Ansible - Use default if a variable is not defined

If anybody is looking for an option which handles nested variables, there are several such options in this github issue.

In short, you need to use "default" filter for every level of nested vars. For a variable "a.nested.var" it would look like:

- hosts: 'localhost'
  tasks:
    - debug:
        msg: "{{ ((a | default({})).nested | default({}) ).var | default('bar') }}"

or you could set default values of empty dicts for each level of vars, maybe using "combine" filter. Or use "json_query" filter. But the option I chose seems simpler to me if you have only one level of nesting.

You can use Jinja's default:

- name: Create user
  user:
    name: "{{ my_variable | default('default_value') }}"

Not totally related, but you can also check for both undefined AND empty (for e.g my_variable:) variable. (NOTE: only works with ansible version > 1.9, see: link)

- name: Create user
  user:
    name: "{{ ((my_variable == None) | ternary('default_value', my_variable)) \
    if my_variable is defined else 'default_value' }}"