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. This section documents the module methods. These methods are called without a receiver and thus can be called in functional form.
.to_a.
Array(1..5) → [1, 2, 3, 4, 5]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") → 123.456Fixnum 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) → 1023599977String by calling its
to_s method.
String(self) → "main"
String(self.type) → "Object"
String(123456) → "123456"%x{...} described in the section “Command Expansion” uses this method.
`date` → "Sun Jun 9 00:19:37 CDT 2002\n"
`ls testdir`.split[1] → "main.rb"Kernel.exit(1).
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 ")
exitproduces:
goodbye cruel worldnilKernel::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 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.
def getBinding(param)
return binding
end
b = getBinding("hello")
eval "param", b → "hello"true or falsetrue 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"
endContinuation 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 for more details. Also see Kernel::throw for an alternative mechanism for unwinding a call stack.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 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(aString). See String#chomp.
$_ = "now\n"
chomp → "now"
$_ → "now"
chomp "ow" → "n"
$_ → "n"
chomp "xxx" → "n"
$_ → "n"nil$_.chomp!(aString). See String#chomp!
$_ = "now\n"
chomp! → "now"
$_ → "now"
chomp! "x" → nil
$_ → "now"($_.dup).chop!, except nil is never returned. See String#chop!.
a = "now\r\n"
$_ = a
chop → "now"
$_ → "now"
chop → "no"
chop → "n"
chop → ""
chop → ""
a → "now\r\n"nil$_.chop!.
a = "now\r\n"
$_ = a
chop! → "now"
chop! → "no"
chop! → "n"
chop! → ""
chop! → nil
$_ → ""
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 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 hereSystemExit 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 blockJust prior to termination, Ruby executes any at_exit functions and runs any object finalizers (see ObjectSpace).
at_exit { puts "at_exit function" }
ObjectSpace.define_finalizer(self, proc { puts "in finalizer" })
exitproduces:
at_exit functionKernel::exit, but exception handling,
at_exit functions, and finalizers are bypassed.
Kernel::raise.
nilfork 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.waitproduces:
Parent: 0
Child: 0
Parent: 1
Child: 1
Parent: 2
Child: 2Kernel::sprintf.
nilnil 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 getsproduces:
This is line one
This is line two
This is line three
And so on...global_variables.grep /std/ → ["$stdin", "$stderr", "$stdout"]$_.gsub..., except that $_ receives the modified result.
$_ = "quick brown fox"
gsub /[aeiou]/, '*' → "q**ck br*wn f*x"
$_ → "q**ck br*wn f*x"nilnilKernel::gsub, except nil is returned if $_ is not modified.
$_ = "quick brown fox"
gsub! /cat/, '*' → nil
$_ → "quick brown fox"true or falseKernel::block_given?. The iterator? method will be removed in Ruby 1.8.
Kernel::proc.
false ) → truetrue, 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. fred = 1
for i in 1..10
# ...
end
local_variables → ["fred", "i"]loop {
print "Input: "
break if !gets or $_ =~ /^qQ/
# ...
}nilnilCreates 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). 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 $stdout. The subprocess will be terminated at the end of the block.
open("testfile") do |f|
print f.gets
endproduces:
This is line onecmd = open("|date")
print cmd.gets
cmd.closeproduces:
Sun Jun 9 00:19:39 CDT 2002f = open("|-", "w+")
if f == nil
puts "in Child"
exit
else
puts "Got: #{f.gets}"
endproduces:
Got: in Childopen("|-") do |f|
if f == nil
puts "in Child"
else
puts "Got: #{f.gets}"
end
endproduces:
Got: in ChildnilanObject.inspect followed by the current output record separator to the program's standard output. p bypasses the Ruby I/O libraries.
p selfproduces:
mainnilnil, its contents will appear between each field. If the output record separator ($\) 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"
$, = ", "
$\ = "\n"
print "cat", [1,2,3], 99produces:
cat12399
cat, 1, 2, 3, 99nilnilwrite sprintf(aString, anObject ...)$defout.write sprintf(aString, anObject ...)
Proc.new.
aProc = proc { "hello" }
aProc.call → "hello"$defout.putc(anInteger).
nil$defout.puts(args).
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.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]Kernel::gets, except readline raises
EOFError at end of file.
Kernel.gets(aString) until the end of file.
true or falsetrue 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. See String#scan.
nilselect 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( [$stdin], nil, nil, 1.5 ) → [[#<IO:0x401ba090>], [], []]nil ) → nilEstablishes aProc as the handler for tracing, or disables tracing if the parameter is nil. aProc takes up to six parameters: an event name, a filename, a line number, an object id, a binding, and the name of a class. aProc is invoked whenever an event occurs. Events are: c-call (call a C-language routine), c-return (return from a C-language routine), call (call a Ruby method), class (start a class or module definition), end (finish a class or module definition), line (execute code on a new line), raise (raise an exception), and return (return from a Ruby method). Tracing is disabled within the context of aProc.
See the example in “Tracing Your Program's Execution” for more information.
nilKernel ignores this, but subclasses may override the method to provide specialized functionality.
class Test
def Test.singleton_method_added(id)
puts "Added #{id.id2name} to Test"
end
def a() end
def Test.b() end
end
def Test.c() endproduces:
Added singleton_method_added to Test
Added b to Test
Added c to TestFloat with fractional seconds). Returns the actual number of seconds slept (rounded), which may be less than that asked for if the thread was interrupted by a SIGALRM, or if another thread calls Thread#run. An argument of zero causes sleep to sleep forever.
Time.new → Sun Jun 09 00:19:40 CDT 2002
sleep 1.2 → 1
Time.new → Sun Jun 09 00:19:41 CDT 2002
sleep 1.9 → 2
Time.new → Sun Jun 09 00:19:43 CDT 2002$_.split(pattern, limit). See String#split.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, 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.)
| Flag | Applies to | Meaning |
|---|---|---|
‿ (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. |
| 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 [‿-]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‿007b"
sprintf("%08b '%4s'", 123, 123) → "01111011‿'‿123'"
sprintf("%*2$s %d", "hello", 10) → "‿‿‿‿‿hello‿10"
sprintf("%*2$s %d", "hello", -10) → "hello‿‿‿‿‿‿-10"
sprintf("%+g:% g:%-g", 1.23, 1.23, 1.23) → "+1.23:‿1.23:1.23"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.$_.sub(args), except that $_
will be updated if substitution occurs.
nilnil$_.sub!(args).
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 boxproduces:
hellotrue or falsetrue 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.
system("echo *")
system("echo", "*")produces:
config.h main.rb
*Uses the integer aCmd to perform various tests on file1 (Table 23.3) or on file1 and file2 (Table 23.4).
| 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 |
| 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 |
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.nilnilProc 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'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.waitproduces:
Terminating: 1425
Child died
Terminating: 1424nilnil. If no command is specified, removes all tracing for that variable and returns an array containing the commands actually removed.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.