Ansible Variables and When Conditional Guide | Generated by AI

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Question: How do I set variables and use the when conditional in an Ansible playbook?

Answer:

Here are the most common and recommended ways to set variables and use the when clause in modern Ansible playbooks:

Setting Variables (vars or set_fact)

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- hosts: all
  tasks:
    # 1. Set variable at play level
    - name: Define a variable directly in the play
      vars:
        my_app_port: 8080
      debug:
        msg: "Port is "

    # 2. Set variable at task level
    - name: Set a simple variable
      set_fact:
        environment: production
        app_version: "1.2.3"

    # 3. Set variable conditionally
    - name: Set fact based on OS
      set_fact:
        package_manager: apt
      when: ansible_os_family == "Debian"

    - name: Set fact based on OS (alternative)
      set_fact:
        package_manager: yum
      when: ansible_os_family == "RedHat"

    # 4. Set variable from registered command output
    - name: Check if service is running
      command: systemctl is-active nginx
      register: nginx_status
      ignore_errors: yes

    - name: Set fact from register
      set_fact:
        nginx_running: ""

Using when Conditional

    - name: Install Nginx only on Ubuntu
      apt:
        name: nginx
        state: present
      when: ansible_distribution == "Ubuntu"

    - name: Restart Nginx only if it was running before
      service:
        name: nginx
        state: restarted
      when: nginx_running | default(false)

    - name: Complex when conditions
      debug:
        msg: "This runs only on prod servers with app_version > 2.0"
      when:
        - environment == "production"
        - app_version is version("2.0", ">")

    - name: Using OR logic
      debug:
        msg: "Runs on either CentOS or Ubuntu"
      when: ansible_distribution in ["CentOS", "Ubuntu"]

    - name: Skip task with custom fact
      debug:
        msg: "This task is skipped on Windows"
      when: ansible_os_family != "Windows"

Best Practices Summary

Method	Scope	Cacheable	Recommended Use Case
vars: (play)	Play level	Yes	Static configuration values
vars: (task)	Task level	Yes	Small, task-specific values
set_fact:	Host level	Yes (if cacheable: yes)	Dynamic values, computed during execution
register: + set_fact:	Host level	Yes	Store command/output results for later use

Use set_fact with cacheable: yes if you want the variable to persist across plays in the same run.

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