mjhjmtu

Ansible in 1.4 and later provides a way to specify this behavior as follows:

- name: Fail task when the command error output prints FAILED
 command: /usr/bin/example-command -x -y -z
 register: command_result
 failed_when: "'FAILED' in command_result.stderr"

but the common Return Values already include .failed and .changed.

It seems to me these features contradict each other. Is there an official resolution to it? Does Ansible define one of failed_when or changed_when as being evaluated first and affecting the latter? Does it define that both are evaluated without seeing the effect of the other?

E.g. what is the defined behaviour of

1. 
   command_result.failed within a failed_when expression


2. 
   command_result.changed within a changed_when expression


3. 
   command_result.failed within a changed_when expression


4. 
   command_result.changed within a changed_when expression

To be more clear, e.g. for the third one I'm interested in how well-defined (reliable) the result of the following would be:

- package:
 name: systemd
 state: present
 check_mode: yes
 register: command_result
 failed_when: command_result.changed

Let's say I'm interested in Ansible 2.0 and above.

This looks wrong

Your example adds another path to your playbook. If systemd is not installed, the output will be different than if it is installed. And after the first run it will be installed. This goes against the ansible priciple that says:

Idempotency

An operation is idempotent if the result of performing it once is exactly the same as the result of performing it repeatedly without any
intervening actions.

If you still do it, make it as explicit as possible

I suggest you run a command which systemctl and register the output. Check the output to install systemd and fail with a fail task.

This is still a very interesting question

I guess there is no real documentation and we have to investigate.

I hope I catched all cases :) but I cannot fill the chart right now.

playbook.yml:

---

- name: test some stuff
 hosts: all
 tasks:
 - include_tasks: tasks.yml
 with_items:
 - data: ping, changed: true 
 - data: ping, changed: false 
 - data: crash, changed: true 
 - data: crash, changed: false 

tasks.yml

---

- name: Check for command_result is defined and command_result
 ping:
 data: " item.data "
 register: command_result
 changed_when: item.changed
 failed_when: command_result is defined and command_result
 ignore_errors: true

- name: Check for command_result is defined and command_result
 file:
 path: ./file
 register: command_result
 changed_when: item.changed
 failed_when: command_result is defined and command_result
 ignore_errors: true

- name: Check for command_result
 ping:
 data: " item.data "
 register: command_result
 changed_when: item.changed
 failed_when: command_result
 ignore_errors: true

- name: Check for command_result
 file:
 path: ./file
 register: command_result
 changed_when: item.changed
 failed_when: command_result
 ignore_errors: true

- name: Check for command_result.changed is defined and command_result.changed
 ping:
 data: " item.data "
 register: command_result
 changed_when: item.changed
 failed_when: command_result.changed is defined and command_result.changed

- name: Check for command_result.changed is defined and command_result.changed
 ping:
 data: " item.data "
 register: command_result
 changed_when: item.changed
 failed_when: command_result.changed is defined and command_result.changed
 ignore_errors: true

- name: Check for command_result.changed
 ping:
 data: " item.data "
 register: command_result
 changed_when: item.changed
 failed_when: command_result.changed
 ignore_errors: true

- name: Check for command_result.changed
 file:
 path: ./file
 register: command_result
 changed_when: item.changed
 failed_when: command_result.changed
 ignore_errors: true