linux-service-triage

# Linux & service basics: logs, systemd/PM2, permissions, Nginx reverse proxy, DNS checks

## PURPOSE
Diagnoses common Linux service issues using logs, systemd/PM2, file permissions, Nginx reverse proxy checks, and DNS sanity checks.

## WHEN TO USE
- TRIGGERS:
  - Show me why this service is failing using logs, then give the exact fix commands.
  - Restart this app cleanly and confirm it is listening on the right port.
  - Fix the permissions on this folder so the service can read and write safely.
  - Set up Nginx reverse proxy for this port and verify DNS and TLS are sane.
  - Create a systemd service for this script and make it survive reboots.
- DO NOT USE WHEN…
  - You need kernel debugging or deep performance profiling.
  - You want to exploit systems or bypass access controls.

## INPUTS
- REQUIRED:
  - Service type: systemd unit name or PM2 process name.
  - Observed symptom: error message, status output, or logs (pasted by user).
- OPTIONAL:
  - Nginx config snippet, domain name, expected upstream port.
  - Filesystem paths used by the service.
- EXAMPLES:
  - `systemctl status myapp` output + `journalctl` excerpt
  - Nginx server block + domain + upstream port

## OUTPUTS
- Default: triage report (likely cause, evidence from logs, minimal fix plan).
- If explicitly requested and safe: exact shell commands to apply the fix.
Success = service runs, listens on expected port, and reverse proxy/DNS path is correct.


## WORKFLOW
1. Confirm scope and safety:
   - identify service name and whether changes are permitted.
2. Gather evidence:
   - status output + recent logs (see `references/triage-commands.md`).
3. Classify failure:
   - config error, dependency missing, permission denied, port conflict, upstream unreachable, DNS mismatch.
4. Propose minimal fix + verification steps.
5. Validate network path (if web service):
   - app listens → Nginx proxies → DNS resolves → (TLS sanity if applicable).
6. Provide restart/reload plan and confirm health checks.
7. STOP AND ASK THE USER if:
   - logs/status output are missing,
   - actions require privileged access not confirmed,
   - TLS/cert management is required but setup is unknown.


## OUTPUT FORMAT
```text
TRIAGE REPORT
- Symptom:
- Evidence (what you provided):
- Most likely cause:
- Fix plan (minimal steps):
- Exact commands (ONLY if user approved changes):
- Verification:
- Rollback:
```


## SAFETY & EDGE CASES
- Read-only by default: diagnose from provided outputs; do not assume you can run commands.
- Avoid destructive changes; require explicit confirmation for anything risky.
- Prefer `nginx -t` before reload and verify ports with `ss`.


## EXAMPLES
- Input: “journal shows permission denied on /var/app/uploads.”  
  Output: path permission analysis + safe chown/chmod plan + verification.

- Input: “App works locally but domain returns 502.”  
  Output: upstream port checks + nginx error log interpretation + proxy_pass fix plan.