Why register is registering even if the module when condition is false? - ansible

Question

I was having a task file which will get container name which has particular volume mounted. The result is stored in container_id variable. This task file need to be executed in linux as well windows. So i added conditional execution but the 'register' variable is behaving weird.
- name: Get the container ID for volume "{{vol}}" mounted
shell: "docker ps -a -q --filter volume={{vol}}"
register: container_id
when: ansible_os_family != "Windows"
- name: Get the container ID for volume "{{vol}}" mounted
win_shell: "docker ps -a -q --filter volume={{vol}}"
register: container_id
when: ansible_os_family == "Windows"
- debug:
var: container_id
When i run this in linux node , i thought it will get all the container name in container_id variable. But to my surprise the output is
ok: [remotenode] => {
"container_id": {
"changed": false,
"skip_reason": "Conditional result was False",
"skipped": true
}
}
which means the skipped output from windows module is registered into container_id variable.
Then i placed the debug below linux module as below , which ensured that register at that point of time has proper value
- name: Get the container ID for volume "{{vol}}" mounted
shell: "docker ps -a -q --filter volume={{vol}}"
register: container_id
when: ansible_os_family != "Windows"
- debug:
var: container_id
- name: Get the container ID for volume "{{vol}}" mounted
win_shell: "docker ps -a -q --filter volume={{vol}}"
register: container_id
when: ansible_os_family == "Windows"
The output is as below
ok: [remotenode] => {
"container_id": {
"changed": true,
"cmd": "docker ps -a -q --filter volume=origvolfd48c6",
"delta": "0:00:00.036279",
"end": "2019-07-30 02:32:13.203036",
"failed": false,
"rc": 0,
"start": "2019-07-30 02:32:13.166757",
"stderr": "",
"stderr_lines": [],
"stdout": "fb81938cdbbe",
"stdout_lines": [
"fb81938cdbbe"
]
}
}
So the register is having value even if the module is skipped. Why it behaves this way. Is there any workaround?

Answer 1

If you take a look at the "Using variables" section of Ansible's Docs you'll see this note:
If a task fails or is skipped, the variable still is registered with a failure or skipped status, the only way to avoid registering a variable is using tags.
So what is happening to you is Ansible's default behaviour. You could modify your playbook to use tags like this:
- name: Get the container ID for volume "{{vol}}" mounted
shell: "docker ps -a -q --filter volume={{vol}}"
register: container_id
tags:
- linux
- name: Get the container ID for volume "{{vol}}" mounted
win_shell: "docker ps -a -q --filter volume={{vol}}"
register: container_id
tags:
- windows
- debug:
var: container_id
Then you can run it using the --tags or --skip-tags options.