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.
The encoding to eval
The optional filename argument passed to Kernel#eval
when the ERB
code is run
The optional lineno argument passed to Kernel#eval
when the ERB
code is run
# File tmp/rubies/ruby-3.2.0/lib/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
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'.gsub(/^\s+/, ""), trim_mode: "", eoutvar: "@product").result b <%= PRODUCT[:name] %> <%= PRODUCT[:desc] %> END_PRODUCT ERB.new(<<-'END_PRICE'.gsub(/^\s+/, ""), 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 tmp/rubies/ruby-3.2.0/lib/erb.rb, line 266
def self.version
VERSION
end
Returns revision information for the erb.rb module.
# File tmp/rubies/ruby-3.2.0/lib/erb.rb, line 500
def def_class(superklass=Object, methodname='result')
cls = Class.new(superklass)
def_method(cls, methodname, @filename || '(ERB)')
cls
end
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 tmp/rubies/ruby-3.2.0/lib/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
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 tmp/rubies/ruby-3.2.0/lib/erb.rb, line 481
def def_module(methodname='erb')
mod = Module.new
def_method(mod, methodname, @filename || '(ERB)')
mod
end
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 tmp/rubies/ruby-3.2.0/lib/erb.rb, line 394
def location=((filename, lineno))
@filename = filename
@lineno = lineno if lineno
end
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 tmp/rubies/ruby-3.2.0/lib/erb.rb, line 361
def make_compiler(trim_mode)
ERB::Compiler.new(trim_mode)
end
Creates a new compiler for ERB
. See ERB::Compiler.new for details
# File tmp/rubies/ruby-3.2.0/lib/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
Returns a new binding each time near TOPLEVEL_BINDING for runs that do not specify a binding.
# File tmp/rubies/ruby-3.2.0/lib/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
# File tmp/rubies/ruby-3.2.0/lib/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
Render a template on a new toplevel binding with local variables specified by a Hash
object.
# File tmp/rubies/ruby-3.2.0/lib/erb.rb, line 412
def run(b=new_toplevel)
print self.result(b)
end
Generate results and print them. (see ERB#result
)
# File tmp/rubies/ruby-3.2.0/lib/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