Added how to use ruby in inspec doc

docs/dsl_inspec.rst

@@ -232,7 +232,7 @@ The following example illustrates various ways to add tags and references to `co
 .. |inspec resource| replace:: InSpec Resource
 .. |chef compliance| replace:: Chef Compliance
 .. |ruby| replace:: Ruby
-.. |ruby| replace:: SSH
+.. |ssh| replace:: SSH
 .. |windows| replace:: Microsoft Windows
 .. |postgresql| replace:: PostgreSQL
 .. |apache| replace:: Apache

docs/ruby_usage.rst

@@ -0,0 +1,136 @@
+=====================================================
+Using |ruby| in InSpec
+=====================================================
+
+The |inspec| DSL is a |ruby| DSL for writing audit controls, which includes audit resources that you can invoke.
+Core and custom resources are written as regular |ruby| classes which inherit from `Inspec.resource`.
+
+Execution
+=====================================================
+
+It's important to understand that |ruby| code used in custom resources and controls DSL is executed on the system that runs |inspec|. This allows |inspec| to work without |ruby| and rubygems being required on the target(node or container).
+
+For example, parsing a |csv| file like this to check the content:
+
+.. code-block:: ruby
+
+    require 'CSV'
+    control 'check-interns-group' do
+      impact 0.8
+      title 'Ensure interns are assigned to the correct group'
+      CSV.foreach("/mnt/share/interns.csv") do |row|
+        describe user(raw[3]) do
+          its('group') { should eq 'interns' }
+        end
+      end
+    end
+
+requires file `/mnt/share/interns.csv` to exist on the system running |inspec|.
+
+The |ruby| code can be slightly changed in order to parse the |csv| content from the remote target. This is accomplished using the |inspec| file resource that retrieves the content of the file from the target.
+
+.. code-block:: ruby
+
+    require 'CSV'
+    control 'check-interns-group' do
+      impact 0.8
+      title 'Ensure interns are assigned to the correct group'
+      csv_content = file('/root/scan-reports/cis-cat.csv').content
+      CSV.parse(csv_content) do |row|
+        describe user(row[3]) do
+          its('group') { should eq 'interns' }
+        end
+      end
+    end
+
+Similarly, using ``ls`` or `system('ls')` will result in the `ls` command being run locally and not on the target system.
+In order to process the output of `ls` executed on the target system, use `inspec.command('ls')` or `inspec.powershell('ls')`
+
+You can also use |ruby| variables and logic to instantiate an |inspec| resource once. For example, run a command and use the content in multiple tests:
+
+.. code-block:: ruby
+
+    control 'check-perl' do
+      impact 0.3
+      title 'Check perl compiled options and permissions'
+      perl_out = command('perl -V')
+      #require 'pry'; binding.pry;
+      describe perl_out do
+        its('exit_status') { should eq 0 }
+        its('stdout') { should match (/USE_64_BIT_ALL/) }
+        its('stdout') { should match (/useposix=true/) }
+        its('stdout') { should match (/-fstack-protector/) }
+      end
+
+      # extract an array of include directories
+      perl_inc = perl_out.stdout.partition('@INC:').last.strip.split("\n")
+      # ensure include directories are only writable by 'owner'
+      perl_inc.each do |path|
+        puts "*** testing '#{path.strip}'"
+        describe directory(path.strip) do
+          it { should_not be_writable.by('group') }
+          it { should_not be_writable.by('other') }
+        end
+      end
+    end
+
+
+Interactive Debugging with Pry
+-----------------------------------------------------
+
+An *advanced* but very useful |ruby| tip. In the previous example, I commented out the `require 'pry'; binding.pry;` line. If you remove the `#` and run the control, the execution will stop at that line and give you a `pry` shell. From this `pry` shell, you can print variables, see methods available, etc. For the above example:
+
+.. code-block:: ruby
+
+    [1] pry> perl_out.exit_status
+    => 0
+    [2] pry> perl_out.stderr
+    => ""
+    [3] pry> ls perl_out
+    Inspec::Plugins::Resource#methods: inspect
+    Inspec::Resources::Cmd#methods: command  exist?  exit_status  result  stderr  stdout  to_s
+    Inspec::Plugins::ResourceCommon#methods: resource_skipped  skip_resource
+    Inspec::Resource::Registry::Command#methods: inspec
+    instance variables: @__backend_runner__  @__resource_name__  @command  @result
+    [4] pry> perl_out.stdout.partition('@INC:').last.strip.split("\n")
+    => ["/Library/Perl/5.18/darwin-thread-multi-2level",
+     "    /Library/Perl/5.18",
+    ...REDACTED...
+    [5] pry> exit    # or abort
+
+You can use `pry` inside both the controls DSL and resources.
+Similarly, for dev and test, you can use `inspec shell` which is based on `pry`, for example:
+
+.. code-block:: ruby
+
+    $ inspec shell
+    Welcome to the interactive InSpec Shell
+    To find out how to use it, type: help
+
+    inspec> command('ls /home/gordon/git/inspec/docs').stdout
+    => "ctl_inspec.rst\ndsl_inspec.rst\ndsl_resource.rst\n"
+    inspec> command('ls').stdout.split("\n")
+    => ["ctl_inspec.rst", "dsl_inspec.rst", "dsl_resource.rst"]
+
+    inspec> help command
+    Name: command
+
+    Description:
+    Use the command InSpec audit resource to test an arbitrary command that is run on the system.
+
+    Example:
+    describe command('ls -al /') do
+      it { should exist }
+      its(:stdout) { should match /bin/ }
+      its('stderr') { should eq '' }
+      its(:exit_status) { should eq 0 }
+    end
+
+.. |inspec| replace:: InSpec
+.. |inspec resource| replace:: InSpec Resource
+.. |chef compliance| replace:: Chef Compliance
+.. |ruby| replace:: Ruby
+.. |csv| replace:: CSV
+.. |windows| replace:: Microsoft Windows
+.. |postgresql| replace:: PostgreSQL
+.. |apache| replace:: Apache