# class ERB

# ERB – Ruby Templating¶ ↑

## Introduction¶ ↑

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 %>
    EOF
    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.

## Options¶ ↑

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\_\_ %>.
    EOF
    puts template.result

Prints: _ENCODING_ is Big5.

## Examples¶ ↑

### 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
      addressed.

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

      <%# 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

Generates:

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

    Community:

    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 = [ ]
      end

      def add_feature( feature )
        @features << feature
      end

      # Support templating of member data.
      def get_binding
        binding
      end

      # ...
    end

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

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

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

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

        </body>
      </html>
    }.gsub(/^  /, '')

    rhtml = ERB.new(template)

    # Set up template data.
    toy = Product.new( "TZ-1002",
                       "Rubysapien",
                       "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.
    rhtml.run(toy.get_binding)

Generates (some blank lines removed):

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

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

        <ul>
            <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>
        </ul>

        <p>
             Call for a price, today!
        </p>

      </body>
    </html>

## Notes¶ ↑

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 RubyGems.org or The Ruby Toolbox.

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

### Attributes

encoding[R]

The encoding to eval

filename[RW]

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

lineno[RW]

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

src[R]

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 %>
    -  omit blank 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.

### Example¶ ↑

    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
      end

      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] %>
        END_PRODUCT
        ERB.new(<<-'END_PRICE'.gsub(/^\s+/, ""), 0, "", "@price").result b
          <%= PRODUCT[:name] %> -- <%= PRODUCT[:cost] %>
          <%= PRODUCT[:desc] %>
        END_PRICE
      end
    end

    # setup template data
    listings = Listings.new
    listings.build

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

Generates

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

    Chicken Fried Steak -- 9.95
    A well messages pattie, breaded and fried.

    # File lib/erb.rb, line 807
    def initialize(str, safe_level=nil, trim_mode=nil, eoutvar='_erbout')
      @safe_level = safe_level
      compiler = make_compiler(trim_mode)
      set_eoutvar(compiler, eoutvar)
      @src, @encoding, @frozen_string = *compiler.compile(str)
      @filename = nil
      @lineno = 0
    end

version() click to toggle source

Returns revision information for the erb.rb module.

    # File lib/erb.rb, line 263
    def self.version
      "erb.rb [2.1.0 #{ERB::Revision.split[1]}]"
    end

### 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.

example:

    class MyClass_
      def initialize(arg1, arg2)
        @arg1 = arg1;  @arg2 = arg2
      end
    end
    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 lib/erb.rb, line 947
    def def_class(superklass=Object, methodname='result')
      cls = Class.new(superklass)
      def_method(cls, methodname, @filename || '(ERB)')
      cls
    end

def_method(mod, methodname, fname='(ERB)') click to toggle source

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

example:

    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 lib/erb.rb, line 911
    def def_method(mod, methodname, fname='(ERB)')
      src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
      mod.module_eval do
        eval(src, binding, fname, -1)
      end
    end

def_module(methodname='erb') click to toggle source

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

example:

    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
    end

    # File lib/erb.rb, line 928
    def def_module(methodname='erb')
      mod = Module.new
      def_method(mod, methodname, @filename || '(ERB)')
      mod
    end

location=((filename, lineno)) click to toggle source

    # File lib/erb.rb, line 837
    def location=((filename, lineno))
      @filename = filename
      @lineno = lineno if lineno
    end

make_compiler(trim_mode) click to toggle source

Creates a new compiler for ERB. See ERB::Compiler.new for details

    # File lib/erb.rb, line 819
    def make_compiler(trim_mode)
      ERB::Compiler.new(trim_mode)
    end

result(b=new_toplevel) click to toggle source

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

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

    # File lib/erb.rb, line 867
    def result(b=new_toplevel)
      if @safe_level
        proc {
          $SAFE = @safe_level
          eval(@src, b, (@filename || '(erb)'), @lineno)
        }.call
      else
        eval(@src, b, (@filename || '(erb)'), @lineno)
      end
    end

result_with_hash(hash) click to toggle source

Render a template on a new toplevel binding with local variables specified by a Hash object.

    # File lib/erb.rb, line 880
    def result_with_hash(hash)
      b = new_toplevel(hash.keys)
      hash.each_pair do |key, value|
        b.local_variable_set(key, value)
      end
      result(b)
    end

run(b=new_toplevel) click to toggle source

Generate results and print them. (see #result)

    # File lib/erb.rb, line 855
    def run(b=new_toplevel)
      print self.result(b)
    end

set_eoutvar(compiler, eoutvar = '_erbout') click to toggle source

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

    # File lib/erb.rb, line 847
    def set_eoutvar(compiler, eoutvar = '_erbout')
      compiler.put_cmd = "#{eoutvar}.<<"
      compiler.insert_cmd = "#{eoutvar}.<<"
      compiler.pre_cmd = ["#{eoutvar} = +''"]
      compiler.post_cmd = [eoutvar]
    end

### Private Instance Methods

new_toplevel(vars = nil) click to toggle source

Returns a new binding each time near TOPLEVEL_BINDING for runs that do not specify a binding.

    # File lib/erb.rb, line 892
    def new_toplevel(vars = nil)
      b = TOPLEVEL_BINDING
      if vars
        vars = vars.select {|v| b.local_variable_defined?(v)}
        unless vars.empty?
          return b.eval("tap {|;#{vars.join(',')}| break binding}")
        end
      end
      b.dup
    end