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RUBY(1) Ruby Programmers Reference Guide RUBY(1)


ruby — Interpreted object-oriented scripting language


ruby [–copyright] [–version] [-SUacdlnpswvy] [-0[octal]] [-C directory] [-E external[:internal]] [-F[pattern]] [-I directory] [-K[c]] [-T[level]] [-W[level]] [-e command] [-i[extension]] [-r library] [-x[directory]] [–{enable|disable}-FEATURE] [–dump=target] [–verbose] [–] [program_file] [argument …]


Ruby is an interpreted scripting language for quick and easy object-ori‐
ented programming. It has many features to process text files and to do
system management tasks (like in Perl). It is simple, straight-forward,
and extensible.

If you want a language for easy object-oriented programming, or you don’t
like the Perl ugliness, or you do like the concept of LISP, but don’t
like too many parentheses, Ruby might be your language of choice.

Ruby’s features are as follows:

Ruby is an interpreted language, so you don’t have to recompile
programs written in Ruby to execute them.

Variables have no type (dynamic typing)
Variables in Ruby can contain data of any type. You don’t have
to worry about variable typing. Consequently, it has a weaker
compile time check.

No declaration needed
You can use variables in your Ruby programs without any declara‐
tions. Variable names denote their scope – global, class,
instance, or local.

Simple syntax
Ruby has a simple syntax influenced slightly from Eiffel.

No user-level memory management
Ruby has automatic memory management. Objects no longer refer‐
enced from anywhere are automatically collected by the garbage
collector built into the interpreter.

Everything is an object
Ruby is a purely object-oriented language, and was so since its
creation. Even such basic data as integers are seen as objects.

Class, inheritance, and methods
Being an object-oriented language, Ruby naturally has basic fea‐
tures like classes, inheritance, and methods.

Singleton methods
Ruby has the ability to define methods for certain objects. For
example, you can define a press-button action for certain widget
by defining a singleton method for the button. Or, you can make
up your own prototype based object system using singleton meth‐
ods, if you want to.

Mix-in by modules
Ruby intentionally does not have the multiple inheritance as it
is a source of confusion. Instead, Ruby has the ability to share
implementations across the inheritance tree. This is often
called a ‘Mix-in’.

Ruby has iterators for loop abstraction.

In Ruby, you can objectify the procedure.

Text processing and regular expressions
Ruby has a bunch of text processing features like in Perl.

M17N, character set independent
Ruby supports multilingualized programming. Easy to process texts
written in many different natural languages and encoded in many
different character encodings, without dependence on Unicode.

With built-in bignums, you can for example calculate facto‐

Reflection and domain specific languages
Class is also an instance of the Class class. Definition of
classes and methods is an expression just as 1+1 is. So your pro‐
grams can even write and modify programs. Thus you can write
your application in your own programming language on top of Ruby.

Exception handling
As in Java(tm).

Direct access to the OS
Ruby can use most UNIX system calls, often used in system pro‐

Dynamic loading
On most UNIX systems, you can load object files into the Ruby
interpreter on-the-fly.

Rich libraries
Libraries called “builtin libraries” and “standard libraries” are
bundled with Ruby. And you can obtain more libraries via the
package management system called `RubyGems’.

Moreover there are thousands of Ruby projects on GitHub


Ruby interpreter accepts following command-line options (switches). They
are quite similar to those of perl.

–copyright Prints the copyright notice.

–version Prints the version of Ruby interpreter.

-0[octal] (The digit “zero”.) Specifies the input record separator
($/) as an octal number. If no digit is given, the null
character is taken as the separator. Other switches may
follow the digits. -00 turns Ruby into paragraph mode.
-0777 makes Ruby read whole file at once as a single
string since there is no legal character with that value.

-C directory
-X directory Causes Ruby to switch to the directory.

-E external[:internal] –encoding external[:internal] Specifies the default value(s) for external encodings and
internal encoding. Values should be separated with colon

You can omit the one for internal encodings, then the
value (Encoding.default_internal) will be nil.

Specify the default external or internal character encod‐

-F pattern Specifies input field separator ($;).

-I directory Used to tell Ruby where to load the library scripts.
Directory path will be added to the load-path variable

-K kcode Specifies KANJI (Japanese) encoding. The default value for
script encodings (__ENCODING__) and external encodings
(Encoding.default_external) will be the specified one.
kcode can be one of


s Windows-31J (CP932)

u UTF-8


-S Makes Ruby use the PATH environment variable to search for
script, unless its name begins with a slash. This is used
to emulate #! on machines that don’t support it, in the
following manner:

#! /usr/local/bin/ruby
# This line makes the next one a comment in Ruby \
exec /usr/local/bin/ruby -S $0 $*

-T[level=1] Turns on taint checks at the specified level (default 1).

-U Sets the default value for internal encodings
(Encoding.default_internal) to UTF-8.

-W[level=2] Turns on verbose mode at the specified level without
printing the version message at the beginning. The level
can be;

0 Verbose mode is “silence”. It sets the
$VERBOSE to nil.

1 Verbose mode is “medium”. It sets the
$VERBOSE to false.

2 (default) Verbose mode is “verbose”. It sets the
$VERBOSE to true. -W2 is same as -w

-a Turns on auto-split mode when used with -n or -p. In
auto-split mode, Ruby executes
$F = $_.split
at beginning of each loop.

-c Causes Ruby to check the syntax of the script and exit
without executing. If there are no syntax errors, Ruby
will print “Syntax OK” to the standard output.

–debug Turns on debug mode. $DEBUG will be set to true.

-e command Specifies script from command-line while telling Ruby not
to search the rest of the arguments for a script file

–help Prints a summary of the options.

-i extension Specifies in-place-edit mode. The extension, if speci‐
fied, is added to old file name to make a backup copy.
For example:

% echo matz > /tmp/junk
% cat /tmp/junk
% ruby -p -i.bak -e ‘$_.upcase!’ /tmp/junk
% cat /tmp/junk
% cat /tmp/junk.bak

-l (The lowercase letter “ell”.) Enables automatic line-end‐
ing processing, which means to firstly set $\ to the value
of $/, and secondly chops every line read using chop!.

-n Causes Ruby to assume the following loop around your
script, which makes it iterate over file name arguments
somewhat like sed -n or awk.

while gets


-p Acts mostly same as -n switch, but print the value of
variable $_ at the each end of the loop. For example:

% echo matz | ruby -p -e ‘$! “a-z”, “A-Z”‘

-r library Causes Ruby to load the library using require. It is use‐
ful when using -n or -p.

-s Enables some switch parsing for switches after script name
but before any file name arguments (or before a –). Any
switches found there are removed from ARGV and set the
corresponding variable in the script. For example:

#! /usr/local/bin/ruby -s
# prints “true” if invoked with `-xyz’ switch.
print “true\n” if $xyz

On some systems $0 does not always contain the full path‐
name, so you need the -S switch to tell Ruby to search for
the script if necessary (to handle embedded spaces and
such). A better construct than $* would be ${1+”$@”}, but
it does not work if the script is being interpreted by

-v Enables verbose mode. Ruby will print its version at the
beginning and set the variable $VERBOSE to true. Some
methods print extra messages if this variable is true. If
this switch is given, and no other switches are present,
Ruby quits after printing its version.

-w Enables verbose mode without printing version message at
the beginning. It sets the $VERBOSE variable to true.

-x[directory] Tells Ruby that the script is embedded in a message.
Leading garbage will be discarded until the first line
that starts with “#!” and contains the string, “ruby”.
Any meaningful switches on that line will be applied. The
end of the script must be specified with either EOF, ^D
(control-D), ^Z (control-Z), or the reserved word __END__.
If the directory name is specified, Ruby will switch to
that directory before executing script.

–yydebug DO NOT USE.

Turns on compiler debug mode. Ruby will print a bunch of
internal state messages during compilation. Only specify
this switch you are going to debug the Ruby interpreter.

Disables (or enables) the specified FEATURE.
–enable-gems Disables (or enables) RubyGems
libraries. By default, Ruby will load
the latest version of each installed
gem. The Gem constant is true if
RubyGems is enabled, false if other‐

–enable-rubyopt Ignores (or considers) the RUBYOPT
environment variable. By default, Ruby
considers the variable.

–enable-all Disables (or enables) all features.

–dump=target Dump some informations.

Prints the specified target. target can be one of;

version version description same as –version

usage brief usage message same as -h

help Show long help message same as –help

syntax check of syntax same as -c –yydebug

yydebug compiler debug mode, same as –yydebug

Only specify this switch if you are going to
debug the Ruby interpreter.


parsetree_with_comment AST nodes tree

Only specify this switch if you are going to
debug the Ruby interpreter.

insns disassembled instructions

Only specify this switch if you are going to
debug the Ruby interpreter.

–verbose Enables verbose mode without printing version message at
the beginning. It sets the $VERBOSE variable to true. If
this switch is given, and no other switches are present,
Ruby quits after printing its version.

RUBYLIB A colon-separated list of directories that are added to Ruby’s
library load path ($:). Directories from this environment
variable are searched before the standard load path is


RUBYOPT Additional Ruby options.

RUBYOPT=”-w -Ke”

Note that RUBYOPT can contain only -d, -E, -I, -K, -r, -T, -U,
-v, -w, -W, –debug, –disable-FEATURE and –enable-FEATURE.

RUBYPATH A colon-separated list of directories that Ruby searches for
Ruby programs when the -S flag is specified. This variable
precedes the PATH environment variable.

RUBYSHELL The path to the system shell command. This environment vari‐
able is enabled for only mswin32, mingw32, and OS/2 platforms.
If this variable is not defined, Ruby refers to COMSPEC.

PATH Ruby refers to the PATH environment variable on calling Ker‐

And Ruby depends on some RubyGems related environment variables unless
RubyGems is disabled. See the help of gem as below.

% gem help

The Ruby garbage collector (GC) tracks objects in fixed-sized slots, but
each object may have auxiliary memory allocations handled by the malloc
family of C standard library calls ( malloc(3), calloc(3), and
realloc(3)). In this documentatation, the “heap” refers to the Ruby
object heap of fixed-sized slots, while “malloc” refers to auxiliary
allocations commonly referred to as the “process heap”. Thus there are
at least two possible ways to trigger GC:

1 Reaching the object limit.

2 Reaching the malloc limit.

In Ruby 2.1, the generational GC was introduced and the limits are
divided into young and old generations, providing two additional ways to
trigger a GC:

3 Reaching the old object limit.

4 Reaching the old malloc limit.

There are currently 4 possible areas where the GC may be tuned by the the
following 11 environment variables:
RUBY_GC_HEAP_INIT_SLOTS Initial allocation slots. Intro‐
duced in Ruby 2.1, default: 10000.

RUBY_GC_HEAP_FREE_SLOTS Prepare at least this amount of
slots after GC. Allocate this
number slots if there are not
enough slots. Introduced in Ruby
2.1, default: 4096

RUBY_GC_HEAP_GROWTH_FACTOR Increase allocation rate of heap
slots by this factor. Introduced
in Ruby 2.1, default: 1.8, mini‐
mum: 1.0 (no growth)

RUBY_GC_HEAP_GROWTH_MAX_SLOTS Allocation rate is limited to this
number of slots, preventing exces‐
sive allocation due to
Introduced in Ruby 2.1, default: 0
(no limit)

RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR Perform a full GC when the number
of old objects is more than R * N,
where R is this factor and N is
the number of old objects after
the last full GC. Introduced in
Ruby 2.1.1, default: 2.0

RUBY_GC_MALLOC_LIMIT The initial limit of young genera‐
tion allocation from the malloc-
family. GC will start when this
limit is reached. Default: 16MB

RUBY_GC_MALLOC_LIMIT_MAX The maximum limit of young genera‐
tion allocation from malloc before
GC starts. Prevents excessive
malloc growth due to RUBY_GC_MAL‐
duced in Ruby 2.1, default: 32MB.

RUBY_GC_MALLOC_LIMIT_GROWTH_FACTOR Increases the limit of young gen‐
eration malloc calls, reducing GC
frequency but increasing malloc
growth until RUBY_GC_MAL‐
LOC_LIMIT_MAX is reached. Intro‐
duced in Ruby 2.1, default: 1.4,
minimum: 1.0 (no growth)

RUBY_GC_OLDMALLOC_LIMIT The initial limit of old genera‐
tion allocation from malloc, a
full GC will start when this limit
is reached. Introduced in Ruby
2.1, default: 16MB

RUBY_GC_OLDMALLOC_LIMIT_MAX The maximum limit of old genera‐
tion allocation from malloc before
a full GC starts. Prevents exces‐
sive malloc growth due to
duced in Ruby 2.1, default: 128MB

RUBY_GC_OLDMALLOC_LIMIT_GROWTH_FACTOR Increases the limit of old genera‐
tion malloc allocation, reducing
full GC frequency but increasing
malloc growth until RUBY_GC_OLD‐
MALLOC_LIMIT_MAX is reached.
Introduced in Ruby 2.1, default:
1.2, minimum: 1.0 (no growth)

Stack size environment variables are implementation-dependent and subject
to change with different versions of Ruby. The VM stack is used for
pure-Ruby code and managed by the virtual machine. Machine stack is used
by the operating system and its usage is dependent on C extensions as
well as C compiler options. Using lower values for these may allow
applications to keep more Fibers or Threads running; but increases the
chance of SystemStackError exceptions and segmentation faults (SIGSEGV).
These environment variables are available since Ruby 2.0.0. All values
are specified in bytes.

RUBY_THREAD_VM_STACK_SIZE VM stack size used at thread creation.
default: 131072 (32-bit CPU) or 262144

RUBY_THREAD_MACHINE_STACK_SIZE Machine stack size used at thread cre‐
ation. default: 524288 or 1048575

RUBY_FIBER_VM_STACK_SIZE VM stack size used at fiber creation.
default: 65536 or 131072

RUBY_FIBER_MACHINE_STACK_SIZE Machine stack size used at fiber cre‐
ation. default: 262144 or 524288

SEE ALSO The official web site. Comprehensive catalog of Ruby libraries.


Security vulnerabilities should be reported via an email to
⟨⟩. Reported problems will be published after
they’ve been fixed.

And you can report other bugs and feature requests via the Ruby Issue
Tracking System ( Do not report security
vulnerabilities via the system because it publishes the vulnerabilities

Ruby is designed and implemented by Yukihiro Matsumoto .

See ⟨⟩ for con‐
tributors to Ruby.

UNIX October 31, 2015 UNIX

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