Ansible - unable to append correctly using the file plugin - ansible

Question

I'm trying to use the Ansible file plugin to append data to a txt file.
This is my playbook:
- name: Setting some variables.
hosts: "{{ target }}"
gather_facts: no
connection: local
tasks:
- name: Delete newDevices file.
file:
path: newDevices.txt
state: absent
run_once: true
- name: Create newDevices file.
file:
path: newDevices.txt
state: touch
run_once: true
- name: Add new devices to file.
delegate_to: 127.0.0.1
lineinfile:
insertafter: EOF
path: newDevices.txt
line: "{{ inventory_hostname }}"
This is my host file:
[lab]
routerA01.mgt.net
routerA02.mgt.net
routerB01.mgt.net
routerB02.mgt.net
This is how I'm running the playbook:
ansible-playbook myplaybook.yml -i hosts --limit "lab[0-1]" -e target=lab
The newDevices.txt file gets deleted and recreated but the contents of the file vary from having 1 hostname or 2 hostnames. If I change the limit in the run command to --limit "lab[0-2]" then the same thing happens. Sometimes 2 hostnames will print and other times all 3 will print. Seems like there will always be a time when the newDevices.txt file will contain one less hostname in it.
Not sure why this is happening. Tried adding a pause after the file is created to maybe give the processing a little extra time, but that didn't help either.

Answer 1

There is a couple of improvements
Put all tasks into a block delegated to localhost. All of them should run only once on the localhost anyway.
Then, there is no point to declare global connection: local
Set hosts: all if you plan to limit the inventory on the command line. In this case, there is no point to put the group into a variable.
Write the file in a loop. This avoids concurrent writing from multiple instances. (What is probably the main reason for your troubles.)
Given the (simplified) inventory
shell> cat hosts
[lab]
A01
A02
B01
B02
The playbook
shell> cat pb.yml
- hosts: all
gather_facts: false
tasks:
- block:
- file:
path: newDevices.txt
state: absent
- file:
path: newDevices.txt
state: touch
- lineinfile:
insertafter: EOF
path: newDevices.txt
line: "{{ item }}"
loop: "{{ ansible_play_hosts }}"
delegate_to: localhost
run_once: true
gives
shell> ansible-playbook pb.yml --limit lab[0:1]
PLAY [all] **************************************************************************************
TASK [file] *************************************************************************************
changed: [A01 -> localhost]
TASK [file] *************************************************************************************
changed: [A01 -> localhost]
TASK [lineinfile] *******************************************************************************
changed: [A01 -> localhost] => (item=A01)
changed: [A01 -> localhost] => (item=A02)
PLAY RECAP **************************************************************************************
A01: ok=3 changed=3 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
shell> cat newDevices.txt
A01
A02
The above playbook is not idempotent. The file is always recreated. The result is that the files will always keep the hosts from the last play only.
The simplified playbook below is idempotent. The list of the hosts in the file, however, will be cumulative
shell> cat pb.yml
- hosts: all
gather_facts: false
tasks:
- lineinfile:
create: true
insertafter: EOF
path: newDevices.txt
line: "{{ item }}"
loop: "{{ ansible_play_hosts }}"
delegate_to: localhost
run_once: true
gives
shell> ansible-playbook pb.yml --limit lab[0:1]
PLAY [all] **************************************************************************************
TASK [lineinfile] *******************************************************************************
changed: [A01 -> localhost] => (item=A01)
changed: [A01 -> localhost] => (item=A02)
PLAY RECAP **************************************************************************************
A01: ok=1 changed=1 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
shell> cat newDevices.txt
A01
A02
shell> ansible-playbook pb.yml --limit lab[0:1]
PLAY [all] **************************************************************************************
TASK [lineinfile] *******************************************************************************
ok: [A01 -> localhost] => (item=A01)
ok: [A01 -> localhost] => (item=A02)
PLAY RECAP **************************************************************************************
A01: ok=1 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
shell> ansible-playbook pb.yml --limit lab[0:2]
PLAY [all] **************************************************************************************
TASK [lineinfile] *******************************************************************************
ok: [A01 -> localhost] => (item=A01)
ok: [A01 -> localhost] => (item=A02)
changed: [A01 -> localhost] => (item=B01)
PLAY RECAP **************************************************************************************
A01: ok=1 changed=1 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
shell> cat newDevices.txt
A01
A02
B01
shell> ansible-playbook pb.yml --limit lab[0]
PLAY [all] **************************************************************************************
TASK [lineinfile] *******************************************************************************
ok: [A01 -> localhost] => (item=A01)
PLAY RECAP **************************************************************************************
A01: ok=1 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
shell> cat newDevices.txt
A01
A02
B01
The playbook below is idempotent and the file will keep the list of hosts from the last play only
shell> cat pb.yml
- hosts: all
gather_facts: false
tasks:
- copy:
dest: newDevices.txt
content: |-
{% for host in ansible_play_hosts %}
{{ host }}
{% endfor %}
delegate_to: localhost
run_once: true

Answer 2

Another way to do this would be to have a play targeting localhost itself since the file is to be created on the controller anyway. The file can be created using the "items" of lab inventory group.
Something like below:
- hosts: localhost
connection: local
tasks:
- file:
path: newDevices.txt
state: absent
- lineinfile:
path: newDevices.txt
create: yes
insertafter: EOF
line: "{{ item }}"
loop: "{{ groups['lab'][0:2] }}"
In the above example, the file gets created by lineinfile task itself. You can also remove/change the list slice of [0:2] as per your requirement.
Then run it with:
ansible-playbook myplaybook.yml -i hosts