Chapter 3 RH294 RHEL Automation with Ansible

ONLY FOR SELF STUDY, NO COMMERCIAL USAGE!!!

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Chapter 3. Managing Variables and Facts

Managing Variables

Variables provide a convenient way to manage dynamic values for a given environment in your Ansible project. Examples of values that variables might contain include:

• Users to create
• Packages to install
• Services to restart
• Files to remove
• Archives to retrieve from the internet

Naming Variables

Variable names must start with a letter, and they can only contain letters, numbers, and underscores.

The following table illustrates the difference between invalid and valid variable names.

Table 3.1. Examples of Invalid and Valid Ansible Variable Names

Invalid variable names	Valid variable names
web server	web_server
remote.file	remote_file
1st file	file_1``file1
remoteserver$1	remote_server_1 or remote_server1

Defining Variables

The following simplified list shows ways to define a variable, ordered from the lowest precedence to the highest:

• Group variables defined in the inventory (lowest)

• Group variables defined in files in a group_vars subdirectory in the same directory as the inventory or the playbook

• Host variables defined in the inventory

• Host variables defined in files in a host_vars subdirectory in the same directory as the inventory or the playbook

• Host facts, discovered at runtime

• Play variables in the playbook (vars and vars_files)

  • One common method is to place a variable in a vars block at the beginning of a play:

    - hosts: all
      vars:
        user: joe
        home: /home/joe
    or
    
    - hosts: all
      vars_files:
        - vars/users.yml
    

  • Using Variables in Playbooks

    Variables are referenced by placing the variable name in double braces ({ { }}). Ansible substitutes the variable with its value when the task is executed.(quotes are mandatory if the variable is the first element to start)

    vars:
      user: joe n
    
    tasks:
      # This line will read: Creates the user joe
      - name: Creates the user {
           
           {
           
            user }}
        user:
          # This line will create the user named Joe
          name: "{
           
           { user }}"
    

• Task variables

• Extra variables defined on the command line (highest)

  • by using the --extra-vars or -e option and specifying those variables

  • [user@demo ~]$ ansible-navigator run main.yml -e "package=apache"
    

For the Directories to Populate Host and Group Variables

The following examples illustrate this approach in more detail. Consider a scenario where you need to manage two data centers, and the data center hosts are defined in the ~/project/inventory inventory file:

[datacenter1]
demo1.example.com
demo2.example.com

[datacenter2]
demo3.example.com
demo4.example.com

[datacenters:children]
datacenter1
datacenter2

• If you need to define a general value for all servers in both data centers, set a group variable for the datacenters host group:

  [admin@station project]$ cat ~/project/group_vars/datacenters
  package: httpd
  

• If you need to define difference values for each data center, set a group variable for each data center host group:

  [admin@station project]$ cat ~/project/group_vars/datacenter1
  package: httpd
  [admin@station project]$ cat ~/project/group_vars/datacenter2
  package: apache
  

• If you need to define different values for each managed host in every data center, then define the variables in separate host variable files:

  [admin@station project]$ cat ~/project/host_vars/demo1.example.com
  package: httpd
  [admin@station project]$ cat ~/project/host_vars/demo2.example.com
  package: apache
  [admin@station project]$ cat ~/project/host_vars/demo3.example.com
  package: mariadb-server
  [admin@station project]$ cat ~/project/host_vars/demo4.example.com
  package: mysql-server
  

The directory structure for the example project, project, if it contained all the example files above, would appear as follows:

project
├── ansible.cfg
├── group_vars
│   ├── datacenters
│   ├── datacenters1
│   └── datacenters2
├── host_vars
│   ├── demo1.example.com
│   ├── demo2.example.com
│   ├── demo3.example.com
│   └── demo4.example.com
├── inventory
└── playbook.yml

Using Dictionaries as Variables

For example, consider the following snippet:

user1_first_name: Bob
user1_last_name: Jones
user1_home_dir: /users/bjones
user2_first_name: Anne
user2_last_name: Cook
user2_home_dir: /users/acook

This could be rewritten as a dictionary called users:

users:
  bjones:
    first_name: Bob
    last_name: Jones
    home_dir: /users/bjones
  acook:
    first_name: Anne
    last_name: Cook
    home_dir: /users/acook

You can then use the following variables to access user data:

# Returns 'Bob'
users.bjones.first_name

# Returns '/users/acook'
users.acook.home_dir

Because the variable is defined as a Python dictionary, an alternative syntax is available.

# Returns 'Bob'
users['bjones']['first_name']

# Returns '/users/acook'
users['acook']['home_dir']

Capturing Command Output with Registered Variables

You can use the register statement to capture the output of a command or other information about the execution of a module.

The following play demonstrates how to capture the output of a command for debugging purposes:

---
- name: Installs a package and prints the result
  hosts: all
  tasks:
    - name: Install the package
      ansible.builtin.dnf:
        name: httpd
        state: installed
      register: install_result

    - debug:
        var: install_result

When you run the play, the debug module dumps the value of the install_result registered variable to the terminal.

[user@demo ~]$ ansible-navigator run playbook.yml -m stdout
PLAY [Installs a package and prints the result] ****************************

TASK [setup] ***************************************************************
ok: [demo.example.com]

TASK [Install the package] *************************************************
ok: [demo.example.com]

TASK [debug] ***************************************************************
ok: [demo.example.com] => {
    "install_result": {
        "changed": false,
        "msg": "",
        "rc": 0,
        "results": [
            "httpd-2.4.51-7.el9_0.x86_64 providing httpd is already installed"
        ]
    }
}

PLAY RECAP *****************************************************************
demo.example.com    : ok=3    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

REFERENCES

How to build your inventory — Ansible Documentation

Using Variables — Ansible Documentation

Variable precedence: Where should I put a variable?

YAML Syntax — Ansible Documentation

Data-variable example

---
- name: Deploy and start Apache HTTPD service
  hosts: webserver
  vars:
    web_pkg: httpd
    firewall_pkg: firewalld
    web_service: httpd
    firewall_service: firewalld
    rule: http

  tasks:
    - name: Required packages are installed and up to date
      ansible.builtin.dnf:
        name:
          - "{
   
   { web_pkg  }}"
          - "{
   
   { firewall_pkg }}"
        state: latest

    - name: The {
   
   {
   
    firewall_service }} service is started and enabled
      ansible.builtin.service:
        name: "{
   
   { firewall_service }}"
        enabled: true
        state: started

    - name: The {
   
   {
   
    web_service }} service is started and enabled
      ansible.builtin.service:
        name: "{
   
   { web_service }}"
        enabled: true
        state: started

    - name: Web content is in place
      ansible.builtin.copy:
        content: "Example web content"
        dest: /var/www/html/index.html

    - name: The firewall port for {
   
   {
   
    rule }} is open
      ansible.posix.firewalld:
        service: "{
   
   { rule }}"
        permanent: true
        immediate: true
        state: enabled