Ansible: How to test that a registered variable is not empty?

How can I test that stderr is non empty::

- name: Check script
  shell: . {{ venv_name }}/bin/activate && myscritp.py
  args:
    chdir: "{{ home }}"
  sudo_user: "{{ user }}"
  register: test_myscript

- debug: msg='myscritp is Ok'
  when: not test_myscript.stderr

So if there is no error I could read::

  TASK: [deploy | debug msg='critp is Ok] *******
  ok: [vagrant] => {
      "msg": "myscritp is Ok"
  }

In case the stderr is not empty a FATAl error occurs.

^{(ansible 2.9.6 ansible-lint 5.3.2)}

See ansible-lint rules. The condition below results in error: 'empty-string-compare: Don't compare to empty string'

    when: test_myscript.stderr != ""

Correct syntax is

    when: test_myscript.stderr | length > 0

Quoting from source code

> Use when: var|length > 0 rather than when: var != "" (or ' 'conversely when: var|length == 0 rather than when: var == "")

---

Notes

1. Test empty bare variable e.g.

    - debug:
        msg: "Empty string '{{ var }}' evaluates to False"
      when: not var
      vars:
        var: ''

    - debug:
        msg: "Empty list {{ var }} evaluates to False"
      when: not var
      vars:
        var: []

give

  msg: Empty string '' evaluates to False
  msg: Empty list [] evaluates to False

2. But, testing non-empty bare variable string depends on CONDITIONAL_BARE_VARS. Setting ANSIBLE_CONDITIONAL_BARE_VARS=false the condition works fine but setting ANSIBLE_CONDITIONAL_BARE_VARS=true the condition will fail

    - debug:
        msg: "String '{{ var }}' evaluates to True"
      when: var
      vars:
        var: 'abc'

gives

fatal: [localhost]: FAILED! => 
  msg: |-
    The conditional check 'var' failed. The error was: error while 
    evaluating conditional (var): 'abc' is undefined

Explicit cast to Boolean prevents the error but evaluates to False i.e. will be always skipped (unless var='True'). When the filter bool is used the options ANSIBLE_CONDITIONAL_BARE_VARS=true and ANSIBLE_CONDITIONAL_BARE_VARS=false have no effect

    - debug:
        msg: "String '{{ var }}' evaluates to True"
      when: var|bool
      vars:
        var: 'abc'

gives

skipping: [localhost]

3. Quoting from Porting guide 2.8 Bare variables in conditionals

  - include_tasks: teardown.yml
    when: teardown

  - include_tasks: provision.yml
    when: not teardown

> " based on a variable you define as a string (with quotation marks around it):"

• In Ansible 2.7 and earlier, the two conditions above are evaluated as True and False respectively if teardown: 'true'

• In Ansible 2.7 and earlier, both conditions were evaluated as False if teardown: 'false'

• In Ansible 2.8 and later, you have the option of disabling conditional bare variables, so when: teardown always evaluates as True, and when: not teardown always evaluates as False when teardown is a non-empty string (including 'true' or 'false')

4. Quoting from CONDITIONAL_BARE_VARS

> Expect that this setting eventually will be deprecated after 2.12

You can check for empty string (when stderr is empty)

- name: Check script
  shell: . {{ venv_name }}/bin/activate && myscritp.py
  args:
    chdir: "{{ home }}"
  sudo_user: "{{ user }}"
  register: test_myscript

- debug: msg='myscritp is Ok'
  when: test_myscript.stderr == ""

If you want to check for fail:

- debug: msg='myscritp has error: {{test_myscript.stderr}}'
  when: test_myscript.stderr != ""

Also look at this stackoverflow question

when: myvar | default('', true) | trim != ''

I use | trim != '' to check if a variable has an empty value or not. I also always add the | default(..., true) check to catch when myvar is undefined too.

When a var is defined but null the when: myvar | default('', true) | trim != '' was the only option that worked for me. (And i tested a lot of different approaches)