Ansible 'command' task missing 'stdout' for 'nginx' config verification

After deploy new configuration of Nginx, I would like to verify configuration before Nginx handler for reload is applied. This is output of command line:

nginx -t && echo $?
nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful
0

Based on output above, I configured Ansible command task like this:

- name: Verify Nginx config
  become: yes
  command: nginx -t
  register: nginx_verify_config
  check_mode: no
  changed_when: "'syntax is ok' not in nginx_verify_config.stdout"

- debug:
    msg: "{{nginx_verify_config.stdout}}"

changed_when should show something changed only if output of command is mising syntax is ok. This does not work and I found out, the message in nginx_verify_config looks like this:

ok: [DevNgx] => {
    "msg": {
        "changed": true,
        "cmd": [
            "nginx",
            "-t"
        ],
        "delta": "0:00:00.018083",
        "end": "2023-04-14 14:44:16.656261",
        "failed": false,
        "msg": "",
        "rc": 0,
        "start": "2023-04-14 14:44:16.638178",
        "stderr": "nginx: the configuration file /etc/nginx/nginx.conf syntax is ok\nnginx: configuration file /etc/nginx/nginx.conf test is successful",
        "stderr_lines": [
            "nginx: the configuration file /etc/nginx/nginx.conf syntax is ok",
            "nginx: configuration file /etc/nginx/nginx.conf test is successful"
        ],
        "stdout": "",
        "stdout_lines": []
    }
}

Why is message stored under stderr sections msg and stdout are empty?

I tried to add 2>&1 at the end of the command, but it raised error nginx: invalid option: "2>&1".

Instead of relying on string output you can check the return code itself (nginx will return non-zero rc if config is not valid):

- name: check nginx configuration
  become: yes
  command: "nginx -t"
  register: nginx_verify_config
  check_mode: no
  changed_when: "nginx_verify_config.rc != 0"

You should also consider checking a config file pre-deployment, that is run nginx -t -c /path/to/conf-file in your validator before you deploy the config, as otherwise your state change is inconsistent and you have to deal with rolling back the changes that may result in a whole lot of other problems..

Why is message stored under stderr sections msg and stdout are empty?

Because

This is how it works since introduction - nginx consistently uses stderr for all output

according Why write nginx version on stderr?

Other software packages like Java or Python are reporting his version to stderr too.

For validating the nginx.conf during a deployment with Ansible you may also have a look into

• Validate nginx.conf during Ansible deploy
• template module – Template a file out to a target host
  • Parameter: validate

I tried to add 2>&1 at the end of the command, but it raised error nginx: invalid option: "2>&1".

That is also the expected behavior for command module – Execute commands on targets

The command(s) will not be processed through the shell, so variables like $HOSTNAME and operations like *, <, >, |, ; and & will not work. Use the ansible.builtin.shell module if you need these features.