Guards

A guard property can be used to evaluate the state of a node during the execution phase of a Chef Infra Client run. Based on the results of this evaluation, a guard property is then used to tell Chef Infra Client if it should continue executing a resource. A guard property accepts either a string value or a Ruby block value:

A string is executed as a shell command. If the command returns 0, the guard is applied. If the command returns any other value, then the guard property is not applied.

String guards in a powershell_script run Windows PowerShell commands and may return true in addition to 0.

A block is executed as Ruby code that must return either true or false. If the block returns true, the guard property is applied. If the block returns false, the guard property is not applied.

	# Condistions Based on if-else statement with Chef Attributes attribute? A useful method that is related to attributes is the attribute? method. This method will check for the existence of an attribute, so that processing can be done in an attributes file or recipe, but only if a specific attribute exists. Using attribute?() in an attributes file: if attribute?('ec2') # ... set stuff related to EC2 end Using attribute?() in a recipe: if node.attribute?('ec2') # ... do stuff on EC2 nodes end Condistions Based on if statement if node[:platform_family].include?("rhel") ... end Condistions Based on if statement
	if node['platform'] == 'debian' || node['platform'] == 'ubuntu'
	execute "apt-get update" do
	command "apt-get update"
	end
	end
	# Condistions Based on include_recipe
	include_recipe 'python::repository' if node['python']['installrepo']
	# Condistions Based on if-else and include_recipe
	if node['platform_family'] == 'windows'
	include_recipe 'python::install-windows'
	else
	include_recipe 'python::install-linux'
	end
	# Condistions Based on Attributes
	if node['platform'] == 'debian' || node['platform'] == 'ubuntu'
	execute "apt-get update" do
	command "apt-get update"
	end
	end
	if node['platform'] == 'redhat'
	execute "yum git" do
	command "yum install git -y"
	end
	end
	# ==========================================================================
	# not_if
	# ==========================================================================
	apt_package "apache2" do
	action :install
	not_if { node['platform'] == 'redhat' }
	end
	file '/tmp/somefile.txt' do
	mode '0755'
	not_if { File.exist?('/etc/passwd' )}
	end
	execute 'bundle install' do
	cwd '/myapp'
	not_if 'bundle check' # This is run from /myapp
	end
	template '/tmp/somefile' do
	mode '0755'
	source 'somefile.erb'
	not_if { node[:some_value] }
	end
	template '/tmp/somefile' do
	mode '0755'
	source 'somefile.erb'
	not_if do
	File.exist?('/etc/passwd')
	end
	end
	template '/tmp/somefile' do
	mode '0755'
	source 'somefile.erb'
	not_if { File.exist?('/etc/passwd' )}
	end
	template '/tmp/somefile' do
	mode '0755'
	source 'somefile.erb'
	not_if 'test -f /etc/passwd'
	end
	Example
	:user
	Specify the user that a command will run as. For example:
	not_if 'grep adam /etc/passwd', :user => 'adam'
	:group
	Specify the group that a command will run as. For example:
	not_if 'grep adam /etc/passwd', :group => 'adam'
	:environment
	Specify a Hash of environment variables to be set. For example:
	not_if 'grep adam /etc/passwd', :environment => {
	'HOME' => '/home/adam'
	}
	:cwd
	Set the current working directory before running a command. For example:
	not_if 'grep adam passwd', :cwd => '/etc'
	:timeout
	Set a timeout for a command. For example:
	not_if 'sleep 10000', :timeout => 10
	apt_package "php5" do
	action :install
	not_if { node['platform'] == 'centos' }
	end
	# want to ensure that we have the right JAVA_HOME path set before we go about
	triggering the command to start the app or check the status.
	bash "some_app" do
	environment { "JAVA_HOME" => "/usr/java/default" }
	code "java /apps/some_app/app start"
	not_if "java /apps/some_app/app status"
	end
	# However, this isn't the right way to go about handling our situation because the environment variable JAVA_HOME isn't available to the java some_app status
	command. One way to do it correctly is this:
	bash "some_app" do
	environment { "JAVA_HOME" => "/usr/java/default" }
	code "java /apps/some_app/app start"
	not_if "java /apps/some_app/app status", :environment => {
	'JAVA_HOME' => '/usr/java/default' }
	end
	bash "some_app" do
	guard_interpreter :bash
	environment { "JAVA_HOME" => "/usr/java/default" }
	code "java /apps/some_app/app start"
	not_if "java /apps/some_app/app status"
	end
	# we are installing a package called package_name and we want to install it only on systems running RHEL 6.x.
	package "package_name" do
	action :install
	not_if { platform_family?('rhel') && node['platform_version'].to_f < 6.0 }
	end
	# ==========================================================================
	# only_if
	# ==========================================================================
	only_if
	Allow a resource to execute only if the condition returns true.
	file '/tmp/infy.txt' do
	only_if { File.exist?('/etc/passwd2' ) }
	end
	package "httpd" do
	action :install
	only_if { platform_family?('rhel') && node['platform_version'].to_f >= 6.0 }
	end
	template '/tmp/somefile' do
	mode '0755'
	source 'somefile.erb'
	only_if { node[:some_value] }
	end
	template '/tmp/somefile' do
	mode '0755'
	source 'somefile.erb'
	only_if do ! File.exist?('/etc/passwd') end
	end
	template '/tmp/somefile' do
	mode '0755'
	source 'somefile.erb'
	only_if 'test -f /etc/passwd'
	end
	file '/var/www/html/login.php' do
	only_if { ::File.exist?('/var/www/html/login.php') }
	action :touch
	end
	file '/path/foo' do
	action :delete
	only_if { File.exist? '/path/foo' }
	end
	apt_package "php-pear" do
	action :install
	only_if "which php"
	end
	# we are installing a package called package_name and
	we want to install it only on systems running RHEL 6.x.
	package "package_name" do
	action :install
	only_if { platform_family?('rhel') && node['platform_version'].to_f >= 6.0 }
	end
	include_recipe "postfix::server" do
	only_if node["defaults"]["postfix_server"] = true
	end

view raw chef-guards.rb hosted with by GitHub

Rajesh Kumar

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