" }
Process.wait
produces: 
Parent: 0
Child: 0
Parent: 1
Child: 1
Parent: 2
Child: 2

format format( aString [, anObject ]* ) -> aString

Synonym for Kernel::sprintf .

gets gets( aString=$/ ) -> aString or nil

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 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...

global_variables global_variables -> anArray

Returns an array of the names of global variables.

global_variables.grep /std/ » ["$stdin", "$stderr", "$stdout"]

gsub gsub( pattern, replacement ) -> aString
gsub( pattern ) {| | block }

-> aString

Equivalent to $_.gsub..., except that $_ receives the modified result.

$_ = "quick brown fox"
gsub /[aeiou]/, '*' » "q**ck br*wn f*x"
$_ » "q**ck br*wn f*x"

gsub! gsub!( pattern, replacement ) -> aString or nil
gsub!( pattern ) {| | block }

-> aString or nil

Equivalent to Kernel::gsub , except nil is returned if $_ is not modified.

$_ = "quick brown fox"
gsub! /cat/, '*' » nil
$_ » "quick brown fox"

iterator? iterator? -> true or false

Synonym for Kernel::block_given? . The iterator? method will be removed in Ruby 1.8.

lambda lambda {| | block } -> aProc

Synonym for Kernel::proc .

load load( aFileName, wrap=false ) -> true

Loads and executes the Ruby program in the file aFileName. If the filename does not resolve to an absolute path, the file is searched for in the library directories listed in $:. If the optional wrap parameter is true, the loaded script will be executed under an anonymous module, protecting the calling program's global namespace. Any local variables in the loaded file will not be propagated to the loading environment.

local_variables local_variables -> anArray

Returns the names of the current local variables.

fred = 1
for i in 1..10
   # ...
end
local_variables » ["fred", "i"]

loop loop {| | block }

Repeatedly executes the block. 

loop {
  print "Input: "
  break if !gets or $_ =~ /^qQ/
  # ...
}

open open( aString [, aMode [ perm ] ] ) -> anIO or nil
open( aString [, aMode [ perm ] ] ) {| anIO | block }

-> nil

Creates an IO object connected to the given stream, file, or subprocess.

If aString does not start with a pipe character (``|''), treat it as the name of a file to open using the specified mode defaulting to ``r'' (see the table of valid modes on page 326). If a file is being created, its initial permissions may be set using the integer third parameter.

If a block is specified, it will be invoked with the File object as a parameter, and the file will be automatically closed when the block terminates. The call always returns nil in this case.

If aString starts with a pipe character, a subprocess is created, connected to the caller by a pair of pipes. The returned IO object may be used to write to the standard input and read from the standard output of this subprocess. If the command following the ``|'' is a single minus sign, Ruby forks, and this subprocess is connected to the parent. In the subprocess, the open call returns nil. If the command is not ``-'', the subprocess runs the command. If a block is associated with an open("|-") call, that block will be run twice---once in the parent and once in the child. The block parameter will be an IO object in the parent and nil in the child. The parent's IO object will be connected to the child's $stdin and $stdout. The subprocess will be terminated at the end of the block. 

open("testfile") do |f|
  print f.gets
end
produces: 
This is line one

Open a subprocess and read its output: 

cmd = open("|date")
print cmd.gets
cmd.close
produces: 
Sun Jun  9 00:19:39 CDT 2002

Open a subprocess running the same Ruby program: 

f = open("|-", "w+")
if f == nil
  puts "in Child"
  exit
else
  puts "Got: #{f.gets}"
end
produces: 
Got: in Child

Open a subprocess using a block to receive the I/O object: 

open("|-") do |f|
  if f == nil
    puts "in Child"
  else
    puts "Got: #{f.gets}"
  end
end
produces: 
Got: in Child

p p( [ anObject ]+ ) -> nil

For each object, directly writes anObject.inspect followed by the current output record separator to the program's standard output. p bypasses the Ruby I/O libraries. 

p self
produces: 
main

print print( [ anObject ]* ) -> nil

Prints each object in turn to $defout. If the output field separator ($,) is not nil, its contents will appear between each field. If the output record separator ($\) is not nil, it will be appended to the output. If no arguments are given, prints $_. Objects that aren't strings will be converted by calling their to_s method. 

print "cat", [1,2,3], 99, "\n"
$, = ", "
$\ = "\n"
print "cat", [1,2,3], 99
produces: 
cat12399
cat, 1, 2, 3, 99

printf printf( anIO, aString [, anObject ]* ) -> nil
printf( aString [, anObject ]* ) -> nil
Equivalent to:

anIO. write sprintf( aString, anObject ...)
or
$defout.write sprintf( aString, anObject ...)

proc proc { block } -> aProc

Creates a new procedure object from the given block. Equivalent to Proc.new .

aProc = proc { "hello" }
aProc.call » "hello"

putc putc( anInteger ) -> anInteger

Equivalent to $defout.putc( anInteger ).

puts puts( [ args ]* ) -> nil

Equivalent to $defout.puts( args ).

raise raise
raise( aString )
raise( anException [, aString [ anArray ] ] )

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 the name of an Exception class (or an object that returns an Exception when sent exception). The optional second parameter sets the message associated with the exception, and the third parameter is an array of callback information. Exceptions are caught by the rescue clause of begin...end blocks. 

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

rand rand( max=0 ) -> aNumber
Converts max to an integer using max1 = max.to_i.abs. If the result is zero, returns a pseudorandom floating point number greater than or equal to 0.0 and less than 1.0. Otherwise, returns a pseudorandom integer greater than or equal to zero and less than max1. Kernel::srand may be used to ensure repeatable sequences of random numbers between different runs of the program.

srand 1234 » 0
[ rand,  rand ] » [0.7408769294, 0.2145348572]
[ rand(10), rand(1000) ] » [3, 323]
srand 1234 » 1234
[ rand,  rand ] » [0.7408769294, 0.2145348572]

readline readline( [ aString=$/ ] ) -> aString

Equivalent to Kernel::gets , except readline raises EOFError at end of file.

readlines readlines( [ aString=$/ ] ) -> anArray

Returns an array containing the lines returned by calling Kernel.gets(aString) until the end of file.

require require( aString ) -> true or false
Ruby tries to load the library named aString, returning true if successful. If the filename does not resolve to an absolute path, it will be searched for in the directories listed in $:. If the file has the extension ``.rb'', it is loaded as a source file; if the extension is ``.so'', ``.o'', or ``.dll'',[Or whatever the default shared library extension is 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. The name of the loaded feature is added to the array in $". A feature will not be loaded if it already appears in $". require returns true if the feature was successfully loaded. 

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

scan scan( pattern ) -> anArray
scan( pattern ) {| | block } -> $_

Equivalent to calling $_.scan. See String#scan on page 373.

select select( readArray [, writeArray [ errorArray [ timeout ] ] ] ) -> anArray or nil

Performs a low-level select call, which waits for data to become available from input/output devices. The first three parameters are arrays of IO objects or nil. The last is a timeout in seconds, which should be an Integer or a Float. The call waits for data to become available for any of the IO objects in readArray, for buffers to have cleared sufficiently to enable writing to any of the devices in writeArray, or for an error to occur on the devices in errorArray. If one or more of these conditions are met, the call returns a three-element array containing arrays of the IO objects that were ready. Otherwise, if there is no change in status for timeout seconds, the call returns nil. If all parameters are nil, the current thread sleeps forever.

select( [$stdin], nil, nil, 1.5 ) » [[#<IO:0x401ba090>], [], []]

set_trace_func set_trace_func( aProc ) -> aProc
set_trace_func( nil ) -> nil
Establishes aProc as the handler for tracing, or disables tracing if the parameter is nil. aProc takes up to six parameters: an event name, a filename, a line number, an object id, a binding, and the name of a class. aProc 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), and return (return from a Ruby method). Tracing is disabled within the context of aProc.

See the example starting on page 267 for more information.

singleton_method_added singleton_method_added( aFixnum ) -> nil

Invoked with a symbol id whenever a singleton method is added to a module or a class. The default implementation in Kernel ignores this, but subclasses may override the method to provide specialized functionality. 

class Test
  def Test.singleton_method_added(id)
    puts "Added #{id.id2name} to Test"
  end
  def a() end
  def Test.b() end
end
def Test.c() end
produces: 
Added singleton_method_added to Test
Added b to Test
Added c to Test

sleep sleep( [ aNumeric ] ) -> aFixnum

Suspends the current thread for aNumber seconds (which may be a Float with fractional seconds). Returns the actual number of seconds slept (rounded), which may be less than that asked for if the thread was interrupted by a SIGALRM, or if another thread calls Thread#run . An argument of zero causes sleep to sleep forever.

Time.new » Sun Jun 09 00:19:40 CDT 2002
sleep 1.2 » 1
Time.new » Sun Jun 09 00:19:41 CDT 2002
sleep 1.9 » 2
Time.new » Sun Jun 09 00:19:43 CDT 2002

split split( [ pattern [ limit ] ] ) -> anArray

Equivalent to $_.split(pattern, limit). See String#split on page 374.

sprintf sprintf( aFormatString [, arguments ]* ) -> aString

Returns the string resulting from applying  aFormatString to any additional arguments. Within the format string, any characters other than format sequences are copied to the result. A format sequence consists of a percent sign, followed by optional flags, width, and precision indicators, then terminated with a field type character. The field type controls how the corresponding sprintf argument is to be interpreted, while the flags modify that interpretation. The flag characters are shown in Table 23.1 on page 424, and the field type characters are listed in Table 23.2.

The field width is an optional integer, followed optionally by a period and a precision. The width specifies the minimum number of characters that will be written to the result for this field. For numeric fields, the precision controls the number of decimal places displayed. For string fields, the precision determines the maximum number of characters to be copied from the string. (Thus, the format sequence %10.10s will always contribute exactly ten characters to the result.)

sprintf flag characters
Flag Applies to Meaning
[visible space] (space) bdeEfgGioxXu Leave a space at the start of positive numbers.
# beEfgGoxX Use an alternative format. For the conversions `o', `x', `X', and `b', prefix the result with ``0'', ``0x'', ``0X'', and ``0b'', respectively. For `e', `E', `f', `g', and 'G', force a decimal point to be added, even if no digits follow. For `g' and 'G', do not remove trailing zeros.
+ bdeEfgGioxXu Add a leading plus sign to positive numbers.
- all Left-justify the result of this conversion.
0 (zero) all Pad with zeros, not spaces.
* all Use the next argument as the field width. If negative, left-justify the result. If the asterisk is followed by a number and a dollar sign, use the indicated argument as the width.

sprintf field types
Field Conversion
b Convert argument as a binary number.
c Argument is the numeric code for a single character.
d Convert argument as a decimal number.
E Equivalent to `e', but uses an uppercase E to indicate the exponent.
e Convert floating point argument into exponential notation with one digit before the decimal point. The precision determines the number of fractional digits (defaulting to six).
f Convert floating point argument as [[visible space]-]ddd.ddd, where the precision determines the number of digits after the decimal point.
G Equivalent to `g', but use an uppercase `E' in exponent form.
g Convert a floating point number using exponential form if the exponent is less than -4 or greater than or equal to the precision, or in d.dddd form otherwise.
i Identical to `d'.
o Convert argument as an octal number.
s Argument is a string to be substituted. If the format sequence contains a precision, at most that many characters will be copied.
u Treat argument as an unsigned decimal number.
X Convert argument as a hexadecimal number using uppercase letters.
x Convert argument as a hexadecimal number.

sprintf("%d %04x", 123, 123) » "123[visible space]007b"
sprintf("%08b '%4s'", 123, 123) » "01111011[visible space]'[visible space]123'"
sprintf("%*2$s %d", "hello", 10) » "[visible space][visible space][visible space][visible space][visible space]hello[visible space]10"
sprintf("%*2$s %d", "hello", -10) » "hello[visible space][visible space][visible space][visible space][visible space][visible space]-10"
sprintf("%+g:% g:%-g", 1.23, 1.23, 1.23) » "+1.23:[visible space]1.23:1.23"

srand srand( [ aNumber ] ) -> oldSeed

Seeds the pseudorandom number generator to the value of aNumber.to_i.abs. If aNumber is omitted or zero, seeds the generator using a combination of the time, the process id, and a sequence number. (This is also the behavior if Kernel::rand is called without previously calling srand, but without the sequence.) By setting the seed to a known value, scripts can be made deterministic during testing. The previous seed value is returned. Also see Kernel::rand on page 421.

sub sub( pattern, replacement ) -> $_
sub( pattern ) { block } -> $_

Equivalent to $_.sub(args), except that $_ will be updated if substitution occurs.

sub! sub!( pattern, replacement ) -> $_ or nil
sub!( pattern ) { block } -> $_ or nil

Equivalent to $_.sub!(args).

syscall syscall( aFixnum [, args ]* ) -> anInteger

Calls the operating system function identified by aFixnum, passing in the arguments, which must be either String objects, or Integer objects that ultimately fit within a native long. Up to nine parameters may be passed (14 on the Atari-ST). The function identified by Fixnum is system dependent. On some Unix systems, the numbers may be obtained from a header file called syscall.h. 

syscall 4, 1, "hello\n", 6   # '4' is write(2) on our box
produces: 
hello

system system( aCmd [, args ]* ) -> true or false

Executes aCmd in a subshell, returning true if the command was found and ran successfully, false otherwise. A detailed error code is available in $?. The arguments are processed in the same way as for Kernel::exec on page 415. 

system("echo *")
system("echo", "*")
produces: 
config.h main.rb
*

test test(aCmd, file1 [, file2 ] ) -> anObject

Uses the integer aCmd to perform various tests on file1 (Table 23.3 on page 426) or on file1 and file2 (Table 23.4).
File tests with a single argument
Integer Description Returns
?A Last access time for file1 Time
?b True if file1 is a block device true or false
?c True if file1 is a character device true or false
?C Last change time for file1 Time
?d True if file1 exists and is a directory true or false
?e True if file1 exists true or false
?f True if file1 exists and is a regular file true or false
?g True if file1 has the setgid bit set (false under NT) true or false
?G True if file1 exists and has a group ownership equal to the caller's group true or false
?k True if file1 exists and has the sticky bit set true or false
?l True if file1 exists and is a symbolic link true or false
?M Last modification time for file1 Time
?o True if file1 exists and is owned by the caller's effective uid true or false
?O True if file1 exists and is owned by the caller's real uid true or false
?p True if file1 exists and is a fifo true or false
?r True if file is readable by the effective uid/gid of the caller true or false
?R True if file is readable by the real uid/gid of the caller true or false
?s If file1 has nonzero size, return the size, otherwise return nil Integer or nil
?S True if file1 exists and is a socket true or false
?u True if file1 has the setuid bit set true or false
?w True if file1 exists and is writable by the effective uid/gid true or false
?W True if file1 exists and is writable by the real uid/gid true or false
?x True if file1 exists and is executable by the effective uid/gid true or false
?X True if file1 exists and is executable by the real uid/gid true or false
?z True if file1 exists and has a zero length true or false

File tests with two arguments
Integer Description
?- True if file1 is a hard link to file2
?= True if the modification times of file1 and file2 are equal
?< True if the modification time of file1 is prior to that of file2
?> True if the modification time of file1 is after that of file2

throw throw( aSymbol [, anObject ] )

Transfers control to the end of the active catch block waiting for aSymbol. Raises NameError if there is no catch block for the symbol. The optional second parameter supplies a return value for the catch block, which otherwise defaults to nil. For examples, see Kernel::catch on page 413.

trace_var trace_var( aSymbol, aCmd ) -> nil
trace_var( aSymbol ) {| val | block }

-> nil
Controls tracing of assignments to global variables. The parameter aSymbol 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 Kernel::untrace_var . 

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

trap trap( signal, cmd ) -> anObject
trap( signal ) {| | block } -> anObject

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 operating system's default handler will be invoked. If the command is ``EXIT'', the script will be terminated by the signal. 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. 

trap 0, proc { puts "Terminating: #{$$}" }
trap("CLD") { puts "Child died" }
fork && Process.wait
produces: 
Terminating: 1425
Child died
Terminating: 1424

untrace_var untrace_var( aSymbol [, aCmd ] ) -> anArray or nil
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.

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Extracted from the book "Programming Ruby - The Pragmatic Programmer's Guide"
Copyright © 2001 by Addison Wesley Longman, Inc. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/)).

Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder.

Distribution of the work or derivative of the work in any standard (paper) book form is prohibited unless prior permission is obtained from the copyright holder.