chef

https://github.com/opscode/chef

Ruby

A systems integration framework, built to bring the benefits of configuration management to your entire infrastructure.

Chef::Mixin::WhyRun::ResourceRequirements::Assertion#whyrun

Defines a message and optionally provides a code block to execute
when the requirement is not met and Chef is executing in why run
mode

If no failure_message is provided (above), then execution
will be allowed to continue in both whyrun and non-whyrun
mode

With a service resource that requires /etc/init.d/service-name to exist:
  # in a provider
  assert(:start, :restart) do |a|
    a.assertion { ::File.exist?("/etc/init.d/service-name") }
    a.whyrun("Init script '/etc/init.d/service-name' doesn't exist, assuming a prior action would have created it.") do
      # blindly assume that the service exists but is stopped in why run mode:
      @new_resource.status(:stopped)
    end
  end

Source | Google | Stack overflow

Edit

git clone [email protected]:opscode/chef.git

cd chef

open lib/chef/mixin/why_run.rb

Contribute

# Make a new branch

git checkout -b -your-name--update-docs-Chef--Mixin--WhyRun--ResourceRequirements--Assertion-whyrun-for-pr

# Commit to git

git add lib/chef/mixin/why_run.rbgit commit -m "better docs for Chef::Mixin::WhyRun::ResourceRequirements::Assertion#whyrun"

# Open pull request

gem install hub # on a mac you can `brew install hub`

hub fork

git push <your name> -your-name--update-docs-Chef--Mixin--WhyRun--ResourceRequirements--Assertion-whyrun-for-pr

hub pull-request

# Celebrate!