ansible get the last character of a fact

I would like to have the last character of a fact. My playbook looks like this:

- name: sf
  set_fact:
    root_dev: "{{ ansible_mounts|json_query('[?mount == `/`].device') }}"

- name: e
  debug:
    msg: "{{ root_dev[:-1] }}"

The problem is the output in this case always:

"msg": []

or if I use the playbook without semicolon:

  debug:
    msg: "{{ root_dev[-1] }}"

then the whole partition will be the output:

"msg": "/dev/sda1"

I also can't quote the whole root_dev because it is a fact and I would like to get the last character of it's value. The split filter also not working in this case, because the device can be /dev/sda or /dev/mapper/root_part_0 and so. What would be the best option in this case?

Solution

Explanation of the problem:

You have to understand how the slicing works. Given the lists l1 and l2 for testing

  l1: [a]
  l2: [a, b, c]

The index [-1] gives you the last item (which is also the first item)

  l1[-1]: a

The slice [:-1] gives you all items but the last one. The result is an empty list because there is only one item

l1[:-1]: []

Test it by slicing l2

 l2[:-1]: [a, b]

Solution:

The value of root_dev is a list

  root_dev: "{{ ansible_mounts | json_query('[?mount == `/`].device') }}"

In your case

  root_dev: ['/dev/sda1']

To get the number of the partition, index the first item in the list and the last character in the item

  root_dev[0][-1]: 1

^{Example of a complete playbook for testing

- hosts: localhost

vars:

root_dev: "{{ ansible_mounts | json_query('[?mount == `/`].device') }}"

tasks:

- setup:
gather_subset: mounts
# - debug:
# var: ansible_mounts

- debug:
var: root_dev
- debug:
var: root_dev[0][-1]

The above solution works for a single-digit partitions only. Use regex_replace to cover also multiple-digit options

part_no: "{{ root_dev.0 | regex_replace('^(.+?)(\\d*)$', '\\2') }}"}