Module 03: Variables & Facts¶
🎯 Learning Objectives¶
By the end of this module, you will:
- Understand variable types, scoping, and precedence rules
- Master fact gathering and utilization in playbooks
- Organize variables using host_vars and group_vars
- Use magic variables for advanced automation logic
- Register task outputs for use in subsequent tasks
- Implement variable inheritance and override patterns
- Debug variable content and troubleshoot precedence issues
📋 Why Variables Transform Automation¶
Static vs Dynamic Playbooks¶
Static Approach: Hard-coded values limit reusability
# Limited reusability
- name: Install web server
ansible.builtin.dnf:
name: httpd # Hard-coded package name
state: present
Dynamic Approach: Variables enable flexibility
# Highly reusable
- name: Install web server
ansible.builtin.dnf:
name: "{{ web_package }}" # Variable package name
state: present
Variable Benefits¶
- Flexibility: Same playbook works across environments
- Maintainability: Change values in one place
- Reusability: Generic playbooks for multiple scenarios
- Security: Sensitive data separation via Ansible Vault
🔢 Variable Types and Definition¶
Variable Naming Rules¶
Valid Names:
- Letters, numbers, and underscores only
- Must start with letter or underscore
- Case-sensitive
# Valid variable names
web_package: httpd
db_port: 3306
_private_key: /path/to/key
environment2: production
# Invalid variable names
# 2environment: production # Cannot start with number
# web-package: httpd # Cannot contain hyphens
# web package: httpd # Cannot contain spaces
Variable Definition Locations¶
1. Playbook Variables:
---
- name: Web server setup
hosts: webservers
vars:
web_package: httpd
web_port: 80
web_user: apache
tasks:
- name: Install {{ web_package }}
ansible.builtin.dnf:
name: "{{ web_package }}"
state: present
2. External Variable Files:
# vars/web_vars.yml
---
web_package: httpd
web_port: 80
web_service: httpd
web_config_dir: /etc/httpd/conf
web_document_root: /var/www/html
# Playbook usage
---
- name: Web server setup
hosts: webservers
vars_files:
- vars/web_vars.yml
tasks:
- name: Install web server
ansible.builtin.dnf:
name: "{{ web_package }}"
state: present
3. Command Line Variables:
# Override variables at runtime
ansible-navigator run site.yml -e "web_port=8080" --mode stdout
ansible-navigator run site.yml -e "env=production" --mode stdout
ansible-navigator run site.yml -e "@vars/production.yml" --mode stdout
4. Host Variables (host_vars/hostname.yml):
# host_vars/web01.example.com.yml
---
web_port: 8080
max_clients: 150
ssl_enabled: true
custom_modules:
- mod_ssl
- mod_rewrite
5. Group Variables (group_vars/groupname.yml):
# group_vars/webservers.yml
---
web_package: httpd
web_service: httpd
web_port: 80
document_root: /var/www/html
# group_vars/production.yml
---
environment: production
backup_enabled: true
monitoring: true
log_level: warn
6. Inventory Variables:
# INI format
[webservers]
web01.example.com web_port=8080 ssl_enabled=yes
web02.example.com web_port=80 ssl_enabled=no
[webservers:vars]
web_package=httpd
web_service=httpd
# YAML format
all:
children:
webservers:
hosts:
web01.example.com:
web_port: 8080
ssl_enabled: yes
web02.example.com:
web_port: 80
ssl_enabled: no
vars:
web_package: httpd
web_service: httpd
Variable Data Types¶
Strings:
server_name: web01.example.com
config_path: "/etc/httpd/conf/httpd.conf"
message: 'Single quotes preserve content literally'
Numbers:
web_port: 80
max_connections: 1000
timeout: 30.5
Booleans:
ssl_enabled: true
debug_mode: false
backup_enabled: yes # Ansible converts to boolean
maintenance: no # Ansible converts to boolean
Lists:
packages:
- httpd
- php
- mysql-server
ports: [80, 443, 8080]
users:
- name: alice
shell: /bin/bash
- name: bob
shell: /bin/zsh
Dictionaries:
database:
host: db.example.com
port: 3306
name: webapp
user: appuser
# Alternative syntax
database: {host: db.example.com, port: 3306, name: webapp}
📊 Variable Precedence (16 Levels)¶
Complete Precedence Hierarchy¶
Higher number = Higher precedence
- role defaults (lowest precedence)
- 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 / cached set_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 specific task)
- include_vars
- set_facts / registered vars
- role (and include_role) params
- include params
- extra vars (
ansible-navigator run -e) (highest precedence)
Practical Precedence Examples¶
Scenario: Same variable defined at multiple levels
# group_vars/all.yml (precedence 3)
web_port: 80
# host_vars/web01.example.com.yml (precedence 8)
web_port: 8080
# play vars (precedence 11)
---
- name: Configure web server
hosts: web01.example.com
vars:
web_port: 443
tasks:
- name: Show final port value
ansible.builtin.debug:
var: web_port # Will show 443 (play vars win)
Command Line Override (highest precedence):
# Command line variables always win
ansible-navigator run site.yml -e "web_port=9090" --mode stdout
# web_port will be 9090 regardless of other definitions
Testing Variable Precedence¶
---
- name: Variable precedence demonstration
hosts: all
vars:
test_var: "from play vars"
tasks:
- name: Show variable from multiple sources
ansible.builtin.debug:
msg: |
test_var value: {{ test_var }}
Source: {{ test_var_source | default('unknown') }}
- name: Set task-level variable
ansible.builtin.debug:
var: test_var
vars:
test_var: "from task vars" # This wins over play vars
🔍 Facts (System Information)¶
Understanding Facts¶
What are Facts?: Automatically gathered system information including:
- Operating system details
- Hardware information
- Network configuration
- Filesystem information
- Service status
Fact Gathering Control¶
---
- name: Control fact gathering
hosts: all
gather_facts: yes # Default behavior
tasks:
- name: Manual fact gathering
ansible.builtin.setup: # Equivalent to automatic gathering
# Disable automatic fact gathering
- name: Skip fact gathering
hosts: all
gather_facts: no
tasks:
- name: Gather facts only when needed
ansible.builtin.setup:
when: gather_system_info | default(false)
Essential Facts¶
System Information:
- name: Display system facts
ansible.builtin.debug:
msg: |
OS: {{ ansible_facts['distribution'] }} {{ ansible_facts['distribution_version'] }}
Architecture: {{ ansible_facts['architecture'] }}
Kernel: {{ ansible_facts['kernel'] }}
Python: {{ ansible_facts['python_version'] }}
Hostname: {{ ansible_facts['hostname'] }}
FQDN: {{ ansible_facts['fqdn'] }}
Hardware Facts:
- name: Display hardware facts
ansible.builtin.debug:
msg: |
Memory: {{ ansible_facts['memtotal_mb'] }} MB
Processors: {{ ansible_facts['processor_count'] }}
CPU: {{ ansible_facts['processor'][0] }}
Virtualization: {{ ansible_facts['virtualization_type'] | default('none') }}
Network Facts:
- name: Display network facts
ansible.builtin.debug:
msg: |
Default IP: {{ ansible_facts['default_ipv4']['address'] }}
Default Interface: {{ ansible_facts['default_ipv4']['interface'] }}
All IPs: {{ ansible_facts['all_ipv4_addresses'] | join(', ') }}
Hostname: {{ ansible_facts['fqdn'] }}
Storage Facts:
- name: Display storage facts
ansible.builtin.debug:
msg: |
Disks: {{ ansible_facts['devices'].keys() | list | join(', ') }}
Root filesystem: {{ ansible_facts['mounts'] | selectattr('mount', 'equalto', '/') | first }}
Available space: {{ ansible_facts['mounts'] | selectattr('mount', 'equalto', '/') | map(attribute='size_available') | first }}
Fact Filtering¶
# Gather specific fact subsets
ansible all -m setup -a "filter=ansible_distribution*"
ansible all -m setup -a "filter=ansible_default_ipv4"
ansible all -m setup -a "filter=ansible_mounts"
# Gather only essential facts
ansible all -m setup -a "gather_subset=hardware,network"
# Exclude specific fact subsets
ansible all -m setup -a "gather_subset=all,!facter,!ohai"
Custom Facts¶
Local Facts Directory: /etc/ansible/facts.d/
# Create custom fact script
sudo mkdir -p /etc/ansible/facts.d
sudo cat > /etc/ansible/facts.d/application.fact << 'EOF'
#!/bin/bash
echo '{
"app_name": "myapp",
"version": "1.2.3",
"config_path": "/opt/myapp/config",
"data_path": "/var/lib/myapp"
}'
EOF
sudo chmod +x /etc/ansible/facts.d/application.fact
Using Custom Facts:
- name: Use custom facts
ansible.builtin.debug:
msg: |
App: {{ ansible_facts['local']['application']['app_name'] }}
Version: {{ ansible_facts['local']['application']['version'] }}
🔮 Magic Variables¶
Built-in Magic Variables¶
Inventory Variables:
- name: Display inventory information
ansible.builtin.debug:
msg: |
Current host: {{ inventory_hostname }}
Short hostname: {{ inventory_hostname_short }}
Groups this host belongs to: {{ group_names | join(', ') }}
All groups: {{ groups.keys() | list | join(', ') }}
Webserver hosts: {{ groups['webservers'] | default([]) | join(', ') }}
Play Variables:
- name: Display play information
ansible.builtin.debug:
msg: |
Hosts in current play: {{ play_hosts | join(', ') }}
Current batch: {{ ansible_play_batch | join(', ') }}
Play name: {{ ansible_play_name | default('unnamed') }}
Hostvars Access:
- name: Access other host variables
ansible.builtin.debug:
msg: |
Web01 IP: {{ hostvars['web01.example.com']['ansible_default_ipv4']['address'] }}
Database host: {{ hostvars[groups['databases'][0]]['ansible_fqdn'] }}
All web server IPs: {% for host in groups['webservers'] %}{{ hostvars[host]['ansible_default_ipv4']['address'] }}{% if not loop.last %}, {% endif %}{% endfor %}
Advanced Magic Variable Usage¶
Cross-Host Information Sharing:
---
- name: Collect information from all hosts
hosts: all
tasks:
- name: Register hostname info
ansible.builtin.set_fact:
server_info:
hostname: "{{ inventory_hostname }}"
ip: "{{ ansible_default_ipv4.address }}"
role: "{{ server_role | default('unknown') }}"
- name: Configure load balancer with all server info
hosts: loadbalancers
tasks:
- name: Generate backend configuration
ansible.builtin.template:
src: backends.conf.j2
dest: /etc/haproxy/backends.conf
vars:
backend_servers: |
{% for host in groups['webservers'] %}
server {{ hostvars[host]['inventory_hostname_short'] }} {{ hostvars[host]['ansible_default_ipv4']['address'] }}:80 check
{% endfor %}
📝 Registering Variables¶
Basic Registration¶
---
- name: Variable registration examples
hosts: all
tasks:
- name: Check service status
ansible.builtin.systemd:
name: httpd
register: service_status
- name: Display service information
ansible.builtin.debug:
msg: |
Service active: {{ service_status.status.ActiveState }}
Service enabled: {{ service_status.status.UnitFileState }}
Service running: {{ service_status.status.SubState }}
Command Registration¶
- name: Execute command and capture output
ansible.builtin.command: whoami
register: current_user
- name: Execute shell command with pipes
ansible.builtin.shell: ps aux | grep httpd | wc -l
register: httpd_processes
- name: Display command results
ansible.builtin.debug:
msg: |
Current user: {{ current_user.stdout }}
Apache processes: {{ httpd_processes.stdout }}
Return code: {{ current_user.rc }}
Command failed: {{ current_user.failed }}
Advanced Registration Patterns¶
Conditional Logic Based on Registration:
- name: Check web service availability
ansible.builtin.uri:
url: "http://{{ inventory_hostname }}/health"
method: GET
status_code: [200, 503]
register: health_check
ignore_errors: yes
- name: Restart service if unhealthy
ansible.builtin.systemd:
name: httpd
state: restarted
when: health_check.status == 503
- name: Report service status
ansible.builtin.debug:
msg: |
Health check status: {{ health_check.status }}
Service is {{ 'healthy' if health_check.status == 200 else 'unhealthy' }}
Loop Registration:
- name: Check multiple services
ansible.builtin.systemd:
name: "{{ item }}"
register: service_results
loop:
- httpd
- mysql
- sshd
- name: Display all service states
ansible.builtin.debug:
msg: "{{ item.item }} is {{ item.status.ActiveState }}"
loop: "{{ service_results.results }}"
🗂️ Variable Organization Strategies¶
Directory Structure¶
inventory/
├── group_vars/
│ ├── all.yml # Variables for all hosts
│ ├── webservers.yml # Web server group variables
│ ├── databases.yml # Database group variables
│ └── production/ # Environment-specific variables
│ ├── all.yml
│ ├── webservers.yml
│ └── databases.yml
├── host_vars/
│ ├── web01.example.com.yml
│ ├── web02.example.com.yml
│ └── db01.example.com.yml
└── inventory.yml
Variable Organization Best Practices¶
Environment Separation:
# group_vars/all/common.yml
---
app_name: myapp
app_user: webapp
log_level: info
# group_vars/all/development.yml
---
environment: development
debug_enabled: true
log_level: debug
database_host: dev-db.internal.com
# group_vars/all/production.yml
---
environment: production
debug_enabled: false
log_level: warn
database_host: prod-db.internal.com
Service-Specific Variables:
# group_vars/webservers/apache.yml
---
apache:
package: httpd
service: httpd
config_dir: /etc/httpd/conf
document_root: /var/www/html
modules:
- mod_ssl
- mod_rewrite
ports:
- 80
- 443
# group_vars/databases/mysql.yml
---
mysql:
package: mysql-server
service: mysqld
config_file: /etc/my.cnf
data_dir: /var/lib/mysql
port: 3306
root_password: "{{ vault_mysql_root_password }}"
Variable Validation¶
---
- name: Validate required variables
hosts: all
tasks:
- name: Ensure required variables are defined
ansible.builtin.assert:
that:
- app_name is defined
- environment is defined
- database_host is defined
fail_msg: "Required variables are not defined"
success_msg: "All required variables are present"
- name: Validate variable values
ansible.builtin.assert:
that:
- environment in ['development', 'staging', 'production']
- app_port | int > 1024
- database_host | length > 0
fail_msg: "Variable validation failed"
🛠️ Variable Debugging¶
Debug Strategies¶
Basic Variable Display:
- name: Debug variable content
ansible.builtin.debug:
var: variable_name
- name: Debug with custom message
ansible.builtin.debug:
msg: "The value of web_port is {{ web_port }}"
- name: Debug multiple variables
ansible.builtin.debug:
msg: |
Environment: {{ environment }}
Web port: {{ web_port }}
SSL enabled: {{ ssl_enabled }}
Fact Exploration:
- name: Display all facts
ansible.builtin.debug:
var: ansible_facts
- name: Display specific fact categories
ansible.builtin.debug:
var: ansible_facts['network']
- name: Search facts by pattern
ansible.builtin.debug:
msg: "{{ ansible_facts | dict2items | selectattr('key', 'match', 'ansible_eth.*') | list }}"
Variable Source Investigation:
- name: Check if variable is defined
ansible.builtin.debug:
msg: "web_port is {{ 'defined' if web_port is defined else 'undefined' }}"
- name: Show variable type
ansible.builtin.debug:
msg: "web_port is of type {{ web_port | type_debug }}"
- name: Display hostvars for debugging
ansible.builtin.debug:
var: hostvars[inventory_hostname]
🧪 Practical Lab Exercises¶
Exercise 1: Variable Precedence Testing¶
Create a comprehensive precedence test:
- Define the same variable at multiple levels
- Use debug tasks to show final values
- Override with command-line variables
- Document the results
# group_vars/all.yml
test_var: "from group_vars all"
# host_vars/testhost.yml
test_var: "from host_vars"
# Playbook
---
- name: Variable precedence test
hosts: testhost
vars:
test_var: "from play vars"
tasks:
- name: Show variable value
ansible.builtin.debug:
var: test_var
vars:
test_var: "from task vars"
Exercise 2: Fact-Based Configuration¶
Create environment-specific configuration using facts:
---
- name: Configure based on system facts
hosts: all
tasks:
- name: Install appropriate packages for OS
ansible.builtin.dnf:
name: "{{ packages[ansible_facts['distribution'] | lower] }}"
state: present
vars:
packages:
redhat:
- httpd
- php
centos:
- httpd
- php
fedora:
- httpd
- php
Exercise 3: Cross-Host Information Sharing¶
Build configuration files using information from multiple hosts:
---
- name: Gather web server information
hosts: webservers
tasks:
- name: Set web server facts
ansible.builtin.set_fact:
web_server_info:
name: "{{ inventory_hostname }}"
ip: "{{ ansible_default_ipv4.address }}"
port: "{{ web_port | default(80) }}"
- name: Configure load balancer
hosts: loadbalancers
tasks:
- name: Generate load balancer config
ansible.builtin.template:
src: haproxy.cfg.j2
dest: /etc/haproxy/haproxy.cfg
Exercise 4: Dynamic Variable Loading¶
Load variables based on conditions:
---
- name: Dynamic variable loading
hosts: all
tasks:
- name: Load environment-specific variables
ansible.builtin.include_vars: "vars/{{ environment }}.yml"
- name: Load OS-specific variables
ansible.builtin.include_vars: "vars/{{ ansible_facts['distribution'] | lower }}.yml"
🎯 Key Takeaways¶
Variable Management Excellence¶
- Precedence understanding: Know which variable definitions take priority
- Organization strategy: Use logical directory structures for variables
- Data typing: Understand how Ansible handles different data types
- Validation: Always validate critical variables before use
Fact Utilization Mastery¶
- System intelligence: Use facts to make decisions about configuration
- Performance consideration: Disable fact gathering when not needed
- Custom facts: Extend system information with application-specific data
- Fact filtering: Gather only needed information for efficiency
Magic Variable Proficiency¶
- Inventory access: Use magic variables to access host and group information
- Cross-host communication: Share data between hosts using hostvars
- Play context: Understand play-level variables and their scope
- Dynamic targeting: Use magic variables for dynamic host selection
Registration and Debugging Skills¶
- Output capture: Register command and module outputs for later use
- Conditional logic: Make decisions based on registered results
- Debugging techniques: Efficiently troubleshoot variable issues
- Testing strategies: Validate variable behavior during development
🔗 Next Steps¶
With solid variable and fact mastery, you're ready for:
- Module 04: Task Control - Advanced logic with conditionals and loops
- Complex automation logic using when statements and loops
- Error handling strategies with blocks and failure management
- Advanced task delegation and execution control
Your variable skills enable sophisticated automation scenarios!
Next Module: Module 04: Task Control →