Ansible: regex_search filter comparison and how to debug when clauses

Question

I spent a bit of time today trying to write some Ansible plays to only run commands if the corresponding lines weren't present in the relevant command output. After a bit of trial & error I've got something that works for me, but I'm not clear as to why my initial comparison with an empty string doesn't work.

Here is a playbook demonstrating my problem:

- name: test
  hosts: localhost
  tasks:
  - shell: "cat /tmp/cmdoutput"
    register: cmdoutput

  - debug: var=filtered_output
    vars:
      filtered_output: "{{ cmdoutput.stdout | regex_search(item) }}"
    with_items:
      - "aa .* xx"
      - "bb .* yy"

  - debug: msg="do action that inserts {{ item }}"
    with_items:
      - "aa .* xx"
      - "bb .* yy"
    when: cmdoutput.stdout | regex_search(item) == ""

  - debug: msg="do action that inserts {{ item }}"
    with_items:
      - "aa .* xx"
      - "bb .* yy"
    when: not cmdoutput.stdout | regex_search(item)

cat /tmp/cmdoutput
aa b c d xx
aa f g h yy
bb i j k xx

This results in the following output:

$ ansible-playbook test.yml 
 [WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'


PLAY [test] **********************************************************************************************************************

TASK [Gathering Facts] ***********************************************************************************************************
ok: [localhost]

TASK [shell] *********************************************************************************************************************
changed: [localhost]

TASK [debug] *********************************************************************************************************************
ok: [localhost] => (item=None) => {
    "filtered_output": "aa b c d xx"
}
ok: [localhost] => (item=None) => {
    "filtered_output": ""
}

TASK [debug] *********************************************************************************************************************
skipping: [localhost] => (item=None) 
skipping: [localhost] => (item=None) 

TASK [debug] *********************************************************************************************************************
skipping: [localhost] => (item=None) 
ok: [localhost] => (item=None) => {
    "msg": "do action that inserts bb .* yy"
}

PLAY RECAP ***********************************************************************************************************************
localhost                  : ok=4    changed=1    unreachable=0    failed=0   

i.e. "filtered_output": "", but the following when comparison doesn't match.

So my questions are:

• why doesn't the second debug when condition match ""?
• what type of object is the output of the regex_search? - a string or an array or something else?
• is there any, or where could I find more info on the regex_search filter? - (at this point in time AFAICT) it's not in the jinja doco and only appears in the filters section of the official docs as brief examples

My Ansible version: 2.5.1

Thanks

Answer 1

To answer your question

why doesn't the second debug when condition match ""?

When there is no regex match object of type 'NoneType' is returned. This type has no length. Instead of testing empty string ( off-topic see E602)

when: cmdoutput.stdout|regex_search(item) == ""

use (you already have it in your example).

when: not cmdoutput.stdout|regex_search(item)