Facts and variables

When connecting to an host, ansible creates a dictionary with a huge set of useful informations.

This step is named fact gathering.

You can inspect facts using the setup module.



In [ ]:

    
cd /notebooks/exercise-03



In [ ]:

    
# Let's run the setup module

!ansible -i inventory -m setup localhost

Exercise

run the following command
what does it do?
where's its output ?
use the json module to reindent it



In [ ]:

    
!ansible >/dev/null -i inventory -m setup localhost --tree host_status



In [ ]:

    
# Solution
!tree host_status
import json
ret = json.load(open('host_status/localhost'))
f = ret['ansible_facts'] # .keys()
f['ansible_all_ipv4_addresses']

Exercise

Gather the following infos from the output of the previous command.

hostname
first ip address
distribution
kernel release



In [ ]:

    
# use this cell for the exercise

Exercise

modify this playbook to print:

hostname
first ip address
distribution
kernel release



In [ ]:

    
!ansible-playbook get-facts.yml

Facts vs Variables

Essentially, facts are per-host variables gathered by ansible.

In the vault lesson we've seen variables in action loaded it via -e @variables.yml.

Now we'll define them with

- hosts: localhost
  vars:
  - one_variable: 1
  - another_variable: 
      that_is: a_map

Templates & filters

Ansible can create files using the jinja2 template engine.

Filters are jinja2 functions to process facts and variables.

jinja2

Here's a template example

<html>
  <body>

# template.j2
This is a static line while the following one
expands the ansible_hostname variable {{ansible_hostname}} 

Now we process a simple number {{ 3.1415 | int }}

Or take the first letter from a variable: {{ ansible_hostname[0] }}
  </body>
</html>

You can process entries inside a template with filters.



In [ ]:

    
# Use the debug module to process a variable
# Modify the command to print the last two letters.
!ansible localhost -mdebug -a"var=ansible_user[:2]"



In [ ]:

    
!cat vars-and-facts.yml



In [ ]:

    
# This playbook mixes variables and facts
!ansible-playbook vars-and-facts.yml --tags variables

filters

Inside {{ braces }} you can process expressions using filters.


  - debug:
      msg: >
        Floor it  {{ ( one + pi ) | int }}

A simple filter using debug in a playbook.

- name: We already found a simple filter mapping entries to int
  debug: msg="{{ 3.1415 | int }}"

A filter is essentially a function returning a function, like a lambda.

int_filter = lambda x: int(x)

A more complex filter:

- name: This is a getter
  debug: 
    msg: >
      {{ ['host1', 'host2'] | 
            map('extract', hostvars, ['key1', .. , 'keyN']) 
      }}

where

hostvars_getter = lambda host: hostvars[host]['key1'][..]['keyN']

You can pipeline filters and test them incrementally.

We'll see further examples when processing hostvars



In [ ]:

    
# This playbook mixes variables and facts
!ansible-playbook vars-and-facts.yml --tags filters

Exercise

Edit the last section of vars-and-facts.yml so that it shows:

- the free percentage of every device

Hints:

- use the playbook as a reference
- iterate thru server facts



In [ ]:

    
!ansible-playbook vars-and-facts.yml --tags exercise



In [ ]: