Glossary

Linux Server Administration

Essential terms every Linux system administrator and DevOps engineer should know.

60 Terms Published: 18 Apr, 2026

Linux #linux #server #administration #sysadmin

Configuration

Environment Variable

A named value stored in the shell environment that affects how processes behave, commonly used to pass configuration like paths, credentials, and flags to applications.

Example

Setting PATH to include a custom binary directory for the current session.

Code Snippet

1
export PATH=$PATH:/opt/myapp/bin

/etc directory

The standard Linux directory that contains system-wide configuration files for the operating system and installed applications, editable by the root user.

Example

Editing /etc/nginx/nginx.conf to update the web server configuration.

Code Snippet

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ls /etc/

/var directory

A Linux directory that holds variable data files that change during normal system operation, including logs, spools, caches, and runtime data.

Example

Inspecting /var/log to find application and system log files.

Code Snippet

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ls /var/log/

File Management

tar

A command-line utility for creating, extracting, and managing archive files, often combined with compression tools like gzip or bzip2 for backup and distribution.

Example

Creating a compressed tarball of the /etc directory as a configuration backup.

Code Snippet

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tar -czf etc-backup-$(date +%F).tar.gz /etc

rsync

A fast and versatile file synchronization and transfer tool that efficiently copies files locally or over SSH, transferring only the differences between source and destination.

Example

Syncing a local web directory to a remote server over SSH, deleting files removed locally.

Code Snippet

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rsync -avz --delete /var/www/ user@server:/var/www/

Filesystem

Symbolic Link (symlink)

A special file that acts as a pointer to another file or directory, allowing the same resource to be accessed from multiple locations in the filesystem.

Example

Creating a symlink so that /usr/local/bin/python points to the desired Python version.

Code Snippet

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ln -s /usr/bin/python3.11 /usr/local/bin/python

Logging

Syslog

The standard logging system on Linux that collects messages from the kernel and applications, routing them to log files or remote logging servers based on facility and severity.

Example

Checking /var/log/syslog for system-level messages after an unexpected reboot.

Code Snippet

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tail -f /var/log/syslog

logrotate

A utility that automates the rotation, compression, and deletion of log files to prevent them from filling up disk space, configured via /etc/logrotate.conf.

Example

Configuring logrotate to rotate Nginx access logs weekly and keep 4 weeks of history.

Code Snippet

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```
2
/var/log/nginx/*.log {
3
  weekly
4
  rotate 4
5
  compress
6
  missingok
7
  notifempty
8
}
9
```

rsyslog

A high-performance system logging daemon that extends syslog with features like filtering, TCP/TLS transport, and forwarding logs to remote centralized servers.

Example

Configuring rsyslog to forward all auth logs to a centralized Graylog server.

Code Snippet

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```
2
*.* @@logs.example.com:514
3
```

Networking

SSH (Secure Shell)

A cryptographic network protocol for securely logging into and executing commands on remote servers over an unsecured network, replacing older protocols like Telnet.

Example

Connecting to a remote AWS EC2 instance using an SSH key pair.

Code Snippet

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ssh -i ~/.ssh/my-key.pem ubuntu@192.168.1.10

SSH Key Pair

A pair of cryptographic keys a private key kept locally and a public key placed on the remote server used to authenticate SSH connections without a password.

Example

Generating an Ed25519 SSH key pair and copying the public key to a remote server.

Code Snippet

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ssh-keygen -t ed25519 -C "[email protected]"

sshd_config

The main configuration file for the OpenSSH server daemon, controlling options such as permitted authentication methods, port, and allowed users.

Example

Disabling root login and password authentication in sshd_config to harden a server.

Code Snippet

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```
2
PermitRootLogin no
3
PasswordAuthentication no
4
Port 2222
5
```

Firewall

A security system that monitors and controls incoming and outgoing network traffic based on predetermined rules, protecting the server from unauthorized access.

Example

Allowing only port 22 and 443 through the firewall and blocking all other inbound traffic.

Code Snippet

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ufw allow 22/tcp && ufw allow 443/tcp && ufw enable

ufw (Uncomplicated Firewall)

A user-friendly frontend for managing iptables firewall rules on Ubuntu and Debian-based systems, designed to simplify firewall configuration.

Example

Enabling ufw and allowing HTTP, HTTPS, and SSH traffic on a new server.

Code Snippet

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ufw allow OpenSSH && ufw allow "Nginx Full" && ufw enable

iptables

A powerful Linux command-line utility for configuring kernel-level packet filtering rules that control network traffic flowing through the system.

Example

Blocking all incoming traffic from a specific IP address using an iptables DROP rule.

Code Snippet

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iptables -A INPUT -s 203.0.113.5 -j DROP

netstat / ss

Command-line tools for displaying network connections, routing tables, and interface statistics. ss is the modern replacement for the deprecated netstat.

Example

Listing all processes listening on TCP ports to verify which services are running.

Code Snippet

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ss -tlnp

DNS (Domain Name System)

A hierarchical system that translates human-readable domain names into IP addresses, enabling servers and clients to locate each other on a network.

Example

Using dig to resolve a domain name and inspect its DNS records from the command line.

Code Snippet

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dig example.com A

/etc/hosts

A local file that maps hostnames to IP addresses, taking precedence over DNS resolution and useful for custom hostname overrides or blocking domains.

Example

Adding a local mapping so that myapp.local resolves to 127.0.0.1 during development.

Code Snippet

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echo '127.0.0.1 myapp.local' >> /etc/hosts

Performance

Memory (RAM)

The physical random-access memory available to the system for storing running processes and kernel data, directly affecting system performance and capacity.

Example

Checking total, used, and available RAM on a server to diagnose a memory pressure issue.

Code Snippet

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free -h

Swap

A designated area on disk that the Linux kernel uses as an overflow for RAM when physical memory is exhausted, allowing the system to continue operating at reduced performance.

Example

Creating and enabling a 4GB swap file on a server that ran out of RAM.

Code Snippet

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```bash
2
fallocate -l 4G /swapfile
3
chmod 600 /swapfile
4
mkswap /swapfile
5
swapon /swapfile
6
```

Load Average

A metric that represents the average number of processes waiting for CPU time over the last 1, 5, and 15 minutes, used to assess overall system load.

Example

Checking the load average to determine if a server is overloaded relative to its CPU count.

Code Snippet

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uptime

vmstat

A command that reports virtual memory statistics, including processes, memory, swap, I/O, and CPU activity, useful for diagnosing performance bottlenecks.

Example

Running vmstat every second to observe memory and CPU usage patterns during a load test.

Code Snippet

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vmstat 1 10

strace

A diagnostic tool that traces system calls and signals made by a process, helping debug application issues and understand how a program interacts with the kernel.

Example

Attaching strace to a running process to see which files it is trying to open.

Code Snippet

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strace -p 1234 -e trace=openat

Kernel Parameters (sysctl)

Runtime configuration variables that control kernel behavior such as network stack settings, memory management, and security policies, managed via the sysctl command.

Example

Tuning the network stack to increase the maximum number of open file descriptors.

Code Snippet

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```bash
2
sysctl -w net.core.somaxconn=65535
3
echo 'net.core.somaxconn=65535' >> /etc/sysctl.conf
4
```

Process Management

Process

An instance of a running program on a Linux system, each identified by a unique Process ID (PID) and managed by the kernel for CPU and memory allocation.

Example

Listing all running processes on a server to identify which ones are consuming the most CPU.

Code Snippet

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ps aux --sort=-%cpu | head -20

PID (Process ID)

A unique numerical identifier assigned by the kernel to every running process, used to reference and manage processes with commands like kill and nice.

Example

Finding the PID of a running Nginx process to send it a reload signal.

Code Snippet

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pidof nginx

systemd

The standard init system and service manager for most modern Linux distributions, responsible for bootstrapping the system and managing services, sockets, and timers.

Example

Using systemctl to start, stop, enable, and check the status of system services.

Code Snippet

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systemctl status nginx

systemctl

The primary command-line tool for interacting with systemd, used to manage services starting, stopping, enabling, disabling, and reloading them.

Example

Enabling the Nginx service to start automatically on boot and starting it immediately.

Code Snippet

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systemctl enable --now nginx

journalctl

A command-line tool for querying and viewing logs collected by the systemd journal, supporting filtering by service, time range, priority, and more.

Example

Following live logs for the SSH daemon to monitor login attempts in real time.

Code Snippet

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journalctl -u sshd -f

kill / killall

Commands used to send signals to processes, most commonly SIGTERM (graceful stop) or SIGKILL (force stop), to terminate or control running processes.

Example

Forcefully terminating a frozen process by sending SIGKILL using its PID.

Code Snippet

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kill -9 1234

top / htop

Interactive command-line utilities that display real-time information about running processes, CPU usage, memory consumption, and system load.

Example

Using htop to identify a rogue process consuming 100% CPU on a production server.

Code Snippet

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htop

cron

A time-based job scheduler in Linux that runs commands or scripts automatically at specified intervals, configured through crontab files.

Example

Scheduling a database backup script to run every day at 3:00 AM.

Code Snippet

1
```
2
# m h dom mon dow command
3
0 3 * * * /opt/scripts/backup.sh
4
```

crontab

The configuration file and command used to define and manage cron jobs for a specific user, with each line specifying a schedule and command to run.

Example

Editing the crontab for the current user to add a new scheduled task.

Code Snippet

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crontab -e

Security

SELinux / AppArmor

Mandatory Access Control (MAC) security frameworks for Linux that enforce fine-grained policies restricting what processes can access, going beyond standard file permissions.

Example

Using AppArmor to confine Nginx so it can only read files within /var/www and /etc/nginx.

Code Snippet

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aa-status

PAM (Pluggable Authentication Modules)

A flexible authentication framework in Linux that decouples authentication logic from applications, allowing system-wide policies for login, password strength, and MFA.

Example

Configuring PAM to enforce a minimum password length and complexity policy.

Code Snippet

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cat /etc/pam.d/common-password

Software Management

Package Manager

A tool that automates the installation, upgrade, configuration, and removal of software packages on a Linux system, resolving dependencies automatically.

Example

Using apt to install, update, and remove packages on an Ubuntu server.

Code Snippet

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apt install curl git vim -y

apt / apt-get

The package management tools for Debian-based Linux distributions like Ubuntu, used to install, update, upgrade, and remove software packages from repositories.

Example

Updating the package index and upgrading all installed packages on an Ubuntu server.

Code Snippet

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apt update && apt upgrade -y

yum / dnf

Package managers for Red Hat-based Linux distributions such as CentOS, RHEL, and Fedora. dnf is the modern replacement for yum with improved performance.

Example

Installing the Apache web server on a CentOS server using dnf.

Code Snippet

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dnf install httpd -y

Storage

Filesystem

The method and structure used by the operating system to organize, store, and retrieve files on a storage device, defining how data is laid out on disk.

Example

Formatting a new disk with the ext4 filesystem before mounting it.

Code Snippet

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mkfs.ext4 /dev/sdb1

Mount

The process of making a filesystem accessible at a specific directory path (mount point) in the Linux directory tree so the OS and users can read and write to it.

Example

Mounting an additional EBS volume to /data on an EC2 instance.

Code Snippet

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mount /dev/sdb1 /data

/etc/fstab

A system configuration file that defines how disk partitions and storage devices are automatically mounted at boot time, including their mount points and options.

Example

Adding an entry to /etc/fstab to automatically mount an NFS share on every boot.

Code Snippet

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echo '/dev/sdb1 /data ext4 defaults 0 2' >> /etc/fstab

df

A command that reports the amount of disk space used and available on all mounted filesystems, helping identify storage capacity issues.

Example

Checking disk usage across all mounted filesystems in human-readable format.

Code Snippet

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df -h

du

A command that estimates the disk space used by files and directories, useful for identifying large files and directories consuming storage.

Example

Finding the top 10 largest directories under /var to diagnose a full disk.

Code Snippet

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du -sh /var/* | sort -rh | head -10

LVM (Logical Volume Manager)

A storage abstraction layer in Linux that allows flexible management of disk space by grouping physical volumes into volume groups and carving them into logical volumes that can be resized dynamically.

Example

Extending a logical volume and its filesystem online without downtime.

Code Snippet

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lvextend -L +10G /dev/vg0/data && resize2fs /dev/vg0/data

NFS (Network File System)

A distributed filesystem protocol that allows a server to share directories over a network, enabling client machines to mount and access remote storage as if it were local.

Example

Mounting a shared NFS export from a file server to /mnt/shared on a client node.

Code Snippet

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mount -t nfs fileserver:/exports/shared /mnt/shared

System Architecture

Kernel

The core component of the Linux operating system that manages hardware resources, process scheduling, memory, and system calls, acting as a bridge between hardware and user-space applications.

Example

The Linux kernel allocating CPU time between running processes and handling a disk I/O interrupt.

Code Snippet

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uname -r

Shell

A command-line interpreter that provides an interface between the user and the kernel, accepting commands, executing programs, and scripting automation tasks.

Example

Using Bash to write a script that automates log rotation and sends an alert email.

Code Snippet

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echo $SHELL

Bash (Bourne Again SHell)

The most widely used Unix shell and scripting language on Linux systems, supporting variables, loops, conditionals, and functions for automation.

Example

Writing a Bash script to back up a directory and compress it with a timestamp.

Code Snippet

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```bash
2
#!/bin/bash
3
tar -czf backup-$(date +%F).tar.gz /var/www
4
```

Terminal / TTY

A text-based interface through which users interact with the shell and operating system, either physically on the server or virtually over a network connection.

Example

Opening a TTY session on a headless server by connecting via SSH.

Code Snippet

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tty

Text Processing

grep

A command-line tool for searching plain text for lines matching a pattern using regular expressions, widely used for filtering logs and configuration files.

Example

Searching all log files in /var/log for lines containing the word ERROR.

Code Snippet

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grep -r "ERROR" /var/log/

sed

A stream editor for filtering and transforming text, commonly used in scripts to perform find-and-replace operations on files or command output.

Example

Replacing all occurrences of localhost with a production hostname in a config file in place.

Code Snippet

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sed -i 's/localhost/prod.example.com/g' /etc/app/config.conf

awk

A versatile text-processing language used for pattern scanning and reporting, especially useful for parsing structured output and extracting specific fields.

Example

Printing only the process name and memory usage columns from ps output.

Code Snippet

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ps aux | awk '{print $11, $6}'

Users & Permissions

Root User

The superuser account in Linux with UID 0 that has unrestricted access to all commands, files, and system resources. Should be used sparingly for security reasons.

Example

Logging in as root to install a system package or modify a protected configuration file.

Code Snippet

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sudo su -

sudo

A command that allows permitted users to execute commands as the root user or another user, as configured in the /etc/sudoers file, without logging in as root.

Example

Running apt update as root via sudo without switching to the root account.

Code Snippet

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sudo apt update

User Management

The administration of user accounts on a Linux system, including creating, modifying, disabling, and deleting users and managing their group memberships.

Example

Creating a new system user for a web application with no login shell.

Code Snippet

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useradd -m -s /bin/bash -G sudo john

Group

A collection of user accounts that share the same access permissions to files and resources, simplifying permission management across multiple users.

Example

Adding a developer user to the docker group to allow running Docker commands without sudo.

Code Snippet

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usermod -aG docker john

File Permissions

A set of rules in Linux that control who can read, write, or execute a file or directory, defined for three classes owner, group, and others.

Example

Making a shell script executable by its owner while keeping it read-only for others.

Code Snippet

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chmod 750 deploy.sh

chmod

A Linux command used to change the read, write, and execute permissions of files and directories for the owner, group, and others.

Example

Granting full permissions to the owner and read-only to everyone else on a config file.

Code Snippet

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chmod 644 /etc/app/config.yaml

chown

A Linux command used to change the ownership of a file or directory, assigning a new owner user and optionally a new group.

Example

Transferring ownership of a web root directory to the www-data user and group.

Code Snippet

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chown -R www-data:www-data /var/www/html

/etc/passwd

A plain text file that stores essential information about each user account on the system, including username, UID, GID, home directory, and default shell.

Example

Inspecting /etc/passwd to verify the home directory and shell assigned to a new user.

Code Snippet

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cat /etc/passwd | grep john