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

Shell & Environment Hygiene

PATH, profiles, non-interactive shells, and avoiding env drift in production.
On this page

Why Environment Hygiene Matters

In development, environment variables are convenient. In production, they are configuration boundaries. Improper environment management leads to inconsistent behavior, security leaks, and debugging nightmares.

Interactive vs Non-Interactive Shell

Most production services do NOT run inside your interactive shell. They do not load:

  • ~/.bashrc
  • ~/.profile
  • ~/.bash_aliases

systemd services and cron jobs run in controlled, minimal environments. Never assume your shell environment exists in production services.

PATH Risks

The PATH variable determines which binary is executed. Check it: 

echo $PATH

Production rule:

  • Avoid custom PATH modifications globally
  • Never include writable directories in PATH
  • Prefer absolute paths in systemd and sudo rules

Absolute Paths Over Assumptions

Instead of: 

node app.js

Use: 

/usr/bin/node app.js

This prevents execution ambiguity.

systemd Environment Strategy

For production services, use: 

Environment="NODE_ENV=production"
EnvironmentFile=/etc/myapp.env

Keep environment files:

  • Outside application directory
  • Readable only by service user
  • Not world-readable

Cron Environment Pitfall

Cron runs with a minimal environment. If your script works manually but fails in cron, check: 

env

Explicitly define PATH and required variables inside cron jobs.

Environment Variable Security

Environment variables may contain:

  • Database passwords
  • API keys
  • Secrets

Never:

  • Expose them via debug endpoints
  • Store them in world-readable files
  • Echo them in logs

Debugging Environment Drift

printenv
systemctl show myapp | grep Environment
cat /proc/<pid>/environ

Compare expected vs actual environment.

Common Production Mistakes

  • Relying on ~/.bashrc for service variables
  • Setting global PATH modifications
  • Putting secrets in git repository
  • Using different NODE_ENV values across environments

Mental Model

Environment variables are configuration contracts. If they differ between servers, behavior differs between servers. Production reliability depends on environment consistency.

Production Checklist

  • All services define environment explicitly
  • No reliance on interactive shell configs
  • Secrets stored in restricted files
  • Absolute paths used in service definitions
  • Cron jobs define required environment
Editing Configs Safely →