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

Permissions & ACLs (chmod/chown/setuid)

Master rwx, umask, sticky bit, setuid/setgid, and when to use ACLs.
On this page

Why Permissions Matter in Production

Linux permissions are not just about access. They define security boundaries. A single incorrect permission can expose secrets, allow privilege escalation, or break deployments. Production systems must treat permissions as a design decision, not a quick fix.

The Three Layers: User, Group, Others

Every file has three permission scopes:

  • User (owner)
  • Group
  • Others (world)
-rwxr-x---

This means:

  • User: read, write, execute
  • Group: read, execute
  • Others: no access

Numeric (Octal) Mode Explained

Permissions are often written numerically: 

chmod 750 file

Breakdown:

  • 7 = rwx
  • 5 = r-x
  • 0 = ---

Production-safe defaults:

  • Directories: 750
  • Config files: 640
  • Secrets: 600

chmod Is Not a Fix Tool

If your solution is “chmod 777”, you are hiding a design mistake. Fix ownership, not permissions.

Ownership Strategy

chown myapp:myapp /srv/myapp

Correct ownership reduces the need for overly permissive modes.

setuid and setgid

Special permission bits change execution behavior.

setuid (4000)

Executable runs with file owner permissions. Example: 

chmod 4755 binary

Be extremely cautious. Misuse can cause privilege escalation.

setgid (2000)

When applied to directories, new files inherit the directory group. This is useful in deploy groups: 

chmod 2750 /srv/myapp

Sticky Bit (1000)

Used mostly on shared directories like /tmp. 

chmod 1777 /shared

Users can only delete their own files.

When Basic Permissions Are Not Enough: ACLs

ACL (Access Control Lists) allow more granular permissions. Example: 

setfacl -m u:deploy:rwx /srv/myapp
getfacl /srv/myapp

ACLs are useful when:

  • Multiple teams need selective access
  • Deploy user needs write access but not ownership
  • You cannot restructure group hierarchy easily

Common Production Mistakes

  • World-readable .env files
  • Secrets with 644 permissions
  • Log files writable by everyone
  • Shared SSH keys across users

How to Audit Permissions Quickly

find /srv -type f -perm -002
find / -perm -4000 2>/dev/null
ls -lR /srv/myapp

Mental Model

Permissions define containment. If an attacker gains access to one service user, proper permissions prevent lateral movement.

Production Checklist

  • No 777 anywhere
  • No world-readable secrets
  • Service directories owned by service user
  • setuid binaries audited
  • Sticky bit correctly used on shared dirs
Sudoers Safe Defaults →