Using when in ansible tasks

The when allows you to run ansible task only when a condition is met.

Based on variables

Run a certain task only when a certain variable is set.

tasks:
  - name: Configure SELinux to start mysql on any port
    ansible.posix.seboolean:
      name: mysql_connect_any
      state: true
      persistent: true
    when: ansible_selinux.status == "enabled"

Based on magic variables

Run a certain task only when the hosts meets a certain criteria.

tasks:
  - name: my task
    ...
    when: 

    # Host is in the group 'web'
    - "'web' in group_names"

    # Host is the first member of the group 'db'
    - inventory_hostname == groups['db'][0]

    # Host is the last member of the group 'dc'
    - inventory_hostname == hostvars[groups['dc']|last]['inventory_hostname']

Based on ansible facts

Run a certain task only when a certain fact is set.

tasks:
  - name: Shut down Debian systems
    ansible.builtin.command: /sbin/shutdown -t now
    when: ansible_facts['os_family'] == "Debian"

Combine conditions

You can combine conditions using and and or.

tasks:
  - name: my task
    ...
    when: 

    # Host is in the group 'web' and in the group 'db'
    - "'web' in group_names"
    - "'db' in group_names"

    # Host is in the group 'web' or debian based
    - "'web' in group_names or ansible_facts['os_family'] == 'Debian'"

Reference

Ansible docs - conditionals