Name: Linux & service basics
Availability: InStock
Author: kowl64

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

Confirm scope and safety:
- identify service name and whether changes are permitted.
Gather evidence:
- status output + recent logs (see
```
references/triage-commands.md
```
  ).
Classify failure:
- config error, dependency missing, permission denied, port conflict, upstream unreachable, DNS mismatch.
Propose minimal fix + verification steps.
Validate network path (if web service):
- app listens → Nginx proxies → DNS resolves → (TLS sanity if applicable).
Provide restart/reload plan and confirm health checks.
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

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.

Linux & service basics

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

PURPOSE

WHEN TO USE

INPUTS

OUTPUTS

WORKFLOW

OUTPUT FORMAT

SAFETY & EDGE CASES

EXAMPLES

Quick Start

Manual Installation

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