Why doesn't ansible.builtin.lineinfile behave as expected in this particular task?

Question

I'm attempting to update /boot/firmware/cmdline.txt on a few Raspberry Pis to enable cgroup support. Seemed a good fit for ansible.builtin.lineinfile.

The existing contents of /boot/firmware/cmdline.txt are a single line of key-value pairs

console=serial0,115200 dwc_otg.lpm_enable=0 console=tty1 root=LABEL=writable rootfstype=ext4 rootwait fixrtc quiet splash

I want the following key-value pairs to be appended to any existing contents of that file that might exist.

• cgroup_enable=cpuset
• cgroup_enable=memory
• cgroup_memory=1
• swapaccount=1

In the end, I want the file to look like

console=serial0,115200 dwc_otg.lpm_enable=0 console=tty1 root=LABEL=writable rootfstype=ext4 rootwait fixrtc quiet splash cgroup_enable=cpuset cgroup_enable=memory cgroup_memory=1 swapaccount=1

Here's the task I created:

- name: "Modify /boot/firmware/cmdline.txt to enable cGroup support if not already enabled"
    ansible.builtin.lineinfile:
      path: /boot/firmware/cmdline.txt
      regexp: '(.*) ((cgroup_enable=cpuset|cgroup_enable=memory|cgroup_memory=1|swapaccount=1)( {0,}|\n)){4}(.*)'
      line: '$1 cgroup_enable=cpuset cgroup_enable=memory cgroup_memory=1 swapaccount=1 $5'
      state: present
      backrefs: true
      backup: true
      owner: root
      group: root
      mode: '0755'

Unfortunately, it doesn't seem to update the file and I'm struggling to understand why. To clarify, the task executes fine but indicates that the file already matches the regexp with the following output:

TASK [common : Modify /boot/firmware/cmdline.txt to enable cGroup support if not already enabled] **************************
ok: [kube02.local]
ok: [kube03.local]
ok: [kube01.local]

As far as I can tell, it does not and I've tested in a few ways. Increasing verbosity of this doesn't help me figure this out, unfortunately.

I did find this post's answer which has a more robust solution, but I would love to understand why my solution doesn't work.

Answer 1

Put the new items into a list. For example,

  cmdline_add:
    - cgroup_enable=cpuset
    - cgroup_enable=memory
    - cgroup_memory=1
    - swapaccount=1

Add the items in the loop

  - lineinfile:
      path: /boot/firmware/cmdline.txt
      regexp: '^(?!.*?{{ item }})(.*)$'
      line: '\1 {{ item }}'
      backrefs: true
    loop: "{{ cmdline_add }}"

Because of the negative lookahead assertion (?!...) the regexp matches if the item is not in the line. The task is idempotent.

---

Example. Consolidate the configuration into comma-separated lists

Given the file

shell> cat /tmp/cmdline.txt

console=serial0,115200 dwc_otg.lpm_enable=0 console=tty1 root=LABEL=writable rootfstype=ext4 rootwait fixrtc quiet splash

The play

- hosts: localhost

  vars:

    cmdline_path: /tmp/cmdline.txt
    cmdline_add:
      - cgroup_enable=cpuset
      - cgroup_enable=memory
      - cgroup_memory=1
      - swapaccount=1

    cmdline_add_items: "{{ cmdline_add|map('split', '=', 1) }}"
    cmdline_add_list: |
      {% filter from_yaml %}
      {% for i in cmdline_add_items %}
      - {key: {{ i.0 }}, value: {{ i.1|d('')|split(',') }} }
      {% endfor %}
      {% endfilter %}

    cmdline_items: "{{ cmdline_out.stdout|trim|split(' ')|map('split', '=', 1) }}"
    cmdline_list: |
      {% filter from_yaml %}
      {% for i in cmdline_items %}
      - {key: {{ i.0 }}, value: {{ i.1|d('')|split(',') }} }
      {% endfor %}
      {% endfilter %}
     
    cmdline_dict: "{{ [cmdline_list, cmdline_add_list]|
                      community.general.lists_mergeby('key', list_merge='append_rp')|
                      items2dict }}"
    cmdline: >-
      {% for k,v in cmdline_dict.items() -%}
      {% if v.0|length > 0 -%}
      {{ k }}={{ v|join(',') }}
      {% else -%}
      {{ k }}
      {% endif -%}
      {% endfor -%}

  tasks:

    - command: "cat {{ cmdline_path }}"
      register: cmdline_out

    - block:
        - debug:
            var: cmdline_add_items|to_yaml
        - debug:
            var: cmdline_add_list|to_yaml
        - debug:
            var: cmdline_items|to_yaml
        - debug:
            var: cmdline_list|to_yaml
        - debug:
            var: cmdline_dict|to_yaml
        - debug:
            var: cmdline
      when: debug|d(false)|bool

    - copy:
        dest: "{{ cmdline_path }}"
        content: "{{ cmdline }}"

gives

shell> cat /tmp/cmdline.txt

cgroup_enable=cpuset,memory cgroup_memory=1 console=serial0,115200,tty1 dwc_otg.lpm_enable=0 fixrtc quiet root=LABEL=writable rootfstype=ext4 rootwait splash swapaccount=1