Conditionals in Ansible

Ansible supports the use of conditional statements.
This is done mainly with the when keyword, used as a key.

when

when is used to evaluate a condition, written in Jinja2 syntax.
If the condition evaluates to true, the task/block will be executed. If false, it's skipped.

Conditions use Jinja2 syntax, so you can use Jinja2 filters and operators.

when: some_list | length > 5

Syntax of when

tasks: 

  - name: Example conditional task
    debug:
      msg: "This task runs if the condition is true."
    when: some_variable == "value"

---

Conditional Operators (TODO)

Ansible supports the following conditional operators:

• ==
• ``

Examples of Using Conditionals in Ansible

Conditional Task Execution

This task will only execute if the when condition is true.

tasks: 

  - name: Install nginx if the web_server variable is true
    apt:
      name: nginx
      state: present
    when: web_server == true

Using Facts in Conditions

You can use "facts", which are variables that Ansible gathers about the remote host, in your conditions. This is stored as a dictionary, and can be accessed using ansible_facts along with a key (ansible_facts['key']).

tasks:

  - name: Install NGINX on Debian-based systems
    apt: 
      name: nginx
      state: present
    when: ansible_facts['os_family'] == 'Debian'

When you start running Ansible at scale (like 300 machines), you'll want to limit what Ansible gathers. You can run into memory problems.

Using Multiple Conditions

You can use multiple conditions in when statements using and/or.

tasks:

  - name: Task runs if either condition is true
    debug:
      msg: "At least one condition is true."
    when: ansible_facts['os_family'] == 'RedHat' or some_variable == "value"

  - name: Task runs if both conditions are true
    debug:
      msg: "Both conditions are true."
    when: ansible_facts['os_family'] == 'Debian' and some_variable == "value"

Checking for the Existence of a File

Use the stat module to conditionally execute tasks based on the existence of a file.

tasks:

  - name: Check if a file exists
    stat:
      path: /etc/nginx/nginx.conf
    register: nginx_conf

  - name: Run task only if the file exists
    debug: 
      msg: "The nginx config file exists!"
    when: nginx_conf.stat.exists == true

Using Conditional Loops

Use the loop keyword to iterate over a list of items, only if a condition is met.

tasks:

  - name: Install only necessary packages
    apt:
      name: "{{ item }}"
      state: present
    loop:

      - nginx
      - apache2
      - mysql-server
    when: item != "apache2"

Negate Conditions (the not keyword)

Use not to negate a condition.

tasks:

  - name: Skip this task if the variable is true
    debug:
      msg: "This task will run if 'web_server' is false."
    when: not web_server

Using when with Blocks

You can apply conditions to entire blocks of tasks.

tasks:

  - block:
    - name: Install Apache
      apt:
        name: apache2
        state: present

    - name: Start Apache
      service: 
        name: apache2
        state: started
  when: ansible_facts['os_family'] == "Debian"

---

Default Values for Variables in Conditions

If a variable used in a when statement is not defined, the task will always fail.
You can use default values for variables by using the default() filter.

when: my_var | default(false)

Conditionally Include other Playbooks or Tasks

You can use conditions to include other playbooks or tasks.

- import_tasks: tasks.yml
  when: ansible_facts['os_family'] == "Debian"