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ERB – Ruby Templating


ERB provides an easy to use but powerful templating system for Ruby. Using ERB, actual Ruby code can be added to any plain text document for the purposes of generating document information details and/or flow control.

A very simple example is this:

require 'erb'

x = 42
template = ERB.new <<-EOF
  The value of x is: <%= x %>
puts template.result(binding)

Prints: The value of x is: 42

More complex examples are given below.

Recognized Tags

ERB recognizes certain tags in the provided template and converts them based on the rules below:

<% Ruby code -- inline with output %>
<%= Ruby expression -- replace with result %>
<%# comment -- ignored -- useful in testing %>
% a line of Ruby code -- treated as <% line %> (optional -- see ERB.new)
%% replaced with % if first thing on a line and % processing is used
<%% or %%> -- replace with <% or %> respectively

All other text is passed through ERB filtering unchanged.


There are several settings you can change when you use ERB:

  • the nature of the tags that are recognized;

  • the value of $SAFE under which the template is run;

  • the binding used to resolve local variables in the template.

See the ::new and #result methods for more detail.

Character encodings

ERB (or ruby code generated by ERB) returns a string in the same character encoding as the input string. When the input string has a magic comment, however, it returns a string in the encoding specified by the magic comment.

# -*- coding: UTF-8 -*-
require 'erb'

template = ERB.new <<EOF
<%#-*- coding: Big5 -*-%>
  \_\_ENCODING\_\_ is <%= \_\_ENCODING\_\_ %>.
puts template.result

Prints: _ENCODING_ is Big5.


Plain Text

ERB is useful for any generic templating situation. Note that in this example, we use the convenient “% at start of line” tag, and we quote the template literally with %q{...} to avoid trouble with the backslash.

require "erb"

# Create template.
template = %q{
  From:  James Edward Gray II <james@grayproductions.net>
  To:  <%= to %>
  Subject:  Addressing Needs

  <%= to[/\w+/] %>:

  Just wanted to send a quick note assuring that your needs are being

  I want you to know that my team will keep working on the issues,

  <%# ignore numerous minor requests -- focus on priorities %>
  % priorities.each do |priority|
    * <%= priority %>
  % end

  Thanks for your patience.

  James Edward Gray II
}.gsub(/^  /, '')

message = ERB.new(template, 0, "%<>")

# Set up template data.
to = "Community Spokesman <spokesman@ruby_community.org>"
priorities = [ "Run Ruby Quiz",
               "Document Modules",
               "Answer Questions on Ruby Talk" ]

# Produce result.
email = message.result
puts email


From:  James Edward Gray II <james@grayproductions.net>
To:  Community Spokesman <spokesman@ruby_community.org>
Subject:  Addressing Needs


Just wanted to send a quick note assuring that your needs are being addressed.

I want you to know that my team will keep working on the issues, especially:

    * Run Ruby Quiz
    * Document Modules
    * Answer Questions on Ruby Talk

Thanks for your patience.

James Edward Gray II

Ruby in HTML

ERB is often used in .rhtml files (HTML with embedded Ruby). Notice the need in this example to provide a special binding when the template is run, so that the instance variables in the Product object can be resolved.

require "erb"

# Build template data class.
class Product
  def initialize( code, name, desc, cost )
    @code = code
    @name = name
    @desc = desc
    @cost = cost

    @features = [ ]

  def add_feature( feature )
    @features << feature

  # Support templating of member data.
  def get_binding

  # ...

# Create template.
template = %{
    <head><title>Ruby Toys -- <%= @name %></title></head>

      <h1><%= @name %> (<%= @code %>)</h1>
      <p><%= @desc %></p>

        <% @features.each do |f| %>
          <li><b><%= f %></b></li>
        <% end %>

        <% if @cost < 10 %>
          <b>Only <%= @cost %>!!!</b>
        <% else %>
           Call for a price, today!
        <% end %>

}.gsub(/^  /, '')

rhtml = ERB.new(template)

# Set up template data.
toy = Product.new( "TZ-1002",
                   "Geek's Best Friend!  Responds to Ruby commands...",
                   999.95 )
toy.add_feature("Listens for verbal commands in the Ruby language!")
toy.add_feature("Ignores Perl, Java, and all C variants.")
toy.add_feature("Karate-Chop Action!!!")
toy.add_feature("Matz signature on left leg.")
toy.add_feature("Gem studded eyes... Rubies, of course!")

# Produce result.

Generates (some blank lines removed):

  <head><title>Ruby Toys -- Rubysapien</title></head>

    <h1>Rubysapien (TZ-1002)</h1>
    <p>Geek's Best Friend!  Responds to Ruby commands...</p>

        <li><b>Listens for verbal commands in the Ruby language!</b></li>
        <li><b>Ignores Perl, Java, and all C variants.</b></li>
        <li><b>Karate-Chop Action!!!</b></li>
        <li><b>Matz signature on left leg.</b></li>
        <li><b>Gem studded eyes... Rubies, of course!</b></li>

         Call for a price, today!



There are a variety of templating solutions available in various Ruby projects:

  • ERB's big brother, eRuby, works the same but is written in C for speed;

  • Amrita (smart at producing HTML/XML);

  • cs/Template (written in C for speed);

  • RDoc, distributed with Ruby, uses its own template engine, which can be reused elsewhere;

  • and others; search the RAA.

Rails, the web application framework, uses ERB to create views.





The optional filename argument passed to Kernel#eval when the ERB code is run


The Ruby code generated by ERB

Public Class Methods

new(str, safe_level=nil, trim_mode=nil, eoutvar='_erbout') click to toggle source

Constructs a new ERB object with the template specified in str.

An ERB object works by building a chunk of Ruby code that will output the completed template when run. If safe_level is set to a non-nil value, ERB code will be run in a separate thread with $SAFE set to the provided level.

If trim_mode is passed a String containing one or more of the following modifiers, ERB will adjust its code generation as listed:

%  enables Ruby code processing for lines beginning with %
<> omit newline for lines starting with <% and ending in %>
>  omit newline for lines ending in %>

eoutvar can be used to set the name of the variable ERB will build up its output in. This is useful when you need to run multiple ERB templates through the same binding and/or when you want to control where output ends up. Pass the name of the variable to be used inside a String.


require "erb"

# build data class
class Listings
  PRODUCT = { :name => "Chicken Fried Steak",
              :desc => "A well messages pattie, breaded and fried.",
              :cost => 9.95 }

  attr_reader :product, :price

  def initialize( product = "", price = "" )
    @product = product
    @price = price

  def build
    b = binding
    # create and run templates, filling member data variables
    ERB.new(<<-'END_PRODUCT'.gsub(/^\s+/, ""), 0, "", "@product").result b
      <%= PRODUCT[:name] %>
      <%= PRODUCT[:desc] %>
    ERB.new(<<-'END_PRICE'.gsub(/^\s+/, ""), 0, "", "@price").result b
      <%= PRODUCT[:name] %> -- <%= PRODUCT[:cost] %>
      <%= PRODUCT[:desc] %>

# setup template data
listings = Listings.new

puts listings.product + "\n" + listings.price


Chicken Fried Steak
A well messages pattie, breaded and fried.

Chicken Fried Steak -- 9.95
A well messages pattie, breaded and fried.
               # File erb.rb, line 783
def initialize(str, safe_level=nil, trim_mode=nil, eoutvar='_erbout')
  @safe_level = safe_level
  compiler = ERB::Compiler.new(trim_mode)
  set_eoutvar(compiler, eoutvar)
  @src, @enc = *compiler.compile(str)
  @filename = nil
version() click to toggle source

Returns revision information for the erb.rb module.

               # File erb.rb, line 260
def self.version
  "erb.rb [2.1.0 #{ERB::Revision.split[1]}]"

Public Instance Methods

def_class(superklass=Object, methodname='result') click to toggle source

Define unnamed class which has methodname as instance method, and return it.


class MyClass_
  def initialize(arg1, arg2)
    @arg1 = arg1;  @arg2 = arg2
filename = 'example.rhtml'  # @arg1 and @arg2 are used in example.rhtml
erb = ERB.new(File.read(filename))
erb.filename = filename
MyClass = erb.def_class(MyClass_, 'render()')
print MyClass.new('foo', 123).render()
               # File erb.rb, line 886
def def_class(superklass=Object, methodname='result')
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
def_method(mod, methodname, fname='(ERB)') click to toggle source

Define methodname as instance method of mod from compiled ruby source.


filename = 'example.rhtml'   # 'arg1' and 'arg2' are used in example.rhtml
erb = ERB.new(File.read(filename))
erb.def_method(MyClass, 'render(arg1, arg2)', filename)
print MyClass.new.render('foo', 123)
               # File erb.rb, line 849
def def_method(mod, methodname, fname='(ERB)')
  src = self.src
  magic_comment = "#coding:#{@enc}\n"
  mod.module_eval do
    eval(magic_comment + "def #{methodname}\n" + src + "\nend\n", binding, fname, -2)
def_module(methodname='erb') click to toggle source

Create unnamed module, define methodname as instance method of it, and return it.


filename = 'example.rhtml'   # 'arg1' and 'arg2' are used in example.rhtml
erb = ERB.new(File.read(filename))
erb.filename = filename
MyModule = erb.def_module('render(arg1, arg2)')
class MyClass
  include MyModule
               # File erb.rb, line 867
def def_module(methodname='erb')
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
result(b=TOPLEVEL_BINDING) click to toggle source

Executes the generated ERB code to produce a completed template, returning the results of that code. (See ERB#new for details on how this process can be affected by safe_level.)

b accepts a Binding or Proc object which is used to set the context of code evaluation.

               # File erb.rb, line 831
def result(b=TOPLEVEL_BINDING)
  if @safe_level
    proc {
      $SAFE = @safe_level
      eval(@src, b, (@filename || '(erb)'), 0)
    eval(@src, b, (@filename || '(erb)'), 0)
run(b=TOPLEVEL_BINDING) click to toggle source

Generate results and print them. (see #result)

               # File erb.rb, line 819
  print self.result(b)
set_eoutvar(compiler, eoutvar = '_erbout') click to toggle source

Can be used to set eoutvar as described in ERB#new. It's probably easier to just use the constructor though, since calling this method requires the setup of an ERB compiler object.

               # File erb.rb, line 803
def set_eoutvar(compiler, eoutvar = '_erbout')
  compiler.put_cmd = "#{eoutvar}.concat"
  compiler.insert_cmd = "#{eoutvar}.concat"

  cmd = []
  cmd.push "#{eoutvar} = ''"

  compiler.pre_cmd = cmd

  cmd = []

  compiler.post_cmd = cmd