Linux Administration

Core Linux system administration skills for managing production servers, development environments, and infrastructure hosts across Debian/Ubuntu and RHEL/CentOS distributions.

When to Use

• Provisioning and maintaining Linux servers in any environment
• Installing, updating, or removing software packages
• Managing filesystems, disk usage, and mount points
• Investigating runaway processes or high resource consumption
• Scheduling recurring tasks with cron or systemd timers
• Analyzing system and application logs for troubleshooting

Prerequisites

• Root or sudo access on the target system
• SSH access configured (see ssh-configuration skill)
• Familiarity with a terminal text editor (vim, nano)
• Package manager available (apt on Debian/Ubuntu, dnf on RHEL 8+/Fedora)

Package Management

Debian / Ubuntu (apt)

# Update package index and upgrade all installed packages
apt update && apt upgrade -y

# Search for a package by keyword
apt search nginx

# Show detailed package info including dependencies
apt show nginx

# Install a specific version of a package
apt install nginx=1.24.0-1ubuntu1

# Install multiple packages in one command
apt install -y nginx certbot python3-certbot-nginx

# Remove a package but keep its config files
apt remove nginx

# Remove a package and purge all config files
apt purge nginx

# Remove unused dependency packages
apt autoremove -y

# List all installed packages
dpkg -l | grep nginx

# Pin a package to prevent automatic upgrades
cat <<'EOF' > /etc/apt/preferences.d/pin-nginx
Package: nginx
Pin: version 1.24.0-1ubuntu1
Pin-Priority: 1001
EOF

# Add an external repository (example: Docker CE)
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | gpg --dearmor -o /usr/share/keyrings/docker-archive-keyring.gpg
echo "deb [arch=amd64 signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" \
  > /etc/apt/sources.list.d/docker.list
apt update

RHEL / CentOS / Fedora (dnf)

# Update all packages
dnf update -y

# Search for a package
dnf search nginx

# Show package details
dnf info nginx

# Install a package
dnf install -y nginx

# Install a specific version
dnf install nginx-1.24.0-1.el9

# Remove a package
dnf remove nginx

# List installed packages
dnf list installed | grep nginx

# Enable a module stream (example: Node.js 20)
dnf module enable nodejs:20
dnf install -y nodejs

# Add an external repository
dnf install -y epel-release

# View repository list
dnf repolist --all

# Clean cached package data
dnf clean all

System Information

# Kernel and OS release
uname -a
cat /etc/os-release

# Hostname and system metadata
hostnamectl

# CPU information
lscpu
nproc                      # Number of processing units

# Memory usage (human-readable)
free -h

# Disk usage summary
df -hT                     # Include filesystem type
du -sh /var/log/*          # Summarize directory sizes

# Network interfaces and IP addresses
ip addr show
ip route show              # Routing table

# Uptime and load average
uptime
w                          # Who is logged in and load

Filesystem Management

# List block devices and partitions
lsblk
fdisk -l

# Create a new ext4 filesystem on a partition
mkfs.ext4 /dev/sdb1

# Mount a filesystem temporarily
mount /dev/sdb1 /mnt/data

# Add a persistent mount via fstab
echo '/dev/sdb1  /mnt/data  ext4  defaults,noatime  0  2' >> /etc/fstab
mount -a                   # Mount everything in fstab

# Check and repair a filesystem (unmount first)
umount /dev/sdb1
fsck.ext4 -y /dev/sdb1

# Monitor disk I/O in real time
iostat -xz 2

# Find files larger than 100 MB
find / -xdev -type f -size +100M -exec ls -lh {} \;

# Check inode usage (out-of-inodes can mimic out-of-disk)
df -i

Process Management

# List all processes with full details
ps auxf

# Interactive process viewer (prefer htop if installed)
top
htop

# Find processes by name
pgrep -la nginx

# Show process tree
pstree -p

# Send graceful stop signal (SIGTERM)
kill <pid>

# Force kill an unresponsive process (SIGKILL)
kill -9 <pid>

# Kill all processes matching a name
pkill nginx

# Show open files for a process
lsof -p <pid>

# Show which process is listening on a port
ss -tlnp | grep :80
lsof -i :80

# Run a process immune to hangups (persists after logout)
nohup /opt/myapp/start.sh > /var/log/myapp.log 2>&1 &


…