/administering-linux

name: administering-linux
description: Manage Linux systems covering systemd services, process management, filesystems, networking, performance tuning, and troubleshooting. Use when deploying applications, optimizing server performance, diagnosing production issues, or managing users and security on Linux servers.

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:**

   top                             # Quick overview
   up