Step 1 − Create a helper method in cookbook’s library.
vipin@laptop:~/chef-repo $ subl cookbooks/my_cookbook/libraries/ipaddress.rb
class Chef::Recipe
def netmask(ipaddress)
IPAddress(ipaddress).netmask
end
end
Step 2 − Use the helper method.
vipin@laptop:~/chef-repo $ subl cookbooks/my_cookbook/recipes/default.rb
ip = '10.10.0.0/24'
mask = netmask(ip) # here we use the library method
Chef::Log.info("Netmask of #{ip}: #{mask}")
Step 3 − Upload the modified cookbook to the Chef Server.
vipin@laptop:~/chef-repo $ knife cookbook upload my_cookbook
Uploading my_cookbook [0.1.0]

Testing the Library

user@server $ sudo chef-client
...TRUNCATED OUTPUT...
[2013-01-18T14:38:26+00:00] INFO: Netmask of 10.10.0.0/24:
255.255.255.0
...TRUNCATED OUTPUT...

Working Method

Chef library code can open the chef::Recipe class and add new methods as done in Step 1. This step is not the cleanest but the simplest way of doing it.

class Chef::Recipe
def netmask(ipaddress)
...
end
end

Best Practices

Once we open the chef::recipe class, there are changes that it gets polluted. As a best practice, it is always a better way to introduce a new sub class inside the library and define a method as class method. This avoids pulling the chef::recipe namespace.

vipin@laptop:~/chef-repo $ subl cookbooks/my_cookbook/libraries/ipaddress.rb
class Chef::Recipe::IPAddress
def self.netmask(ipaddress)
IPAddress(ipaddress).netmask
end
end
We can use the method inside the recipe like
IPAddress.netmask(ip)

Chef – Definition

Definition can be defined as a logical method of grouping resources, which are used again and again. In this flow, we group the resources and give them a name to regain readability of defined cookbooks.
In order to do this, we should have a recipe. In this case, we are using test_cookbook and a run list of nodes, which includes the cookbook.

Creating a Definition

Step 1 − Create a new definition file in the cookbooks definition folder.
vipin@laptop:~/chef-repo $ subl cookbooks/test_cookbook/definitions/
capistrano_deploy_dirs.rb
define :capistrano_deploy_dirs, :deploy_to => '' do
directory "#{params[:deploy_to]}/releases"
directory "#{params[:deploy_to]}/shared"
directory "#{params[:deploy_to]}/shared/system"
end
Step 2 − Use a definition inside the cookbooks default recipe.
vipin@laptop:~/chef-repo $ subl cookbooks/test_cookbook/recipes/default.rb
capistrano_deploy_dirs do
deploy_to "/srv"
end
Step 3 − Upload the cookbook to the chef server.
vipin@laptop:~/chef-repo $ knife cookbook upload test_cookbook
Uploading test_cookbook [0.1.0]
Step 4 − Run the Chef client on the desired node.
vipin@laptop:~/chef-repuser@server $ sudo chef-client
...TRUNCATED OUTPUT...
[2013-01-18T16:31:11+00:00] INFO: Processing directory[/srv/
releases] action create (my_cookbook::default line 2)
[2013-01-18T16:31:11+00:00] INFO: directory[/srv/releases] created
directory /srv/releases
[2013-01-18T16:31:11+00:00] INFO: Processing directory[/srv/
shared] action create (my_cookbook::default line 3)
[2013-01-18T16:31:11+00:00] INFO: directory[/srv/shared] created
directory /srv/shared
[2013-01-18T16:31:11+00:00] INFO: Processing directory[/srv/
shared/system] action create (my_cookbook::default line 4)
[2013-01-18T16:31:11+00:00] INFO: directory[/srv/shared/system]

Definition in cookbooks are like micros, which group the resources and give them a name. A definition has a name by which one can tell them from which can be called inside the recipe and it has a list of perimeters.
In the definition, we have parameters which in our code looks like the following.

…..
directory "#{params[:deploy_to]}/releases"
directory "#{params[:deploy_to]}/shared"
directory "#{params[:deploy_to]}/shared/system”
……
It can be used inside the default recipe as follows.
capistrano_deploy_dirs do
deploy_to "/srv"`
end

Chef – Environment Variable

Environment variable is a key way to make Chef recipe run on any particular node successfully. There are multiple ways of doing it, either manually setting them up or by using a Shell script. Setting them via recipe is what we need to perform here.
To do this, we need to have a cookbook here we would use test_cookbook and a run list which contains test_cookbook.

Setting Environment Variable Using Chef Recipe

Step 1 − Update the default recipe of cookbook with an environment variable.
vipin@laptop:~/chef-repo $ subl cookbooks/test_cookbook/recipes/default.rb
ENV['MESSAGE'] = 'Testing environment variable update with chef !'
execute 'print value of environment variable $MESSAGE' do
command 'echo $MESSAGE > /tmp/message'
end
Step 2 − Upload the updated cookbook to the server.
vipin@laptop:~/chef-repo $ knife cookbook upload test_cookbook
Uploading my_cookbook [0.1.0]
Step 3 − Running the Chef client to create a temp file.
user@server:~$ sudo chef-client
...TRUNCATED OUTPUT...
[2013-01-25T15:01:57+00:00] INFO: Processing execute[print
value of environment variable $MESSAGE] action run
(my_cookbook::default line 11)
[2013-01-25T15:01:57+00:00] INFO: execute[print value of
environment variable $MESSAGE] ran successfully
...TRUNCATED OUTPUT...

Validating Variable

user@server:~$ cat /tmp/message
Hello from Chef

Working Method

Ruby exposes the current environment variable via ENV –a hash to read and modify the environment variable.

Execute Resource

We can use execute resource to do the same inside the Chef default recipe of cookbook.

mma@laptop:~/chef-repo $ subl cookbooks/test_cookbook/recipes/default.rb
execute 'print value of environment variable $MESSAGE' do
command 'echo $MESSAGE > /tmp/message'
environment 'MESSAGE' => 'Hello from the execute resource'
end

Note − Setting an environment variable using ENV will make that variable available during the whole Chef run. In contrast, passing it to the execute resource will only make it available for that one command executed by the resource.

So, this brings us to the end of blog. This Tecklearn ‘Concept of Libraries , Definition and setting environment variable’ blog helps you with commonly asked questions if you are looking out for a job in DevOps. If you wish to learn Chef and build a career in DevOps domain, then check out our interactive, Continuous Delivery using Chef Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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