4 - Using Variables

Objective

Ansible supports variables to store values that can be used in Ansible playbooks. Variables can be defined in a variety of places and have a clear precedence. Ansible substitutes the variable with its value when a task is executed.

This exercise covers variables, specifically

How to use variable delimiters {{ and }}
What host_vars and group_vars are and when to use them
How to use ansible_facts
How to use the debug module to print variables to the console window

Guide

Variables are referenced in Ansible Playbooks by placing the variable name in double curly braces:

This string contains a {{ variable }}

Variables and their values can be defined in various places: the inventory, additional files, on the command line, etc.

The recommended practice to provide variables in the inventory, is to define them in files located in two directories: host_vars and group_vars.

To define variables for a group "servers", a YAML file named group_vars/servers.yml with the variable definitions is created.
To define variables specifically for a host node1, the file host_vars/node1.yml with the variable definitions is created.

Tip

Host variables take precedence over group variables (more about precedence can be found in the docs).

Step 1 - Create Variable Files

For understanding and practice let’s do a lab. Following up on the theme "Let’s build a web server. Or two. Or even more…", you will change the index.html to show the development environment (dev/prod) a server is deployed in.

On the ansible control host, as the student user, create the directories to hold the variable definitions in ~/ansible-files/:

[student@ansible-1 ansible-files]$ mkdir host_vars group_vars

Now create two files containing variable definitions. We’ll define a variable named stage which will point to different environments, dev or prod:

Create the file ~/ansible-files/group_vars/web.yml with this content:

---
stage: dev

Create the file ~/ansible-files/host_vars/node2.yml with this content:

---
stage: prod

What is this about?

For all servers in the web group the variable stage with value dev is defined. So as default we flag them as members of the dev environment.
For server node2 this is overridden and the host is flagged as a production server.

Step 2 - Create web.html Files

Now create two files in ~/ansible-files/files/:

One called prod_web.html with the following content:

<body>
<h1>This is a production webserver, take care!</h1>
</body>

And the other called dev_web.html with the following content:

<body>
<h1>This is a development webserver, have fun!</h1>
</body>

Step 3 - Create the Playbook

Now you need a Playbook that copies the prod or dev web.html file - according to the "stage" variable.

Create a new Playbook called deploy_index_html.yml in the ~/ansible-files/ directory.

Tip

Note how the variable "stage" is used in the name of the file to copy.

---
- name: Webserver configuration
  hosts: web
  become: true
  tasks:
    - name: Copy web.html
      ansible.builtin.copy:
        src: "{{ stage }}_web.html"
        dest: /var/www/html/index.html
        mode: "0644"
        owner: apache
        group: apache

Run the Playbook:

AnsibleNavigator

[student@ansible-1 ansible-files]$ ansible-playbook deploy_index_html.yml

[student@ansible-1 ansible-files]$ ansible-navigator run deploy_index_html.yml

Step 4 - Test the Result

The Ansible Playbook copies different files as index.html to the hosts, use curl to test it.

For node1:

[student@ansible-1 ansible-files]$ curl http://node1
<body>
<h1>This is a development webserver, have fun!</h1>
</body>

For node2:

[student@ansible-1 ansible-files]$ curl http://node2
<body>
<h1>This is a production webserver, take care!</h1>
</body>

For node3:

[student@ansible-1 ansible-files]$ curl http://node3
<body>
<h1>This is a development webserver, have fun!</h1>
</body>

Tip

If by now you think: There has to be a smarter way to change content in files… you are absolutely right. This lab was done to introduce variables, you are about to learn about templates in one of the next chapters.

Step 5 - Ansible Facts

Ansible facts are variables that are automatically discovered by Ansible from a managed host. Remember the "Gathering Facts" task with the setup module we used with the Ansible ad hoc command?
The facts contain useful information stored into variables that administrators can reuse.

To get an idea what facts Ansible collects by default, on your control node as your student user create the playbook setup.yml and run it to get the setup details of node1:

---
- name: Capture and output facts
  hosts: node1
  gather_facts: false
  tasks:
    - name: Collect only facts returned by facter
      ansible.builtin.setup:
        gather_subset:
          - 'all'
      register: setup_output

    - name: Output variable content
      ansible.builtin.debug:
        msg: "{{ setup_output }}"

AnsibleNavigator

[student@ansible-1 ansible-files]$ ansible-playbook setup.yml

[student@ansible-1 ansible-files]$ ansible-navigator run setup.yml -m stdout

This might be a bit too much, you can use filters to limit the output to certain facts, the expression is shell-style wildcard within your playbook. Create a playbook labeled setup_filter.yml as shown below. In this example, we filter to get the eth0 facts as well as memory details of node1.

---
- name: Capture and output facts
  hosts: node1
  gather_facts: false
  tasks:
    - name: Collect only specific facts
      ansible.builtin.setup:
        filter:
          - 'ansible_eth0'
          - 'ansible_*_mb'
      register: setup_output

    - name: Output variable content
      ansible.builtin.debug:
        msg: "{{ setup_output }}"

Run the playbook:

AnsibleNavigator

[student@ansible-1 ansible-files]$ ansible-playbook setup_filter.yml

[student@ansible-1 ansible-files]$ ansible-navigator run setup_filter.yml -m stdout

Step 6 - Challenge Lab: Facts

Try to find and print the OS family (RedHat) of your managed hosts using a playbook, it should output only this single fact.

Tip

Use an ad-hoc command to output all facts, maybe even filter the output by using grep to find the appropriate fact.

Solution

$ ansible node1 -m setup | grep family
    "ansible_os_family": "RedHat",

---
- name: Capture and output facts
  hosts: node1
  gather_facts: false
  tasks:
    - name: Collect only specific facts, this task can be removed when enabling 'gather_facts' again.
      ansible.builtin.setup:
        filter:
          - '*family'
      register: setup_output

    - name: Output variable content
      ansible.builtin.debug:
        msg: "{{ ansible_os_family }}"

Run the playbook:

[student@ansible-1 ansible-files]$ ansible-playbook setup_filter.yml

Optionally, run the playbook with the ansible-navigator:

[student@ansible-1 ansible-files]$ ansible-navigator run setup_filter.yml -m stdout

Step 7 - Using Facts in Playbooks

Facts can be used in a Playbook like variables, using the proper naming, of course. Create this Playbook as facts.yml in the ~/ansible-files/ directory:

---
- name: Output facts within a playbook
  hosts: all
  tasks:
    - name: Prints Ansible facts
      ansible.builtin.debug:
        msg: From a total of {{ ansible_memtotal_mb }} MB the server {{ ansible_fqdn }} has {{ ansible_memfree_mb }} MB RAM left.

Tip

The "debug" module is handy for e.g. debugging variables or expressions.

Execute it to see how the facts are printed:

AnsibleNavigator

[student@ansible-1 ansible-files]$ ansible-playbook facts.yml

[student@ansible-1 ansible-files]$ ansible-navigator run facts.yml -m stdout

Examine the output:

PLAY [Output facts within a playbook] ******************************************************************************

TASK [Gathering Facts] *********************************************************************************************
ok: [node2]
ok: [node3]
ok: [node1]
ok: [ansible-1]

TASK [Prints Ansible facts] ****************************************************************************************
ok: [ansible-1] => {
    "msg": "From a total of 7937 MB the server ansible-1 has 2856 MB RAM left."
}
ok: [node1] => {
    "msg": "From a total of 7937 MB the server node1 has 3152 MB RAM left."
}
ok: [node2] => {
    "msg": "From a total of 7937 MB the server node2 has 3138 MB RAM left."
}
ok: [node3] => {
    "msg": "From a total of 7937 MB the server node3 has 3247 MB RAM left."
}

PLAY RECAP *********************************************************************************************************
ansible-1                  : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0  
node1                      : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0  
node2                      : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0  
node3                      : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0