Introduction
In the realm of IT automation, Ansible stands out for its simplicity and power. Understanding how to manage variables and gather facts is crucial for creating dynamic, flexible and efficient playbooks. This guide will take you through the essentials of defining and using variables, understanding variable precedence and scope, gathering facts about your hosts and using registered variables to capture command outputs.
Defining and Using Variables in Playbooks
Variables in Ansible allow you to store values that can be reused across your playbooks. They help in making your playbooks dynamic and more manageable. Variables can be defined in various places, including inventory files, playbooks or even external files.
Example of Defining Variables
Here’s how you can define variables within a playbook :-
---
- name: Install Apache on web servers
hosts: webservers
vars:
http_port: 80
max_clients: 200
tasks:
- name: Install Apache
apt:
name: apache2
state: present
- name: Configure Apache
template:
src: /path/to/httpd.conf.j2
dest: /etc/apache2/httpd.conf
notify:
- Restart Apache
In this example http_port and max_clients are variables that can be used within tasks.
Variable Precedence and Scope
Ansible variables can be defined in multiple locations and their precedence determines which value gets applied when there are conflicts. The order of precedence is as follows (from lowest to highest) :-
Role defaults
Inventory file or script group vars
Inventory group_vars/all
Playbook group_vars/all
Inventory group_vars/*
Playbook group_vars/*
Inventory file or script host vars
Inventory host_vars/*
Playbook host_vars/*
Host facts
Play vars
Play vars_prompt
Play vars_files
Role vars (defined in role/vars/main.yml)
Block vars (only for tasks in block)
Task vars (only for the task)
Role (and include) params
Set_facts / registered vars
Command line vars (e.g., -e "var=value")
Example of Variable Precedence
---
- name: Variable precedence
hosts: webservers
vars:
message: "This is from play vars"
tasks:
- name: Print the message
debug:
msg: "{{ message }}"
If you override the message variable using -e "message='This is from command line'" the command line value will take precedence.
Gathering and Using Facts About Your Hosts
Ansible automatically gathers facts about your hosts which are stored as variables. These facts provide useful information such as OS type, IP addresses and available memory.
Gathering Facts
Facts are gathered using the setup module which runs by default at the start of a play. You can also explicitly gather facts using :-
- name: Gather facts
hosts: all
tasks:
- name: Gather facts
setup:
Using Facts
Facts can be used just like any other variable. For example to display the OS type of a host :-
- name: Display OS type
hosts: all
tasks:
- name: Show OS type
debug:
msg: "The OS type is {{ ansible_os_family }}"
Using Registered Variables to Capture Command Outputs
Registered variables capture the output of a task which can then be used in subsequent tasks. This is useful for conditional operations based on the output of a command.
Example of Using Registered Variables
---
- name: Using registered variables
hosts: webservers
tasks:
- name: Check if Apache is installed
shell: dpkg -l | grep apache2
register: apache_installed
ignore_errors: yes
- name: Display installation status
debug:
msg: "Apache is installed"
when: apache_installed.rc == 0
In this example the output of the command is captured in apache_installed and the next task is executed based on whether Apache is installed.
Example Playbook :- Using Variables and Facts
Let's put everything together in an example playbook that defines variables, gathers facts and uses registered variables :-
---
- name: Using variables and facts
hosts: webservers
vars:
http_port: 80
document_root: /var/www/html
tasks:
- name: Gather facts
setup:
- name: Display OS type
debug:
msg: "The OS type is {{ ansible_os_family }}"
- name: Check available memory
debug:
msg: "Available memory is {{ ansible_memtotal_mb }} MB"
- name: Install Apache
apt:
name: apache2
state: present
- name: Start Apache
service:
name: apache2
state: started
- name: Check if Apache is running
shell: systemctl status apache2
register: apache_status
ignore_errors: yes
- name: Display Apache status
debug:
msg: "Apache is running"
when: apache_status.rc == 0
- name: Display HTTP port
debug:
msg: "The HTTP port is {{ http_port }}"
- name: Display Document Root
debug:
msg: "The Document Root is {{ document_root }}"
Running the Playbook
Save the playbook as variables_facts.yml and run it using the following command :-
ansible-playbook -i inventory.ini variables_facts.yml
This playbook demonstrates how to use variables, gather facts and register command outputs.
Conclusion
Understanding how to manage variables and gather facts in Ansible is crucial for creating robust and flexible automation scripts. By leveraging variables you can make your playbooks dynamic and reusable. Gathering facts allows you to make informed decisions based on the state of your hosts and registered variables enable conditional operations based on command outputs.
With this foundational knowledge, you're now equipped to craft more advanced and powerful Ansible playbooks. Let's continue exploring the capabilities of Ansible in future posts diving deeper into advanced topics and real-world use cases to further enhance your automation skills.