How can I define changed_when based on file content?

I have a few installations on our servers and workstations, which are executed via proprietary setup scripts, for example MATLAB.

Do keep the idempotency I would like to analyze the log file, which is created in the process. How can I define the changed_when based on the content of those log files?

So for example the command task would look like the following at the moment:

- name: Run MATLAB installer
  ansible.builtin.command:
    argv:
      - /mnt/matlabiso/install
      - -inputFile
      - /mnt/downloads/Software/MATLAB/R2024a/installer_input_linux.txt
  changed_when: false

So this command creates a log file at /tmp/ML2024.log, which has the line "Succesful installed" as the last line or "Installation failed" if it was not succesfull. The command itself if remember correctly mostly never fails.

If I understand my linter correctly, changed_when: false and than reading the content of the log file and printing via debug is not a real idempotent way of calling a command, since the changes are already done with the command.

So the actual Question: Did I miss a way how command is able to treat file content, without creating a custom script, which is executing and setup and reading the content afterwards.

Solution

Q: "To keep the idempotency I would like to analyze the log file, which is created in the process."

Still, this looks like a job for Custom facts for me.

Q: "How command is able to treat file content, without creating a custom script, which is executing and setup and reading the content afterwards?"

It isn't and it wasn't made for that. See command module – Execute commands on targets - Notes and at least "Use the ansible.builtin.shell module if you need these features."

Q: "This command creates a log file at /tmp/ML2024.log, which has the line Succesful installed as the last line or Installation failed if it was not succesfull."

That sounds more like Define failure and the status of failed_when, because it doesn't tells if it was already installed before or not. See a minimal example playbook

---
- hosts: localhost
  become: false
  gather_facts: false

  tasks:

    - shell:
        cmd: echo 'Installation failed' > /tmp/ML2024.log | grep 'Succesful installed' /tmp/ML2024.log
      register: result

    - debug:
        var: result

resulting into an output of

TASK [shell] *******************************************************************
fatal: [localhost]: FAILED! => changed=true
  cmd: echo 'Installation failed' > /tmp/ML2024.log | grep 'Succesful installed' /tmp/ML2024.log
  delta: '0:00:00.007761'
  end: '2024-11-12 09:30:00.347773'
  msg: non-zero return code
  rc: 1
  start: '2024-11-12 09:30:00.340012'
  stderr: ''
  stderr_lines: <omitted>
  stdout: ''
  stdout_lines: <omitted>

Just Registering result and check on Common Return Values will already provide some of the capabilities in question.