Ruby Guide - Chef 18

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Ruby is a simple programming language:

Chef uses Ruby as its reference language to define the patterns that are found in resources, recipes, and cookbooks
Use these patterns to configure, deploy, and manage nodes across the network

Ruby is also a powerful and complete programming language:

Use the Ruby programming language to make decisions about what should happen to specific resources and recipes
Extend Chef in any manner that your organization requires

To learn more about Ruby, see:

Chef Infra Client 15 ships with Ruby 2.6 and Chef Infra Client 16 ships with Ruby 2.7.

Ruby Basics

This section covers the basics of Ruby.

Verify Syntax

Many people who are new to Ruby often find that it doesn’t take very long to get up to speed with the basics. For example, it’s useful to know how to check the syntax of a Ruby file, such as the contents of a cookbook named my_cookbook.rb:

ruby -c my_cookbook_file.rb

to return:

Syntax OK

Comments

Use a comment to explain code that exists in a cookbook or recipe. Anything after a # is a comment.

# This is a comment.

Local Variables

Assign a local variable:

x = 1

Math

Do some basic arithmetic:

1 + 2           # => 3
2 * 7           # => 14
5 / 2           # => 2   (because both arguments are whole numbers)
5 / 2.0         # => 2.5 (because one of the numbers had a decimal place)
1 + (2 * 3)     # => 7   (you can use parentheses to group expressions)

Strings

Work with strings:

'single quoted'   # => "single quoted"
"double quoted"   # => "double quoted"
'It\'s alive!'    # => "It's alive!" (the \ is an escape character)
'1 + 2 = 5'       # => "1 + 2 = 5" (numbers surrounded by quotes behave like strings)

Convert a string to uppercase or lowercase. For example, a hostname named “Foo”:

node['hostname'].downcase    # => "foo"
node['hostname'].upcase      # => "FOO"

Ruby in Strings

Embed Ruby in a string:

x = 'Bob'
"Hi, #{x}"      # => "Hi, Bob"
'Hello, #{x}'   # => "Hello, \#{x}" Notice that single quotes don't work with #{}

Escape Character

Use the backslash character (\) as an escape character when quotes must appear within strings. However, you do not need to escape single quotes inside double quotes. For example:

'It\'s alive!'                        # => "It's alive!"
"Won\'t you read Grant\'s book?"      # => "Won't you read Grant's book?"

Interpolation

When strings have quotes within quotes, use double quotes (" ") on the outer quotes, and then single quotes (' ') for the inner quotes. For example:

Chef::Log.info("Loaded from aws[#{aws['id']}]")

"node['mysql']['secretpath']"

"#{ENV['HOME']}/chef.txt"

antarctica_hint = hint?('antarctica')
if antarctica_hint['snow']
  "There are #{antarctica_hint['penguins']} penguins here."
else
  'There is no snow here, and penguins like snow.'
end

Truths

Work with basic truths:

true            # => true
false           # => false
nil             # => nil
0               # => true ( the only false values in Ruby are false
                #    and nil; in other words: if it exists in Ruby,
                #    even if it exists as zero, then it is true.)
1 == 1          # => true ( == tests for equality )
1 == true       # => false ( == tests for equality )

Untruths

Work with basic untruths (! means not!):

!true           # => false
!false          # => true
!nil            # => true
1 != 2          # => true (1 is not equal to 2)
1 != 1          # => false (1 is not equal to itself)

Convert Truths

Convert something to either true or false (!! means not not!!):

!!true          # => true
!!false         # => false
!!nil           # => false (when pressed, nil is false)
!!0             # => true (zero is NOT false).

Arrays

Create lists using arrays:

x = ['a', 'b', 'c']   # => ["a", "b", "c"]
x[0]                  # => "a" (zero is the first index)
x.first               # => "a" (see?)
x.last                # => "c"
x + ['d']             # => ["a", "b", "c", "d"]
x                     # => ["a", "b", "c"] ( x is unchanged)
x = x + ['d']         # => ["a", "b", "c", "d"]
x                     # => ["a", "b", "c", "d"]

Whitespace Arrays

The %w syntax is a Ruby shortcut for creating an array without requiring quotes and commas around the elements.

For example:

if %w(debian ubuntu).include?(node['platform'])
  # do debian/ubuntu things with the Ruby array %w() shortcut
end

When %w syntax uses a variable, such as |foo|, double quoted strings should be used.

Right:

%w(openssl.cnf pkitool vars Rakefile).each do |foo|
  template "/etc/openvpn/easy-rsa/#{foo}" do
    source "#{foo}.erb"
    ...
  end
end

Wrong:

%w(openssl.cnf pkitool vars Rakefile).each do |foo|
  template '/etc/openvpn/easy-rsa/#{foo}' do
    source '#{foo}.erb'
    ...
  end
end

Example

WiX includes several tools – such as candle (preprocesses and compiles source files into object files), light (links and binds object files to an installer database), and heat (harvests files from various input formats). The following example uses a whitespace array and the Chef InSpec file audit resource to verify if these three tools are present:

%w(
  candle.exe
  heat.exe
  light.exe
).each do |utility|
  describe file("C:/wix/#{utility}") do
    it { should be_file }
  end
end

Hash

A Hash is a list with keys and values. Sometimes hashes don’t have a set order:

h = {
  'first_name' => 'Bob',
  'last_name'  => 'Jones',
}

And sometimes they do. For example, first name then last name:

h.keys              # => ["first_name", "last_name"]
h['first_name']     # => "Bob"
h['last_name']      # => "Jones"
h['age'] = 23
h.keys              # => ["first_name", "age", "last_name"]
h.values            # => ["Jones", "Bob", 23]

Regular Expressions

Use Perl-style regular expressions:

'I believe'  =~ /I/                       # => 0 (matches at the first character)
'I believe'  =~ /lie/                     # => 4 (matches at the 5th character)
'I am human' =~ /bacon/                   # => nil (no match - bacon comes from pigs)
'I am human' !~ /bacon/                   # => true (correct, no bacon here)
/give me a ([0-9]+)/ =~ 'give me a 7'     # => 0 (matched)

Statements

Use conditions! For example, an if statement

if false
  # this won't happen
elsif nil
  # this won't either
else
  # code here will run though
end

or a case statement:

x = 'dog'
case x
when 'fish'
 # this won't happen
when 'dog', 'cat', 'monkey'
  # this will run
else
  # the else is an optional catch-all
end

if

An if statement can be used to specify part of a recipe to be used when certain conditions are met. else and elsif statements can be used to handle situations where either the initial condition is not met or when there are other possible conditions that can be met. Since this behavior is 100% Ruby, do this in a recipe the same way here as anywhere else.

For example, using an if statement with the platform node attribute:

if node['platform'] == 'ubuntu'
  # do ubuntu things
end

if modifier

if can be used as a modifier that executes the left side of an expression if the right side of the expression is true. The if modifier expression must be a single line, and else and elsif statements are not supported.

In the following example, the do_ubuntu_thing function will execute if the platform on a node is Ubuntu.

do_ubuntu_thing if platform?('ubuntu')

case

A case statement can be used to handle a situation where there are a lot of conditions. Use the when statement for each condition, as many as are required.

For example, using a case statement with the platform node attribute:

case node['platform']
when 'debian', 'ubuntu'
  # do debian/ubuntu things
when 'redhat', 'centos', 'fedora'
  # do redhat/centos/fedora things
end

For example, using a case statement with the platform_family node attribute:

case node['platform_family']
when 'debian'
  # do things on debian-ish platforms (debian, ubuntu, linuxmint)
when 'rhel'
  # do things on RHEL platforms (redhat, centos, scientific, etc)
end

Call a Method

Call a method on something with .method_name():

x = 'My String'
x.split(' ')            # => ["My", "String"]
x.split(' ').join(', ') # => "My, String"

Define a Method

Define a method (or a function, if you like):

def do_something_useless(first_argument, second_argument)
  puts "You gave me #{first_argument} and #{second_argument}"
end

do_something_useless('apple', 'banana')
# => "You gave me apple and banana"
do_something_useless 1, 2
# => "You gave me 1 and 2"
# see how the parentheses are optional if there's no confusion about what to do

Ruby Class

Use the Ruby File class in a recipe. Because Chef has the file resource, use File to use the Ruby File class. For example:

execute 'apt-get-update' do
  command 'apt-get update'
  ignore_failure true
  not_if { ::File.exist?('/var/lib/apt/periodic/update-success-stamp') }
end

Include a Class

Use :include to include another Ruby class. For example:

::Chef::DSL::Recipe.include MyCookbook::Helpers

In non-Chef Ruby, the syntax is include (without the : prefix), but without the : prefix Chef Infra Client will try to find a provider named include. Using the : prefix tells Chef Infra Client to look for the specified class that follows.

Include a Parameter

The include? method can be used to ensure that a specific parameter is included before an action is taken. For example, using the include? method to find a specific parameter:

if %w(debian ubuntu).include?(node['platform'])
  # do debian/ubuntu things
end

or:

if %w(rhel).include?(node['platform_family'])
  # do RHEL things
end

Patterns to Follow

This section covers best practices for cookbook and recipe authoring.

git Etiquette

Although not strictly a Chef style thing, please always ensure your user.name and user.email are set properly in your .gitconfig file.

user.name should be your given name (e.g., “Julian Dunn”)
user.email should be an actual, working e-mail address

This will prevent commit log entries similar to "guestuser <[email protected]>", which are unhelpful.

Use of Hyphens

Cookbook and custom resource names should contain only alphanumeric characters. A hyphen (-) is a valid character and may be used in cookbook and custom resource names, but it is discouraged. Chef Infra Client will return an error if a hyphen is not converted to an underscore (_) when referencing from a recipe the name of a custom resource in which a hyphen is located.

Cookbook Naming

Use a short organizational prefix for application cookbooks that are part of your organization. For example, if your organization is named SecondMarket, use sm as a prefix: sm_postgresql or sm_httpd.

Cookbook Versioning

Use semantic versioning when numbering cookbooks.
Only upload stable cookbooks from master.
Only upload unstable cookbooks from the dev branch. Merge to master and bump the version when stable.
Always update CHANGELOG.md with any changes, with the JIRA ticket and a brief description.

Naming

Name things uniformly for their system and component. For example:

attributes: node['foo']['bar']
recipe: foo::bar
role: foo-bar
directories: foo/bar (if specific to component), foo (if not). For example: /var/log/foo/bar.

Name attributes after the recipe in which they are primarily used. e.g. node['postgresql']['server'].

Parameter Order

Follow this order for information in each resource declaration:

Source
Cookbook
Resource ownership
Permissions
Notifications
Action

For example:

template '/tmp/foobar.txt' do
  source 'foobar.txt.erb'
  owner  'someuser'
  group  'somegroup'
  mode   '0644'
  variables(
    foo: 'bar'
  )
  notifies :reload, 'service[whatever]'
  action :create
end

File Modes

Always specify the file mode with a quoted 3-5 character string that defines the octal mode:

mode '755'

mode '0755'

Wrong:

mode 755

Specify Resource Action?

A resource declaration does not require the action to be specified because Chef Infra Client will apply the default action for a resource automatically if it’s not specified within the resource block. For example:

package 'monit'

will install the monit package because the :install action is the default action for the package resource.

However, if readability of code is desired, such as ensuring that a reader understands what the default action is for a custom resource or stating the action for a resource whose default may not be immediately obvious to the reader, specifying the default action is recommended:

ohai 'apache_modules' do
  action :reload
end

String Quoting

Use single-quoted strings in all situations where the string doesn’t need interpolation.

Whitespace Arrays

When %w syntax uses a variable, such as |foo|, double quoted strings should be used.

Right:

%w(openssl.cnf pkitool vars Rakefile).each do |foo|
  template "/etc/openvpn/easy-rsa/#{foo}" do
    source "#{foo}.erb"
    ...
  end
end

Wrong:

%w(openssl.cnf pkitool vars Rakefile).each do |foo|
  template '/etc/openvpn/easy-rsa/#{foo}' do
    source '#{foo}.erb'
    ...
  end
end

Recipes

A recipe should be clean and well-commented. For example:

###########
# variables
###########

connection_info = {
  host: '127.0.0.1',
  port: '3306',
  username: 'root',
  password: 'm3y3sqlr00t',
}

#################
# Mysql resources
#################

mysql_service 'default' do
  port '3306'
  initial_root_password 'm3y3sqlr00t'
  action [:create, :start]
end

mysql_database 'wordpress_demo' do
  connection connection_info
  action :create
end

mysql_database_user 'wordpress_user' do
  connection connection_info
  database_name 'wordpress_demo'
  password 'w0rdpr3ssdem0'
  privileges [:create, :delete, :select, :update, :insert]
  action :grant
end

##################
# Apache resources
##################

httpd_service 'default' do
  listen_ports %w(80)
  mpm 'prefork'
  action [:create, :start]
end

httpd_module 'php' do
  notifies :restart, 'httpd_service[default]'
  action :create
end

###############
# Php resources
###############

package 'php-gd' do
  action :install
end

package 'php-mysql' do
  action :install
end

directory '/etc/php.d' do
  action :create
end

template '/etc/php.d/mysql.ini' do
  source 'mysql.ini.erb'
  action :create
end

httpd_config 'php' do
  source 'php.conf.erb'
  notifies :restart, 'httpd_service[default]'
  action :create
end

#####################
# wordpress resources
#####################

directory '/srv/wordpress_demo' do
  user 'apache'
  recursive true
  action :create
end

tar_extract 'https://wordpress.org/wordpress-4.1.tar.gz' do
  target_dir '/srv/wordpress_demo'
  tar_flags ['--strip-components 1']
  user 'apache'
  creates '/srv/wordpress_demo/index.php'
  action :extract
end

directory '/srv/wordpress_demo/wp-content' do
  user 'apache'
  action :create
end

httpd_config 'wordpress' do
  source 'wordpress.conf.erb'
  variables(
    servername: 'wordpress',
    server_aliases: %w(computers.biz www.computers.biz),
    document_root: '/srv/wordpress_demo'
  )
  notifies :restart, 'httpd_service[default]'
  action :create
end

template '/srv/wordpress_demo/wp-config.php' do
  source 'wp-config.php.erb'
  owner 'apache'
  variables(
    db_name: 'wordpress_demo',
    db_user: 'wordpress_user',
    db_password: 'w0rdpr3ssdem0',
    db_host: '127.0.0.1',
    db_prefix: 'wp_',
    db_charset: 'utf8',
    auth_key: 'You should probably use randomly',
    secure_auth_key: 'generated strings. These can be hard',
    logged_in_key: 'coded, pulled from encrypted databags,',
    nonce_key: 'or a ruby function that accessed an',
    auth_salt: 'arbitrary data source, such as a password',
    secure_auth_salt: 'vault. Node attributes could work',
    logged_in_salt: 'as well, but you take special care',
    nonce_salt: 'so they are not saved to your chef-server.',
    allow_multisite: 'false'
  )
  action :create
end

Cookstyle Linting

Chef Workstation includes Cookstyle for linting the Ruby-specific and Chef-specific portions of your cookbook code. All cookbooks should pass Cookstyle rules before being uploaded.

cookstyle your-cookbook

should return no offenses detected