Ansible - Use default if a variable is not defined

Question

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' }}"

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.

If you have a single play that you want to loop over the items, define that list in group_vars/all or somewhere else that makes sense:

all_items:
  - first
  - second
  - third
  - fourth

Then your task can look like this:

  - name: List items or default list
    debug:
      var: item
    with_items: "{{ varlist | default(all_items) }}"

Pass in varlist as a JSON array:

ansible-playbook <playbook_name> --extra-vars='{"varlist": [first,third]}'

Prior to that, you might also want a task that checks that each item in varlist is also in all_items:

  - name: Ensure passed variables are in all_items
    fail:
      msg: "{{ item }} not  in all_items list"
    when: item not in all_items
    with_items: "{{ varlist | default(all_items) }}"

In case you using lookup to set default read from environment you have also set the second parameter of default to true:

- set_facts:
    ansible_ssh_user: "{{ lookup('env', 'SSH_USER') | default('foo', true) }}"

You can also concatenate multiple default definitions:

- set_facts:
    ansible_ssh_user: "{{ some_var.split('-')[1] | default(lookup('env','USER'), true) | default('foo') }}"

If you are assigning default value for boolean fact then ensure that no quotes is used inside default().

- name: create bool default
  set_fact:
    name: "{{ my_bool | default(true) }}"

For other variables used the same method given in verified answer.

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

The question is quite old, but what about:

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

It looks less cumbersome to me...