ancoleman/ai-design-components/administering-linux

Linux Administration

Comprehensive Linux system administration for managing servers, deploying applications, and troubleshooting production issues in modern cloud-native environments.

Purpose

This skill teaches fundamental and intermediate Linux administration for DevOps engineers, SREs, backend developers, and platform engineers. Focus on systemd-based distributions (Ubuntu, RHEL, Debian, Fedora) covering service management, process monitoring, filesystem operations, user administration, performance tuning, log analysis, and network configuration.

Modern infrastructure requires solid Linux fundamentals even with containerization. Container hosts run Linux, Kubernetes nodes need optimization, and troubleshooting production issues requires understanding systemd, processes, and logs.

Not Covered:

• Advanced networking (BGP, OSPF) - see network-architecture skill
• Deep security hardening (compliance, pentesting) - see security-hardening skill
• Configuration management at scale (Ansible, Puppet) - see configuration-management skill
• Container orchestration - see kubernetes-operations skill

When to Use This Skill

Use when deploying custom applications, troubleshooting slow systems, investigating service failures, optimizing workloads, managing users, configuring SSH, monitoring disk space, scheduling tasks, diagnosing network issues, or applying performance tuning.

Quick Start

Essential Commands

Service Management:

systemctl start nginx              # Start service
systemctl stop nginx               # Stop service
systemctl restart nginx            # Restart service
systemctl status nginx             # Check status
systemctl enable nginx             # Enable at boot
journalctl -u nginx -f             # Follow service logs

Process Monitoring:

top                                # Interactive process monitor
htop                               # Enhanced process monitor
ps aux | grep process_name         # Find specific process
kill -15 PID                       # Graceful shutdown (SIGTERM)
kill -9 PID                        # Force kill (SIGKILL)

Disk Usage:

df -h                              # Filesystem usage
du -sh /path/to/dir                # Directory size
ncdu /path                         # Interactive disk analyzer

Log Analysis:

journalctl -f                      # Follow all logs
journalctl -u service -f           # Follow service logs
journalctl --since "1 hour ago"    # Filter by time
journalctl -p err                  # Show errors only

User Management:

useradd -m -s /bin/bash username   # Create user with home dir
passwd username                    # Set password
usermod -aG sudo username          # Add to sudo group
userdel -r username                # Delete user and home dir

Core Concepts

Systemd Architecture

Systemd is the standard init system and service manager. Systemd units define services, timers, targets, and other system resources.

Unit File Locations (priority order):

• /etc/systemd/system/ - Custom units (highest priority)
• /run/systemd/system/ - Runtime units (transient)
• /lib/systemd/system/ - System-provided units (don't modify)

Key Unit Types: .service (services), .timer (scheduled tasks), .target (unit groups), .socket (socket-activated)

Essential systemctl Commands:

systemctl daemon-reload            # Reload unit files after changes
systemctl list-units --type=service
systemctl list-timers              # Show all timers
systemctl cat nginx.service        # Show unit file content
systemctl edit nginx.service       # Create override file

For detailed systemd reference, see references/systemd-guide.md.

Process Management

Processes are running programs with unique PIDs. Understanding process states, signals, and resource usage is essential for troubleshooting.

Process States: R (running), S (sleeping), D (uninterruptible sleep/I/O), Z (zombie), T (stopped)

Common Signals: SIGTERM (15) graceful, SIGKILL (9) force, SIGHUP (1) reload config

Process Priority:

nice -n 10 command                 # Start with lower priority
renice -n 5 -p PID                 # Change priority of running process

Filesystem Hierarchy

Essential directories: / (root), /etc/ (config), /var/ (variable data), /opt/ (optional software), /usr/ (user programs), /home/ (user directories), /tmp/ (temporary), /boot/ (boot loader)

Filesystem Types Quick Reference:

• ext4 - General purpose (default)
• XFS - Large files, databases (RHEL default)
• Btrfs - Snapshots, copy-on-write
• ZFS - Enterprise, data integrity, NAS

For filesystem management details including LVM and RAID, see references/filesystem-management.md.

Package Management

Ubuntu/Debian (apt):

apt update && apt upgrade          # Update system
apt install package                # Install package
apt remove package                 # Remove package
apt search keyword                 # Search packages

RHEL/CentOS/Fedora (dnf):

dnf update                         # Update all packages
dnf install package                # Install package
dnf remove package                 # Remove package
dnf search keyword                 # Search packages

Use native package managers for system services; snap/flatpak for desktop apps and cross-distro compatibility.

Decision Frameworks

Troubleshooting Performance Issues

Investigation Workflow:

1. Identify bottleneck:

``bash top # Quick overview uptime # Load averages ``

1. CPU Issues (usage >80%):

``bash top # Press Shift+P to sort by CPU ps aux --sort=-%cpu | head ``

1. Memory Issues (swap used):

``bash free -h # Memory usage top # Press Shift+M to sort by memory ``

1. Disk I/O Issues (high wa%):

``bash iostat -x 1 # Disk statistics iotop # I/O by process ``

1. Network Issues:

``bash ss -tunap # Active connections iftop # Bandwidth monitor ``

For comprehensive troubleshooting, see references/troubleshooting-guide.md.

Filesystem Selection

Quick Decision:

• Default/General → ext4
• Database servers → XFS
• Large file storage → XFS or ZFS
• NAS/File server → ZFS
• Need snapshots → Btrfs or ZFS

Common Workflows

Creating a Systemd Service

Step 1: Create unit file

sudo nano /etc/systemd/system/myapp.service

Step 2: Unit file content

[Unit]
Description=My Web Application
After=network.target postgresql.service
Requires=postgresql.service
 
[Service]
Type=simple
User=myapp
Group=myapp
WorkingDirectory=/opt/myapp
Environment="PORT=8080"
ExecStart=/opt/myapp/bin/server
ExecReload=/bin/kill -HUP $MAINPID
Restart=on-failure
RestartSec=5s
StandardOutput=journal
 
# Security hardening
PrivateTmp=true
NoNewPrivileges=true
ProtectSystem=strict
ReadWritePaths=/var/lib/myapp
 
[Install]
WantedBy=multi-user.target

Step 3: Deploy and start

sudo useradd -r -s /bin/false myapp
sudo mkdir -p /var/lib/myapp
sudo chown myapp:myapp /var/lib/myapp
sudo systemctl daemon-reload
sudo systemctl enable myapp.service
sudo systemctl start myapp.service
sudo systemctl status myapp.service

For complete examples, see examples/systemd-units/.

Systemd Timer (Cron Replacement)

Create service and timer units for scheduled tasks. Timer unit specifies OnCalendar= schedule and Persistent=true for missed jobs. Service unit has Type=oneshot. See examples/systemd-units/backup.timer and backup.service for complete examples.

SSH Hardening

Generate SSH key:

ssh-keygen -t ed25519 -C "admin@example.com"
ssh-copy-id admin@server

Harden sshd_config:

sudo nano /etc/ssh/sshd_config

Key settings:

PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication yes
MaxAuthTries 3
AllowUsers admin deploy
X11Forwarding no
Port 2222                          # Optional

Apply changes:

sudo sshd -t                       # Test
sudo systemctl restart sshd        # Apply (keep backup session!)

For complete SSH configuration, see examples/configs/sshd_config.hardened and references/security-hardening.md.

Performance Tuning

Configure sysctl parameters in /etc/sysctl.d/99-custom.conf for network tuning (tcp buffers, BBR congestion control), memory management (swappiness, cache pressure), and file descriptors. Set ulimits in /etc/security/limits.conf for nofile and nproc. Configure I/O schedulers and CPU governors. For comprehensive tuning, see references/performance-tuning.md and examples/configs/ for templates.

Log Investigation

Use systemctl status myapp and journalctl -u myapp to investigate issues. Filter logs by time --since, severity -p err, or search patterns with grep. Correlate with system metrics using top, df -h, free -h. Check for OOM kills with journalctl -k | grep -i oom. For detailed workflows, see references/troubleshooting-guide.md.

Essential Commands

Interface Management:

ip addr show                       # Show all interfaces
ip link set eth0 up                # Bring interface up
ip addr add 192.168.1.100/24 dev eth0

Routing:

ip route show                      # Show routing table
ip route get 8.8.8.8               # Show route to IP
ip route add 10.0.0.0/24 via 192.168.1.1

Socket Statistics:

ss -tunap                          # All TCP/UDP connections
ss -tlnp                           # Listening TCP ports
ss -ulnp                           # Listening UDP ports
ss -tnp state established          # Established connections

Firewall Configuration

Ubuntu (ufw):

sudo ufw status
sudo ufw enable
sudo ufw allow 22/tcp              # Allow SSH
sudo ufw allow 80/tcp              # Allow HTTP
sudo ufw allow from 192.168.1.0/24 # Allow from subnet
sudo ufw default deny incoming

RHEL/CentOS (firewalld):

firewall-cmd --state
firewall-cmd --list-all
firewall-cmd --add-service=http --permanent
firewall-cmd --add-port=8080/tcp --permanent
firewall-cmd --reload

For complete network configuration including netplan, NetworkManager, and DNS, see references/network-configuration.md.

Scheduled Tasks

Cron Syntax

crontab -e                         # Edit user crontab
 
# Format: minute hour day month weekday command
0 2 * * * /usr/local/bin/backup.sh              # Daily at 2:00 AM
*/5 * * * * /usr/local/bin/check-health.sh      # Every 5 minutes
0 3 * * 0 /usr/local/bin/weekly-cleanup.sh      # Weekly Sunday 3 AM
@reboot /usr/local/bin/startup-script.sh        # Run at boot

Systemd Timer Calendar Syntax

OnCalendar=daily                   # Every day at midnight
OnCalendar=*-*-* 02:00:00          # Daily at 2:00 AM
OnCalendar=Mon *-*-* 09:00:00      # Every Monday at 9 AM
OnCalendar=*-*-01 00:00:00         # 1st of every month
OnBootSec=5min                     # 5 minutes after boot

Essential Tools

Process Monitoring

• top, htop - Real-time process monitor
• ps - Report process status
• pgrep/pkill - Find/kill by name

Log Analysis

• journalctl - Query systemd journal
• grep - Search text patterns
• tail -f - Follow log files

Disk Management

• df - Disk space usage
• du - Directory space usage
• lsblk - List block devices
• ncdu - Interactive disk analyzer

Network Tools

• ip - Network configuration
• ss - Socket statistics
• ping - Test connectivity
• dig/nslookup - DNS queries
• tcpdump - Packet capture