Writing Custom Knife Plugins

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A knife plugin is a set of one (or more) subcommands that can be added to knife to support additional functionality that is not built-in to the base set of knife subcommands. Many of the knife plugins are built by members of the Chef community and several of them are built and maintained by Chef.

The Chef Infra Client will load knife plugins from the following locations:

• The home directory: ~/.chef/plugins/knife/
• A .chef/plugins/knife directory in the cookbook repository
• A plugin installed from RubyGems. (For more information about releasing a plugin on RubyGems, see: http://guides.rubygems.org/make-your-own-gem/.)

This approach allows knife plugins to be reused across projects in the home directory, kept in a repository that is accessible to other team members, and distributable to the community using RubyGems.

Syntax

There are many ways to structure a knife plugin. The following syntax shows a typical knife plugin:

require 'chef/knife'
# other require attributes, as needed

module ModuleName
  class SubclassName < Chef::Knife

    deps do
      require 'chef/dependency'
      # other dependencies, as needed
    end

    banner "knife subcommand argument VALUE (options)"

    option :name_of_option,
      :short => "-l VALUE",
      :long => "--long-option-name VALUE",
      :description => "The description for the option.",
      :proc => Proc.new { code_to_run }
      :boolean => true | false
      :default => default_value

    def run
      # Ruby code goes here
    end
  end
end

where:

• require identifies any other knife subcommands and/or knife plugins required by this plugin
• module ModuleName declares the knife plugin as its own namespace
• class SubclassName < Chef::Knife declares the plugin as a subclass of Knife, which is in the Chef namespace. The capitalization of this name is important. For example, SubclassName would have a knife command of knife subclass name, whereas Subclassname would have a knife command of knife subclassname
• deps do is a list of dependencies
• banner "knife subcommand argument VALUE (options)" is displayed when a user enters knife subclassName --help
• option :name_of_option defines each of the command-line options that are available for this plugin. For example, knife subclass -l VALUE or knife subclass --long-option-name VALUE
• def run is the Ruby code that is executed when the command is run

and where for each command-line option:

• :short defines the short option name
• :long defines the long option name
• :description defines a description that is displayed when a user enters knife subclassName --help
• :boolean defines whether the option is true or false; if the :short and :long names define a VALUE, then this attribute must not be used
• :proc defines code that determines the value for this option
• :default defines a default value

The following example shows part of a knife plugin named knife windows:

require 'chef/knife'
require 'chef/knife/winrm_base'

class Chef
  class Knife
    class Winrm < Knife

      include Chef::Knife::WinrmBase

      deps do
        require 'readline'
        require 'chef/search/query'
        require 'em-winrm'
      end

      attr_writer :password

      banner "knife winrm QUERY COMMAND (options)"

      option :attribute,
        :short => "-a ATTR",
        :long => "--attribute ATTR",
        :description => "The attribute to use for opening the connection - default is fqdn",
        :default => "fqdn"

      ... # more options

      def session
        session_opts = {}
        session_opts[:logger] = Chef::Log.logger if Chef::Log.level == :debug
        @session ||= begin
          s = EventMachine::WinRM::Session.new(session_opts)
          s.on_output do |host, data|
            print_data(host, data)
          end
          s.on_error do |host, err|
            print_data(host, err, :red)
          end
          s.on_command_complete do |host|
            host = host == :all ? 'All Servers' : host
            Chef::Log.debug("command complete on #{host}")
          end
          s
        end

      end

      ... # more def blocks

    end
  end
end

Take a look at all of the code for this plugin on GitHub: https://github.com/chef/knife-windows/blob/main/lib/chef/knife/winrm.rb.

Namespace

A knife plugin should have its own namespace (even though knife will load a command regardless of its namespace). The namespace is declared using the module method, for example:

require 'chef/knife'
## other require attributes, as needed

module MyNamespace
  class SubclassName < Chef::Knife

where module MyNamespace declares that the knife plugin has its own namespace, with a namespace of MyNamespace.

Class Name

The class name declares a plugin as a subclass of both Knife and Chef. For example:

class SubclassName < Chef::Knife

where SubclassName is the class name used by this plugin. The capitalization of this name is important. For example, OMG would have a knife command of knife o m g, whereas Omg would have a knife command of knife omg. Use the capitalization pattern to define the word grouping that best makes sense for the plugin.

A plugin can override an existing knife subcommand by using the same class name as the existing subcommand. For example, to override the current functionality of knife cookbook upload, use the following class name:

class CookbookUpload < Chef::Knife

Banner

A banner displays the syntax for the plugin to users when they enter the --help option. Use the banner method in the class body similar to the following:

module example
  class example < Chef::Knife

  banner "knife example"

  ...
end

and the when a user enters knife --help, the following will be displayed:

**EXAMPLE COMMANDS**
knife example

Dependencies

The functionality of other knife plugins can be accessed from a plugin by using the deps method to ensure the necessary files are available. The deps method acts as a lazy loader, ensuring that dependencies are only loaded into knife when the plugin which requires them is run. Use the following syntax just below the class declaration:

class subclassName < Chef::Knife

deps do
  require 'chef/knife/name_of_command'
  require 'chef/search/query'
  # other dependencies, as needed
end

where the actual path may vary from plugin to plugin, but is typically located in the chef/knife/ directory.

Requirements

The functionality of other knife plugins can be accessed from a plugin by using the require method to ensure the necessary files are available, and then within the code for the plugin, to create a new object of the class of the plugin to be used.

First, ensure that the correct files are available using the following syntax:

require 'chef/knife/name_of_command'

where the actual path may vary from plugin to plugin, but is typically located in the chef/knife/ directory.

For example, use the following to require a plugin named bootstrap:

require 'chef/knife/bootstrap'

Next, for the required plugin, create an object of that plugin, like this:

bootstrap = Chef::Knife::Bootstrap.new

and then pass arguments or options to that object. This is done by altering that object’s config and name_arg variables. For example:

bootstrap.config[:ssh_user] = "myuser"
bootstrap.config[:distro] = "ubuntu10.04-gems"
bootstrap.config[:use_sudo] = true

bootstrap.name_args = "some_host_name"

where the available configuration objects vary from plugin to plugin. Make sure those configuration objects are correct by verifying them in the source files for each plugin.

And then call the object’s run method, like this:

bootstrap.run

Options

Command-line options can be added to a knife plugin using the option method. An option can have a true/false value:

option :true_or_false,
  :short => "-t",
  :long => "--true-or-false",
  :description => "Is this value true? Or is this value false?",
  :boolean => true | false
  :default => true

and it can have a string value:

option :some_type_of_string_value,
  :short => "-s VALUE",
  :long => "--some-type-of-string-value VALUE",
  :description => "This is not a random string value.",
  :default => 47

and can specify code that is run to determine the option’s value:

option :tags,
  :short => "-T T=V[,T=V,...]",
  :long => "--tags Tag=Value[,Tag=Value...]",
  :description => "A list of tags associated with the virtual machine",
  :proc => Proc.new { |tags| tags.split(',') }

where the knife command allows a comma-separated list of values and the :proc attribute converts that list of values into an array.

When a user enters knife --help, the description attributes are displayed as part of the help. Using the previous examples, something like the following will be displayed:

**EXAMPLE COMMANDS**
knife example
  -s, --some-type-of-string-value     This is not a random string value.
  -t, --true-or-false                 Is this value true? Or is this value false?
  -T, --tags                          A list of tags associated with the virtual machine.

When knife runs the command, the options are parsed from the command-line and make the settings available as a hash that can be used to access the config method. For example, the following option:

option :omg,
  :short => '-O',
  :long => '--omg',
  :boolean => true,
  :description => "I'm so excited!"

can be used to update the run method of a class to change its behavior based on the config flag, similar to the following:

def run
  if config[:omg]
    # Oh yeah, we are pumped.
    puts "OMG HELLO WORLD!!!1!!11"
  else
    # meh
    puts "I am just a boring example."
  end
end

For a knife plugin with the --omg option, run knife example --omg to return something like:

OMG HELLO WORLD!!!1!!11

or just knife example to return:

I am just a boring example.

Arguments

A knife plugin can also take command-line arguments that are not specified using the option flag, for example: knife node show NODE. These arguments are added using the name_args method. For example:

banner "knife hello world WHO"

def run
  unless name_args.size == 1
    puts "You need to say hello to someone!"
    show_usage
    exit 1
  end

  who = name_args.first

  if config[:omg]
    puts "OMG HELLO #{who.upcase}!!!1!!11"
  else
    puts "Hello, #{who.capitalize}!"
  end
end

where

• unless name_args.size == 1 is used to check the number of arguments given; the command should fail if the input does not make sense
• who = name_args.first is used to access arguments using name_args
• show_usage is used to display the correct usage before exiting (if the command fails)

For example, the following command:

knife hello world

will return:

You need to say hello to someone!
USAGE: knife hello world WHO

the following command:

knife hello world chefs

will return:

Hello, Chefs!

and the following command:

knife hello world chefs --omg

will return:

OMG HELLO CHEFS!!!1!!11

config.rb Settings

Certain settings defined by a knife plugin can be configured so that they can be set using the config.rb file. This can be done in two ways:

• By using the :proc attribute of the option method and code that references Chef::Config[:knife][:setting_name]
• By specifying the configuration setting directly within the def Ruby blocks using either Chef::Config[:knife][:setting_name] or config[:setting_name]

An option that is defined in this manner may be configured using the config.rb file with the following syntax:

knife[:setting_name]

This approach can be useful when a particular setting is used a lot. The order of precedence for a knife option is:

1. A value passed via the command line
2. A value saved in the config.rb file
3. A default value

The following example shows how the knife bootstrap subcommand checks for a value in the config.rb file by using the :proc attribute:

option :ssh_port,
  :short => "-p PORT",
  :long => "--ssh-port PORT",
  :description => "The ssh port",
  :proc => Proc.new { |key| Chef::Config[:knife][:ssh_port] = key }

where Chef::Config[:knife][:ssh_port] tells knife to check the config.rb file for a setting named knife[:ssh_port].

And the following example shows the knife bootstrap subcommand calling the knife ssh subcommand for the actual SSH part of running a bootstrap operation:

def knife_ssh
  ssh = Chef::Knife::Ssh.new
  ssh.ui = ui
  ssh.name_args = [ server_name, ssh_command ]
  ssh.config[:ssh_user] = Chef::Config[:knife][:ssh_user] || config[:ssh_user]
  ssh.config[:ssh_password] = config[:ssh_password]
  ssh.config[:ssh_port] = Chef::Config[:knife][:ssh_port] || config[:ssh_port]
  ssh.config[:ssh_gateway] = Chef::Config[:knife][:ssh_gateway] || config[:ssh_gateway]
  ssh.config[:identity_file] = Chef::Config[:knife][:identity_file] || config[:identity_file]
  ssh.config[:manual] = true
  ssh.config[:host_key_verify] = Chef::Config[:knife][:host_key_verify] || config[:host_key_verify]
  ssh.config[:on_error] = :raise
  ssh
end

where

• ssh = Chef::Knife::Ssh.new creates a new instance of the Ssh subclass named ssh
• A series of settings in knife ssh are associated with knife bootstrap using the ssh.config[:setting_name] syntax
• Chef::Config[:knife][:setting_name] tells knife to check the config.rb file for various settings
• Raises an exception if any aspect of the SSH operation fails

Search

Use the Chef Infra Server search capabilities from a plugin to return information about the infrastructure to that plugin. Use the require method to ensure that search functionality is available with the following:

require 'chef/search/query'

Create a search query object and assign it to a variable:

variable_name = Chef::Search::Query.new

After the search object is created it can be used by the plugin to execute search queries for objects on the Chef Infra Server. For example, using a variable named query_nodes a plugin could search for nodes with the webserver role and then return the name of each node found:

query = "role:webserver"

query_nodes.search('node', query) do |node_item|
  puts "Node Name: #{node_item.name}"
end

This result can then be used to edit nodes. For example, searching for nodes with the webserver role, and then changing the run_list for those nodes to a role named apache2:

query = "role:webserver"

query_nodes.search('node', query) do |node_item|
  ui.msg "Changing the run_list to role[apache2] for #{node_item.name}"
  node_item.run_list("role[apache2]")
  node_item.save
  ui.msg "New run_list: #{node_item.run_list}"
end

It’s also possible to specify multiple items to add to the run_list:

node_item.run_list("role[apache2]", "recipe[mysql]")

And arguments sent with a plugin command can also be used to search. For example, if the command knife envchange "web*" is sent, then the command will search for any nodes in roles beginning with “web” and then change their environment to “web”:

module MyKnifePlugins

  class Envchange < Chef::Knife

    banner "knife envchange ROLE"

    deps do
      require 'chef/search/query'
    end

    def run
      if name_args.size == 1
        role = name_args.first
      else
        ui.fatal "Please provide a role name to search for"
        exit 1
      end

      query = "role:#{role}"
      query_nodes = Chef::Search::Query.new

      query_nodes.search('node', query) do |node_item|
        ui.msg "Moving #{node_item.name} to the web environment"
        node_item.chef_environment("web")
        node_item.save
      end

    end
  end

User Interaction

The ui object provides a set of methods that can be used to define user interactions and to help ensure a consistent user experience across knife plugins. The following methods should be used in favor of manually handling user interactions:

server_list = [
  ui.color('Instance ID', :bold),
  ui.color('Name', :bold),
  ui.color('Public IP', :bold),
  ui.color('Private IP', :bold),
  ui.color('Flavor', :bold),
  ui.color('Image', :bold),
  ui.color('State', :bold)
]

server = connection.servers.create(server_def)
  puts "#{ui.color("Instance ID", :cyan)}: #{server.id}"
  puts "#{ui.color("Flavor", :cyan)}: #{server.flavor_id}"
  puts "#{ui.color("Image", :cyan)}: #{server.image_id}"
  ...
  puts "#{ui.color("SSH Key", :cyan)}: #{server.key_name}"
print "\n#{ui.color("Waiting for server", :magenta)}"

For example, to show a fatal error in a plugin in the same way that it would be shown in knife do something similar to the following:

unless name_args.size == 1
  ui.fatal "Be sure to say hello to someone!"
  show_usage
  exit 1
end

Create a Plugin

A knife command is a Ruby class that inherits from the Chef::Knife class. A knife command is run by calling the run method on an instance of the command class. For example:

module MyKnifePlugins
  class HelloWorld < Chef::Knife

    def run
      puts "Hello, World!"
    end
  end
end

and is run from the command line using:

knife hello world

Exceptions

In most cases, the exception handling available within knife is enough to ensure that exception handling for a plugin is consistent with how knife ordinarily behaves. That said, exceptions can also be handled within a knife plugin in the same way they are handled in any Ruby program.

Install a Plugin

To install a knife plugin from a file, do one of the following:

• Copy the file to the ~/.chef/plugins/knife directory; the file’s extension must be .rb
• Add the file to the chef-repo at the CHEF_REPO/.chef/plugins/knife; the file’s extension must be .rb
• Install the plugin from RubyGems