class ERB - erb: Ruby Standard Library Documentation

# 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 %> (`<% #` doesn't work. Don't use Ruby comments.)
    % 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 binding used to resolve local variables in the template.

See the `ERB.new` and `ERB#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, trim_mode: "%<>")

    # 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. For example, RDoc, distributed with Ruby, uses its own template engine, which can be reused elsewhere.

Other popular engines could be found in the corresponding Category of The Ruby Toolbox.

### Constants

NOT_GIVEN
VERSION

### 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=NOT_GIVEN, legacy_trim_mode=NOT_GIVEN, legacy_eoutvar=NOT_GIVEN, 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 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', trim_mode: "", eoutvar: "@product").result b
          <%= PRODUCT[:name] %>
          <%= PRODUCT[:desc] %>
        END_PRODUCT
        ERB.new(<<~'END_PRICE', trim_mode: "", eoutvar: "@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 erb.rb, line 334
    def initialize(str, safe_level=NOT_GIVEN, legacy_trim_mode=NOT_GIVEN, legacy_eoutvar=NOT_GIVEN, trim_mode: nil, eoutvar: '_erbout')
      # Complex initializer for $SAFE deprecation at [Feature #14256]. Use keyword arguments to pass trim_mode or eoutvar.
      if safe_level != NOT_GIVEN
        warn 'Passing safe_level with the 2nd argument of ERB.new is deprecated. Do not use it, and specify other arguments as keyword arguments.', uplevel: 1
      end
      if legacy_trim_mode != NOT_GIVEN
        warn 'Passing trim_mode with the 3rd argument of ERB.new is deprecated. Use keyword argument like ERB.new(str, trim_mode: ...) instead.', uplevel: 1
        trim_mode = legacy_trim_mode
      end
      if legacy_eoutvar != NOT_GIVEN
        warn 'Passing eoutvar with the 4th argument of ERB.new is deprecated. Use keyword argument like ERB.new(str, eoutvar: ...) instead.', uplevel: 1
        eoutvar = legacy_eoutvar
      end

      compiler = make_compiler(trim_mode)
      set_eoutvar(compiler, eoutvar)
      @src, @encoding, @frozen_string = *compiler.compile(str)
      @filename = nil
      @lineno = 0
      @_init = self.class.singleton_class
    end

version() click to toggle source

Returns revision information for the erb.rb module.

    # File erb.rb, line 266
    def self.version
      VERSION
    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 erb.rb, line 500
    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 erb.rb, line 464
    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 erb.rb, line 481
    def def_module(methodname='erb')
      mod = Module.new
      def_method(mod, methodname, @filename || '(ERB)')
      mod
    end

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

Sets optional filename and line number that will be used in `ERB` code evaluation and error reporting. See also `filename=` and `lineno=`

    erb = ERB.new('<%= some_x %>')
    erb.render
    # undefined local variable or method `some_x'
    #   from (erb):1

    erb.location = ['file.erb', 3]
    # All subsequent error reporting would use new location
    erb.render
    # undefined local variable or method `some_x'
    #   from file.erb:4

    # File erb.rb, line 394
    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 erb.rb, line 361
    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 `ERB::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 erb.rb, line 424
    def result(b=new_toplevel)
      unless @_init.equal?(self.class.singleton_class)
        raise ArgumentError, "not initialized"
      end
      eval(@src, b, (@filename || '(erb)'), @lineno)
    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 erb.rb, line 433
    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 `ERB#result`)

    # File erb.rb, line 412
    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 `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 404
    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 erb.rb, line 445
    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