Core Code
Core Code Modern Dev Handbook
LINUX-PRODUCTION Contents
Linux Fundamentals for Production
Filesystem Layout (FHS) in Practice Users, Groups, and Service Accounts Permissions & ACLs (chmod/chown/setuid) Sudoers Safe Defaults Packages: apt/dnf, Repos, and Pinning Shell & Environment Hygiene Editing Configs Safely Time, Timezones, and NTP System Introspection Toolkit
Processes & Signals
Linux Process Model (PID, PPID, Fork/Exec) ps/top/htop: Finding What Hurts Signals in Practice (TERM/KILL/HUP) nice/renice and Scheduling Basics ulimits and Resource Limits Background Jobs (nohup, tmux, systemd-run) /proc and What It Tells You cgroups Basics (CPU/Memory Isolation) Zombies, Orphans, and Stuck Processes
systemd & Services
systemd Unit Types Overview Service Unit Best Practices Dependencies & Ordering (After/Wants/Requires) Restart Policies and Backoff Environment Files and Secrets Handling journald Basics (Reading, Filtering, Retention) Logging Practices with systemd Timers vs Cron Debugging systemd Services
Networking Basics for Production
ip, routes, and interfaces DNS Debugging (dig, resolvectl, hosts) Ports and Sockets with ss curl for HTTP Debugging TCP Basics (Handshake, States, Retries) Firewall Overview (nftables/iptables/ufw) NAT and Port Forwarding Concepts TLS and Certificates Basics Network Troubleshooting Playbook
Storage & Filesystems
Disk Layout and Partitioning (lsblk/fdisk) Filesystems (ext4/xfs) Basics Mounts and /etc/fstab Safety Disk Usage: df vs du (and Why They Differ) Inodes and “Disk Full” Mysteries Swap Strategy in Production Log Rotation (logrotate) Basics Temporary and Runtime Storage Hygiene Backup Storage Considerations
Security Basics
SSH Hardening in Production Users and Least Privilege in Production Firewall Minimum Rules for Production Linux Patching Strategy in Production Managing Secrets on Linux Hosts Linux Audit and Basic Logging in Production Fail2ban and Rate Limiting in Production SELinux and AppArmor Overview in Production Linux Security Baseline Checklist for Production
Observability on Linux
Linux Log Sources Map in Production journalctl Power Usage in Production Log Structure and Correlation in Production Metrics Hooks Overview in Production Health Checks and Readiness in Production Tracing Mindset for Linux and Production Systems Core Dumps Basics in Production Debug Packages and Symbols in Production Linux Observability Checklist for Production
Performance & Debugging
Load Average Explained for Production Debugging CPU Profiling Basics in Linux Production Memory Pressure Basics in Linux Production IO Bottlenecks and iostat in Production lsof and Open Files in Production Debugging Practical strace Usage in Production Sockets Debugging Tools in Linux Production dmesg and Kernel Signals for Production Debugging Production Debugging Workflow for Linux Systems
Deployment Patterns
Environment and Configuration Management in Production Release Directory Pattern for Safe Linux Deployments Zero-Downtime Restarts in Production Running Migrations Safely in Production Secrets Deploy Patterns in Production Backups and Restore Drills in Production Rollback Strategies for Production Deployments Maintenance Windows in Production Operations Production Deployment Checklist for Linux Systems
Production Checklists
Writing Effective Runbooks for Production Systems Failure Modes Mapping for Linux Production Systems Incident Response Basics for Linux Production Break-Glass Access in Production Capacity Planning Basics for Linux Production Systems Monitoring and Alerting Baseline for Linux Production Production Readiness Review for Linux Systems Configuration Drift in Linux Production Systems Go-Live Checklist

Filesystem Layout (FHS) in Practice

Understand /etc, /var, /srv, /opt, /run, and what belongs where on production hosts.
On this page

Why Filesystem Layout Matters in Production

A production Linux server is not a random collection of files. It is a structured system designed to separate configuration, runtime state, logs, binaries, and user data. If you do not respect the Filesystem Hierarchy Standard (FHS), your deployments become fragile, backups become incomplete, and incidents become harder to debug.

The Core Directories You Must Understand

/etc — Configuration Only

This directory contains configuration files. Nothing else. Production rule: if something changes behavior, it should live in /etc.

  • /etc/nginx/nginx.conf
  • /etc/systemd/system/myapp.service
  • /etc/ssh/sshd_config

Never store runtime files, logs, or secrets carelessly here. Treat it as declarative configuration.

/var — Mutable Data

/var is where runtime data lives. If your application writes something during execution, it likely belongs here.

  • /var/log — logs
  • /var/lib — application state
  • /run — runtime pid/socket files

/srv — Application Data

Use /srv for service-owned data exposed externally. 

/srv/myapp/releases/
/srv/myapp/current/

/usr vs /usr/local

/usr contains system-managed binaries. /usr/local contains manually installed software. Production rule: package manager owns /usr, you own /usr/local.

/run — Ephemeral Runtime

/run is cleared on reboot. Never store persistent data here.

Common Production Mistakes

  • Storing logs inside application directory
  • Writing temp files into /home
  • Putting runtime data inside /etc
  • Deploying apps inside /root

How to Inspect a Host Layout Quickly

lsblk
df -h
mount
tree -L 1 /

Production Checklist

  • Configs in /etc
  • Logs in /var/log
  • State in /var/lib
  • App releases in /srv
  • No writable root-owned surprises

Mental Model

Think of a Linux server as: Configuration → Runtime → Logs → State → Binaries. Respecting that separation makes backups, debugging, and rollbacks predictable.

Users, Groups, and Service Accounts →