Conditionally define variable in Ansible - ansible

Question

I want to conditionally define a variable in an Ansible playbook like this:
my_var: "{{ 'foo' if my_condition}}"
I would like the variable to remain undefined if the condition does not resolve to true.
Ansible gives the following error if I try to execute the code:
fatal: [foo.local] => {'msg': 'AnsibleUndefinedVariable: One or more undefined
variables: the inline if-expression on line 1 evaluated
to false and no else section was defined.', 'failed': True}
Why is this an error anyway?
The complete case looks like this:
{role: foo, my_var: "foo"}
If my_var is defined, the role does something special. In some cases, I don't want the role to do this. I could use when: condition, but then I would have to copy the whole role block. I could also use an extra bool variable, but I would like a solution without having to change the "interface" to the role.
Any ideas?

Answer 1

You could use something like this:
my_var: "{{ 'foo' if my_condition else '' }}"
The 'else' will happen if condition not match, and in this case will set a empty value for the variable. I think this is a short, readable and elegant solution.

Answer 2

This code may help you to define a variable with condition.
- hosts: node1
gather_facts: yes
tasks:
- name: Check File
shell: ls -ld /etc/postfix/post-install
register: result
ignore_errors: yes
- name: Define Variable
set_fact:
exists: "{{ result.stdout }}"
when: result|success
- name: Display Variable
debug: msg="{{ exists }}"
ignore_errors: yes
So here the exists will display only if the condition is true.

Answer 3

My example, after https://stackoverflow.com/a/43403229/5025060:
vars:
sudoGroup: "{{ 'sudo' if ansible_distribution == 'Ubuntu' else 'wheel' }}"
Because of the different sudo conventions used by Ubuntu versus other platforms, here I am telling Ansible to set a variable named sudoGroup to sudo if the platform is Ubuntu, otherwise set it to wheel.
Later in my playbook, I combine the variable with Ansible's user module to add either sudo or wheel to an account's secondary groups depending on the OS Ansible is running on:
- name: Add or update bob account
user:
name: bob
uid: 3205
groups: "{{ sudoGroup }}"
append: yes
NOTES:
Double quotes around the {{ variable }} are required in the user: groups: definition above.
Once I define sudoGroup as above in my playbook's global vars: section, Ansible configures it at run time (based on ansible_distribution) for each target I define in my hosts: section.

Answer 4

I believe you're after the default(omit) filter. (Reference).
As per the example, mode will behave like it wasn't set at all for the first two items in the loop.
- name: touch files with an optional mode
file:
dest: "{{item.path}}"
state: touch
mode: "{{item.mode|default(omit)}}"
loop:
- path: /tmp/foo
- path: /tmp/bar
- path: /tmp/baz
mode: "0444"

Answer 5

This can be set as with bool:
- name: Conditional (true and false)
set_fact:
my_boolean_set_to_be: "{{ 'true' if my_var == 'foo' else 'false' }}"
- name: Display Variable
debug: msg="{{ my_boolean_set_to_be }}"
This can be set as for more conditionals like 'if-ifelse-else' statements:
- name: Conditional for 'my_var' (2 options and one default)
set_fact:
my_var_set_to_be: "{{ 'breakfast' if my_var == 'morning' else 'lunch' if my_var == 'afternoon' else 'dinner' }}"
- name: Display Variable
debug: msg="{{ my_var_set_to_be }}"