How do I find which register attribute to use in Ansible? - ansible

Question

I am testing out the results of the register commands and it yields different attribute for various tasks: failures, msg, stderr, err..etc
- yum:
name: packagenotfound
state: present
ignore_errors: yes
register: command_result
- debug:
msg: "{{ command_result }}"
ok: [ansible] => {
"msg": {
"changed": false,
"failed": true,
"failures": [
"No package packagenotfound available."
],
"msg": "Failed to install some of the specified packages",
"rc": 1,
"results": []
}}
And
- lvg:
pvs: /dev/sddnotfound
vg: vgdata
ignore_errors: yes
register: command_result
- debug:
msg: "{{ command_result }}"
ok: [ansible] => {
"msg": {
"changed": false,
"failed": true,
"msg": "Device /dev/sddnotfound not found."
}
And
- shell: thiscommandwontwork
ignore_errors: yes
register: command_result
- debug:
msg: "{{ command_result }}"
ok: [ansible] => {
"msg": {
"changed": true,
"cmd": "thiscommandwontwork",
"delta": "0:00:00.002560",
"end": "2020-02-05 04:24:35.297556",
"failed": true,
"msg": "non-zero return code",
"rc": 127,
"start": "2020-02-05 04:24:35.294996",
"stderr": "/bin/sh: thiscommandwontwork: command not found",
"stderr_lines": [
"/bin/sh: thiscommandwontwork: command not found"
],
"stdout": "",
"stdout_lines": []
}
And
- lvol:
lv: lvdata
vg: vgroup
size: 2000M
ignore_errors: yes
register: command_result
- debug:
msg: "{{ command_result }}"
ok: [ansible] => {
"msg": {
"changed": false,
"err": " Volume group \"vgroup\" not found\n Cannot process volume group vgroup\n",
"failed": true,
"msg": "Volume group vgroup does not exist.",
"rc": 5
}
Now if I tried to use when: '"xxx" in command_result.err' with yum task for example, it will result in dict_object not found error.
Is there a way to find out which attribute to use without testing?

Answer 1

Testing is definitely the easiest and fastest way to have a look at the content of your registered var in several situation and to take decisions on how to use it in your playbook.
Meanwhile, there are ways to have a global knowledge of what is returned in your registered var from the documenations:
There is a page on Common modules return values
Modules returning specific values usually document them on each relevant doc page. Here is an example for the stat module
You should also be aware that the global register structure is changed when using a loop by addind a top level results list, as explained in registering variables
Knowing what could be in your register does not mean it will be. Your example mentions the (undocumented...) err attribute for the lvol module, which will only be available for an lvol task in error. You can work around such cases by using tests (like my_register is failed) or defaulting values with the default filter.