Defining Variables in Ansible

Introduction

Variables are an essential part of any configuration management system. In Ansible, variables allow you to manage dynamic values. This tutorial will guide you through the process of defining variables in Ansible, with detailed explanations and examples.

Basic Variable Definition

Variables in Ansible are defined using YAML syntax. They can be defined in various places such as inventory files, playbooks, or separate variable files. Here's a simple example of a variable definition:

---
- hosts: all
  vars:
    my_variable: "Hello, Ansible!"

Using Variables in Playbooks

Once a variable is defined, it can be used in playbooks. Variables are referenced using the Jinja2 templating system, which uses double curly braces {{ }}. Here's an example:

---
- hosts: all
  vars:
    greeting: "Hello"
    name: "World"
  tasks:
    - name: Print greeting
      debug:
        msg: "{{ greeting }}, {{ name }}!"

Variable Precedence

Ansible has a specific order of precedence for variables. This means that if a variable is defined in multiple places, Ansible will determine which value to use based on where it was defined. The order of precedence is as follows:

1. Extra vars (always win precedence)
2. Task vars (only for the current task)
3. Block vars (only for tasks in the current block)
4. Role and include vars (only for tasks in the current role or include)
5. Play vars
6. Set_facts / registered vars
7. Host facts
8. Playbook group_vars
9. Playbook host_vars
10. Inventory group_vars
11. Inventory host_vars
12. Inventory file or script group vars
13. Inventory file or script host vars
14. Role defaults

Defining Variables in Inventory Files

Variables can also be defined in inventory files. This allows you to associate variables with specific hosts or groups of hosts. Here's an example of an inventory file with variables:

[webservers]
web1 ansible_host=192.168.1.10 http_port=80
web2 ansible_host=192.168.1.11 http_port=8080

[databases]
db1 ansible_host=192.168.1.20 db_port=3306
db2 ansible_host=192.168.1.21 db_port=5432

Defining Variables in Separate Files

For better organization, variables can be stored in separate files. These files can then be included in playbooks. Here's an example of a variable file:

---
greeting: "Hello"
name: "Ansible User"

---
- hosts: all
  vars_files:
    - /path/to/variables.yml
  tasks:
    - name: Print greeting
      debug:
        msg: "{{ greeting }}, {{ name }}!"

Host and Group Variables

Host and group variables are defined in the host_vars and group_vars directories. These directories should be at the same level as your inventory file. Here's an example:

inventory/
  |-- hosts
  |-- host_vars/
  |     |-- web1.yml
  |     |-- web2.yml
  |-- group_vars/
        |-- webservers.yml
        |-- databases.yml
                

---
http_port: 80

---
ansible_host: 192.168.1.10

Conclusion

Defining and using variables in Ansible is a powerful way to manage dynamic values in your playbooks. By understanding the different ways to define variables and the order of precedence, you can write more flexible and maintainable automation scripts.