ruby man page

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-oriented 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 declarations. 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 referenced 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 features 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 methods, 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 factorial(400).
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 programs 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 programming.
Dynamic loading
On most UNIX systems, you can load object files into the Ruby interpreter on-the-fly.
Rich libraries
In addition to the “builtin libraries” and “standard libraries” that are bundled with Ruby, a vast amount of third-party libraries (“gems”) are available via the package management system called ‘RubyGems’, namely the gem(1) command. Visit ( to find the gems you need, and explore GitHub ( to see how they are being developed and used.


Ruby interpreter accepts following command-line options (switches). They are quite similar to those of perl(1).

Prints the copyright notice.

Prints the version of Ruby interpreter.

(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 encoding

-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
Windows-31J (CP932)

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 $*

Turns on taint checks at the specified level (default 1).

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

Turns on verbose mode at the specified level without printing the version message at the beginning. The level can be;
Verbose mode is "silence". It sets the $VERBOSE to nil.
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

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

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.

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 name.

Prints a summary of the options.

-i extension
Specifies in-place-edit mode. The extension, if specified, 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 

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

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 

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 useful when using -n or -p.

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 pathname, 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 csh(1).

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.

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

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.


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.
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 otherwise.

Ignores (or considers) the RUBYOPT environment variable. By default, Ruby considers the variable.

Disables (or enables) all features.

Dump some informations.

Prints the specified target. target can be one of;

version description same as --version
brief usage message same as -h
Show long help message same as --help
check of syntax same as -c --yydebug
compiler debug mode, same as --yydebug

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

AST nodes tree

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

disassembled instructions

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

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.


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 searched.



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.

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.

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

Ruby refers to the PATH environment variable on calling Kernel#system.

And Ruby depends on some RubyGems related environment variables unless RubyGems is disabled. See the help of gem(1) as below.

% gem help

Gc Environment

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:

Reaching the object limit.
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:

Reaching the old object limit.
Reaching the old malloc limit.

There are currently 4 possible areas where the GC may be tuned by the following 11 environment variables:

Initial allocation slots. Introduced in Ruby 2.1, default: 10000.

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

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

Allocation rate is limited to this number of slots, preventing excessive allocation due to RUBY_GC_HEAP_GROWTH_FACTOR. Introduced in Ruby 2.1, default: 0 (no limit)

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

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

The maximum limit of young generation allocation from malloc before GC starts. Prevents excessive malloc growth due to RUBY_GC_MALLOC_LIMIT_GROWTH_FACTOR. Introduced in Ruby 2.1, default: 32MB.

Increases the limit of young generation malloc calls, reducing GC frequency but increasing malloc growth until RUBY_GC_MALLOC_LIMIT_MAX is reached. Introduced in Ruby 2.1, default: 1.4, minimum: 1.0 (no growth)

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

The maximum limit of old generation allocation from malloc before a full GC starts. Prevents excessive malloc growth due to RUBY_GC_OLDMALLOC_LIMIT_GROWTH_FACTOR. Introduced in Ruby 2.1, default: 128MB

Increases the limit of old generation malloc allocation, reducing full GC frequency but increasing malloc growth until RUBY_GC_OLDMALLOC_LIMIT_MAX is reached. Introduced in Ruby 2.1, default: 1.2, minimum: 1.0 (no growth)

Stack Size Environment

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.

VM stack size used at thread creation. default: 131072 (32-bit CPU) or 262144 (64-bit)

Machine stack size used at thread creation. default: 524288 or 1048575

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

Machine stack size used at fiber creation. default: 262144 or 524288

See Also
The official web site.
Comprehensive catalog of Ruby libraries.

Reporting Bugs

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 immediately.


Ruby is designed and implemented by Yukihiro Matsumoto ⟨⟩.

See ⟨…⟩ for contributors to Ruby.

Referenced By

erb(1), htags-server(1), irb(1), kf5kross(1), kross(1), nbdkit-ruby-plugin(3), ri(1), shotgun(1).

October 31, 2015