Manage Linux systems covering systemd services, process management, filesystems, networking, performance tuning, and troubleshooting...
Comprehensive Linux system administration for managing servers, deploying applications, and troubleshooting production issues in modern cloud-native environments.
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:
network-architecture skillsecurity-hardening skillconfiguration-management skillkubernetes-operations skillUse 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.
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
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.
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
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:
For filesystem management details including LVM and RAID, see references/filesystem-management.md.
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.
Investigation Workflow:
Identify bottleneck:
top # Quick overview
uptime # Load averages
CPU Issues (usage >80%):
top # Press Shift+P to sort by CPU
ps aux --sort=-%cpu | head
Memory Issues (swap used):
free -h # Memory usage
top # Press Shift+M to sort by memory
Disk I/O Issues (high wa%):
iostat -x 1 # Disk statistics
iotop # I/O by process
Network Issues:
ss -tunap # Active connections
iftop # Bandwidth monitor
For comprehensive troubleshooting, see references/troubleshooting-guide.md.
Quick Decision:
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/.
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.
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.
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.
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.
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
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.
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
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
top, htop - Real-time process monitorps - Report process statuspgrep/pkill - Find/kill by namejournalctl - Query systemd journalgrep - Search text patternstail -f - Follow log filesdf - Disk space usagedu - Directory space usagelsblk - List block devicesncdu - Interactive disk analyzerip - Network configurationss - Socket statisticsping - Test connectivitydig/nslookup - DNS queriestcpdump - Packet captureLinux administration is the foundation for Kubernetes node management. Node optimization (sysctl tuning), kubelet as systemd service, container logs via journald, cgroups for resource limits.
Example:
# /etc/sysctl.d/99-kubernetes.conf
net.bridge.bridge-nf-call-iptables = 1
net.ipv4.ip_forward = 1
For Kubernetes-specific operations, see kubernetes-operations skill.
Linux administration provides knowledge; configuration management automates it. Ansible playbooks automate systemd service creation and system tuning.
For automation at scale, see configuration-management skill.
This skill covers SSH and firewall basics. For advanced security (MFA, certificates, CIS benchmarks, compliance), see security-hardening skill.
CI/CD pipelines deploy to Linux servers using these skills. Uses systemctl for deployment and journalctl for monitoring.
For deployment automation, see building-ci-pipelines skill.
references/systemd-guide.md - Comprehensive systemd reference (unit files, dependencies, targets)references/performance-tuning.md - Complete sysctl, ulimits, cgroups, I/O scheduler guidereferences/filesystem-management.md - LVM, RAID, filesystem types, permissionsreferences/network-configuration.md - ip/ss commands, netplan, NetworkManager, DNS, firewallreferences/security-hardening.md - SSH hardening, firewall, SELinux/AppArmor basicsreferences/troubleshooting-guide.md - Common issues, diagnostic workflows, solutionsexamples/systemd-units/ - Service, timer, and target unit filesexamples/scripts/ - Backup, health check, and maintenance scriptsexamples/configs/ - sshd_config, sysctl.conf, logrotate examplesPackage Manager: apt, Network: netplan, Firewall: ufw, Repositories: /etc/apt/sources.list
Package Manager: dnf, Network: NetworkManager, Firewall: firewalld, Repositories: /etc/yum.repos.d/, SELinux enabled by default
Package Manager: pacman, Network: NetworkManager, Rolling release, AUR for community packages
Official Documentation:
Related Skills:
kubernetes-operations - Container orchestration on Linuxconfiguration-management - Automate Linux admin at scalesecurity-hardening - Advanced security and compliancebuilding-ci-pipelines - Deploy via CI/CDperformance-engineering - Deep performance analysis