variables.md

Variables in Ansible

Variables in Ansible

• Several different types:
  • Inventory variables
  • Play scoped variables
  • Task variables
  • Extra variables

Inventory Variables

• Scoped to a host throughout the execution of a playbook
• Assigned in
  • The inventory file
  • Specific subdirectories
    • host_vars/host
    • group_vars/group
• Can be referenced directly when executing play on host
• Available in hostvars dictionary

Variables in Inventory File

• Assign variable to a single host

[web] web1.mycompany.com ansible_host=152.240.43.12 opt2=arg2 web2.mycompany.com .

• Assign variables to groups of hosts

[web] web1.mycompany.com ansible_host=152.240.43.12 web2.mycompany.com [web:vars] proxy=web.mycompany.com

Using the host_vars directory

• Ansible will automatically load files under host_vars directory
  • Files with same name as particular host
  • Any files in a directory with the same name as a host

    
    [web]
    web1.myhost.com
    web2.myhost.com
    

    
    $INTRO_ANSIBLE_DIR/working-with-playbooks
    └── host_vars
        ├── web1.myhost.com.yml
        └── web2.myhost.com
            └── vars.yml
    

_.yml_ or _.yaml_ suffix is optional. Files can also be JSON

Exercise: create host inventory

• Create inventory variables for myserver host
• Try either way:

     cd $INTRO_ANSIBLE_DIR/working-with-playbooks
     mkdir -p host_vars
     gedit host_vars/myserver.yml

     mkdir -p host_vars/myserver
     gedit host_vars/myserver/vars.yml

    # variables for myserver
    ---
    foo: bar

Using group_vars

• Similar to host_vars except for groups
  • Files with same name as particular group
  • Any files in a directory with the same name as a group

    
    [web]
    web1.myhost.com
    [app]
    app1.myhost.com
    

    
    $INTRO_ANSIBLE_DIR/working-with-playbooks
    └── group_vars
        ├── web.yml
        └── app
            └── vars.yml
    

Exercise: Create group_vars for our app

• Create a group_vars directory and variable file

    cd $INTRO_ANSIBLE_DIR/working-with-playbooks/
    mkdir -p group_vars
    gedit group_vars/web.yml
    

    mkdir -p group_vars/web
    gedit group_vars/web/vars.yml
    

    # variables for "web" group
    ---
    bizz: buzz

Using inventory variables

• Inventory variables available to Ansible throughout playbook run

$ less $INTRO_ANSIBLE_DIR/working-with-playbooks/templates/index.html.j2
.
.
{% if foo is defined %}
<p>
The value for foo is <em>{{ foo }}</em>
</p>
{% endif %}
{% if bizz is defined %}
<p>
The value for bizz is <em>{{ bizz }}</em>
</p>
{% endif %}
.
.

Using Inventory Variables

ansible-playbook static-site.yml

• Run the ansible playbook again
• Note behaviour of Copy up static website html task
• Reload website when finished

hostvars

• Dictionary with variables from each host (including inventory variables)
• Indexed on hostname from inventory
  • hostvars['myserver']['foo'] == hostvars.myserver.foo
• Can be accessed across plays in the same playbook run

Using hostvars

• Have a look at use-hostvars.yml
• Run static-site.yml and use-hostvars.yml

  ansible-playbook static-site.yml use-hostvars.yml
  

• Change foo to hostvars.myserver.foo in playbook-localhost.yml and run playbooks

Exercise: View a group variable form hostvars

• The bizz variable still appears as undefined in playbook run
• How would you find that in hostvars?
• A group variable will be defined for every host in a group

- name: Output of bizz
  debug:
    var: hostvars.myserver.bizz

Play-scoped Variables

• Declared in control section of a play
  • vars - static declaration
  • vars_prompt - interactively prompt user when running playbook
• Can be referenced directly while in play

Play-scoped Variables

• Open static-site.yml in editor
• Edit as shown:
• Rerun ansible-playbook

- name: Set up static website with nginx
  hosts: myserver
  become: true
  vars_prompt:
    - name: "participant_name"
      prompt: "What is your name"
      private: no
  vars:
    base_path: "/"
    course: Introduction to ansible
    lesson:
      name: Working with play variables
      session: 1
  tasks:
  .
  .

Task Variables

• Contain data discovered while executing a task
• Gathering facts phase of play execution
• Tasks that gather user input
• Become part of the host scope once defined
• In a play they can be directly referenced
• In subsequent plays are available in hostvars dictionary for host

Assigning Variables with set_fact

tasks:
  - name: Assign an arbitrary variable
    set_fact:
      person_name: Homer Simpson

  - name: Assign dictionary
    set_fact:
      person_details:
        street: 754 Evergreen Terrace
        city: Springfield
        state: ST

Assigning Variables with register

- name: Execute code and capture the output
  command: uptime
  register: uptime_output

  .
  .
- name: do something with output of uptime
  debug:
    var: {{ uptime_output.stdout }}

Exercise: Assign Task Variables

    tasks:
        - name: set name of person
          set_fact:
            person_name: Homer Simpson

        - name: Get output of uptime
          command: uptime
          register: uptime_output
        .
        .

• Use set_fact and register in static-site.yml to assign task variables
• Rerun ansible playbook

Gathering facts

• Plugin run before play executes
  • setup module
  • Collects detailed information about host and attaches to host scope
  • You can get an idea by running
    • ansible myserver -m setup
  • Filter facts to find variables you are interested in:
    • ansible myserver -m setup -a 'filter=ansible_distribution*'

Exercise: Display System Facts

• Uncomment area in templates/index.html.j2
• Re-run ansible-playbook and visit static website

<!--   remove line
{% if ansible_distribution is defined %}
    <p>  
    System facts: {{ ansible_distribution }} {{ ansible_distribution_major_version }} {{ ansible_architecture }}
    </p>
{% endif %}
-->   remove line

Exercise: Turn off fact gathering

• Edit static-site.yml as follows
• Re-run ansible-playbook and visit static website

    - name: Set up static website with nginx
      hosts: myserver
      become: true
      gather_facts: false
      tasks:
        .
        .

Extra Variables

• Variables passed to ansible or ansible-playbook via command line
  • ansible-playbook --extra-vars="key1=value1 key2=value2" playbook.yml
  • ansible-playbook -e key1=value1 -e key2=value2 playbook.yml
  • ansible-playbook -e @extra-variables.yml playbook.yml

Variable Precedence

• Ansible variables in order of increasing precedence:
• inventory file
• host facts
• play vars_files
• task vars
• include_vars
• set_facts/registered variables
• extra vars
• [complete list](https://docs.ansible.com/ansible/latest/playbooks_variables.html#variable-precedence-where-should-i-put-a-variable)

Exercise: Variable Precedence

• Pass extra variables to ansible playbook to override previous examples
• For example you can create a new file called override-vars.yml

  
     person_name: Marge Simpson
     lesson:
       name: Override variables
       session: 2
  

• Then add this on the command line

  ansible-playbook static-site.yml -e @override-vars.yml
  

Summary

• There are several types of variables in Ansible
  • inventory
  • play scoped
  • task variables
  • extra