Linux Glossary: Terms Explained for Beginners

A

Absolute path: A path that starts at the very top of the filesystem, with /, like /etc/ssh/sshd_config. It means the same thing wherever you are. Compare relative path. Learn more: Moving around: cd
Alias: A short name you make up for a longer command, like alias ll='ls -l'. Put it in ~/.bashrc to keep it. Learn more: Shell skills
AppArmor: Ubuntu's extra security layer: each program gets a profile listing the files and actions it may use, even when it runs as root. The RHEL family uses SELinux for the same job. Learn more: SELinux vs AppArmor
apt: The package manager on Ubuntu and Debian: sudo apt install nginx. It installs .deb packages from repositories. The RHEL family uses dnf. Learn more: Installing software: dnf vs apt
Archive: One file that holds many files and folders, like a .tar or .zip. It's often also compressed (.tar.gz). Learn more: Archives & compression, Backups: tar, rsync & cron
Argument: What a command works on, usually written after its options. In ls -l /home, /home is the argument. Learn more: Meet the terminal
Attack surface: Everything on a computer that someone could reach or abuse: listening services, accounts that can log in, admin rights, old software. Making it smaller is one of the best defences. Learn more: Think like an attacker

B

Bash: The shell you use on both Rocky and Ubuntu: the program that reads your commands and runs them. Also the language of most shell scripts. Learn more: Meet the terminal, Shell scripting: if, loops & functions
Branch: In Git, a separate line of work. You try an idea on a branch, then merge it back when it works. Learn more: Git: branches, merges & sharing

C

Cloud: Computers you rent from a provider like AWS instead of owning them. You start and stop servers with a few commands and pay for the time you use. Learn more: The cloud & AWS: getting started
Command: The first word you type in the terminal: what to do. In ls -l /home, ls is the command. Learn more: Meet the terminal
Commit: In Git, a saved snapshot of your project with a message saying what changed. You can always go back to it. Learn more: Git: save points for your work
Compression: Making a file smaller by storing it more cleverly: gzip (.gz), xz (.xz), bzip2 (.bz2). Text shrinks a lot; photos and videos hardly at all. Learn more: Archives & compression
Container: An app packed together with everything it needs, running in its own sealed-off space on a Linux machine. Lighter than a virtual machine because it shares the host's kernel. Built from an image. Learn more: Containers: build & ship your own image, Alpine Linux: tiny Linux for containers
Continuous integration (CI): Automatically testing every change as soon as it's pushed, so mistakes are caught in minutes instead of weeks. Learn more: CI pipelines: test every push
cron: The service that runs commands on a schedule: every night, every Monday at 9, every 5 minutes. Schedules live in a crontab. Learn more: Scheduling with cron

D

Daemon: A program that runs in the background without a window or terminal, like the web server or sshd. Names often end in d. See service. Learn more: Services & your first website
Debian family: Ubuntu, Debian, Linux Mint, Raspberry Pi OS and Kali: distros that use .deb packages and apt. One of the two big Linux families this site teaches. Learn more: What even is Linux?
Directory: Another word for a folder. pwd shows the one you're in; cd moves you to another. Learn more: Look around: pwd & ls, Moving around: cd
Distribution (distro): A complete Linux operating system: the kernel plus the programs, package manager and settings around it. Rocky Linux and Ubuntu are distros. Learn more: What even is Linux?
dnf: The package manager on Rocky, AlmaLinux, RHEL and Fedora: sudo dnf install nginx. It installs .rpm packages. The Debian family uses apt. Learn more: Installing software: dnf vs apt
DNS: The internet's phone book: it turns names like example.com into IP addresses. Learn more: Networking: ifconfig vs ip, Route 53: names for your cloud
Drop-in file: A small extra config file in a .d folder (like /etc/ssh/sshd_config.d/) that adds to or overrides the main file, so you never have to edit the original. Learn more: Lock down SSH, Limits: cgroups, systemd & the OOM killer

E

Environment variable: A named value that programs can read, like $HOME or $PATH. Set one with export NAME=value. Learn more: Shell skills, Secrets & config: the twelve-factor way
EPEL: Extra Packages for Enterprise Linux: a trusted extra repository for Rocky and RHEL with software that isn't in the default ones. Learn more: Installing software: dnf vs apt
Exit code: The number a command leaves behind when it finishes: 0 means it worked, anything else means something went wrong. See it with echo $?. Learn more: Shell scripting: if, loops & functions, Capturing output

F

Filesystem: How files and folders are organised, starting at /. Also the format a disk is prepared with, like ext4 or XFS. Learn more: The filesystem tour, Disks & mounting
Fingerprint: A short code that identifies a server's SSH key. The first time you connect, you check it so you know you've reached the right machine. Learn more: Connecting with SSH
Firewall: Rules that decide which network traffic may reach a machine. firewalld (firewall-cmd) on Rocky, ufw on Ubuntu. Learn more: Services & your first website, Network & security tools

G

Git: A version control tool: it remembers every saved version of your files, so you can undo mistakes and work with other people. Learn more: Git: save points for your work
Group: A named set of users. Giving a group access to a folder gives it to everyone in the group. Learn more: Managing users & groups, Permissions: who can do what

H

Home directory: Your own folder, like /home/student. ~ is short for it, and cd on its own takes you there. Learn more: Moving around: cd

I

Idempotent: Safe to run again and again: the second run changes nothing if everything is already right. Good automation, like an Ansible playbook, is idempotent. Learn more: Infrastructure as code: Ansible first steps, Testing & linting your automation
Image: A read-only template a container is started from: an operating system base plus your app. Built from a Containerfile or Dockerfile. Learn more: Containers: build & ship your own image
Instance: One virtual server in the cloud. On AWS, an EC2 instance. Learn more: EC2: launch a Linux server
IP address: A machine's number on a network, like 192.168.1.50. See yours with ip a. Learn more: Networking: ifconfig vs ip

J

Job: A command you started from your shell. You can pause it (Ctrl+Z), send it to the background (bg) and bring it back (fg). Learn more: Jobs: pause, background & bring back

K

Kernel: The core of the operating system: it talks to the hardware and shares it out between programs. Linux is, strictly speaking, the name of the kernel. Learn more: What even is Linux?

L

Least privilege: Give every user and program only the access it really needs, and nothing more. If it's ever misused, it can do less damage. Learn more: Think like an attacker, Root, sudo & su
Linux: A free, open-source operating system that runs most of the world's servers, phones (Android) and supercomputers. It comes in many distributions. Learn more: What even is Linux?
Log: A file (or journal) where programs write down what they did and what went wrong. The first place to look when something breaks. Learn more: Logs & troubleshooting

M

Mount: Attaching a disk or partition to a folder so its files appear there, like a new disk at /data. Permanent mounts go in /etc/fstab. Learn more: Disks & mounting

O

Open source: Software whose source code anyone may read, change and share. Linux, Git, Ansible and most tools on this site are open source. Learn more: What even is Linux?
Option (flag): A switch that changes how a command works, usually starting with - or --, like -l in ls -l. Learn more: Meet the terminal

P

Package: A program bundled so it can be installed, updated and removed cleanly: .rpm on the RHEL family, .deb on the Debian family. Learn more: Installing software: dnf vs apt
Package manager: The tool that installs, updates and removes packages and works out what else they need: dnf on Rocky, apt on Ubuntu. Learn more: Installing software: dnf vs apt
Partition: A slice of a disk that acts like a separate disk. Each one usually gets its own filesystem. Learn more: Disks & mounting
Passphrase: A long password that encrypts your SSH private key file, so a stolen copy of the file is useless. It never leaves your computer; ssh-add unlocks the key once per session. Learn more: Lock down SSH
Path: The address of a file or folder, like /etc/hosts or notes/today.txt. See absolute path and relative path. Learn more: Moving around: cd
Permissions: Who may read, write or run a file: the owner, the group, and everyone else. Shown by ls -l, changed with chmod. Learn more: Permissions: who can do what
Pipe: The | symbol: it sends one command's output into the next command, like ls | wc -l. Learn more: Wildcards & your first pipes, Text power tools
Playbook: An Ansible file (YAML) that describes how servers should be set up. Running it makes them match. Learn more: Infrastructure as code: Ansible first steps
Port: A numbered door on a machine that a network service listens on: 22 for SSH, 80 for web, 443 for secure web. Learn more: Networking: ifconfig vs ip, Network & security tools
Process: A program that is running right now. Each one has a number, its PID. See them with ps or top. Learn more: Processes & disk space
Prompt: The text the shell shows when it's ready for a command, like [student@rocky ~]$. It tells you who and where you are. Learn more: Meet the terminal

R

Redirection: Sending a command's output to a file instead of the screen (> replaces, >> adds), or reading input from a file (<). Learn more: Capturing output
Relative path: A path that starts from the folder you're in, like notes/today.txt or ../pictures. Compare absolute path. Learn more: Moving around: cd
Repository (repo): For packages: an online store your package manager installs from. For Git: a project folder with its full history. Learn more: Installing software: dnf vs apt, Git: save points for your work
RHEL family: Red Hat Enterprise Linux and its relatives: Rocky Linux, AlmaLinux, CentOS Stream and Fedora. They use .rpm packages and dnf. One of the two big Linux families this site teaches. Learn more: What even is Linux?, Meet the cousins: Alma, Amazon Linux & more
Rocky Linux: A free distro built to match Red Hat Enterprise Linux, used on this site for the RHEL family. Learn more: What even is Linux?
Root: Linux's all-powerful administrator account. Also the top folder, /. Use root's power only when needed, with sudo. Learn more: Root, sudo & su

S

Script: A text file full of commands that runs them in order, so you can repeat a job with one command. Learn more: Shell scripting: if, loops & functions
SELinux: The RHEL family's extra security layer: every file and process gets a label, and rules decide which labels may touch which, even for root. Ubuntu uses AppArmor for the same job. Learn more: SELinux vs AppArmor, Deploying your company’s app
Service: A program that runs in the background and usually starts at boot, like a web server or SSH. Controlled with systemctl. Learn more: Services & your first website, Services deep dive: systemd & init.d
Shebang: The first line of a script that says which program runs it, like #!/bin/bash. Learn more: Shell scripting: if, loops & functions, POSIX: write once, run anywhere
Shell: The program that reads what you type in the terminal and runs it. On this site's servers it's bash. Learn more: Meet the terminal
Signal: A message sent to a process, like please stop (TERM) or stop now (KILL). Ctrl+C sends one; so does kill. Learn more: Processes & disk space, Jobs: pause, background & bring back
SLO: Service Level Objective: a reliability target, like 99.9% of requests succeed each month. Learn more: SLIs, SLOs, SLAs & KPIs: measuring reliability
SSH: Secure Shell: the safe, encrypted way to log in to another computer and run commands on it: ssh student@server. Learn more: Connecting with SSH, SSH apps: PuTTY, iTerm2 & friends
SSH key: A pair of files that proves who you are without a password: a private key you keep secret and a public key you put on servers. Learn more: Connecting with SSH, SSH apps: PuTTY, iTerm2 & friends
sshd: The SSH server: the program that answers when someone connects with ssh. The service is sshd on Rocky and ssh on Ubuntu; its settings are in /etc/ssh/sshd_config. Learn more: Lock down SSH, Connecting with SSH
sudo: Run one command with root's power: sudo dnf install tree. It asks for your password and keeps a record. Learn more: Root, sudo & su
systemd: The program that starts everything else when Linux boots and looks after services. You talk to it with systemctl and read its logs with journalctl. Learn more: Services deep dive: systemd & init.d, Services & your first website

T

Terminal: The window where you type commands and read the answers. The program inside it that understands the commands is the shell. Learn more: Meet the terminal

U

Ubuntu: The most popular Linux distro for beginners and the cloud, used on this site for the Debian family. Learn more: What even is Linux?
User: An account on the machine, with its own name, home folder and permissions. Learn more: Managing users & groups

V

Version control: Keeping every saved version of your files so you can compare, undo and share changes. Git is the most used tool for it. Learn more: Git: save points for your work
vim: A powerful text editor that runs in the terminal and is on almost every Linux machine. nano is the simpler alternative. Learn more: Text editors: vim (and nano)
Virtual machine (VM): A whole pretend computer running inside a real one, with its own operating system. Great for practising safely. Learn more: What even is Linux?, What’s in this computer?

W

Wildcard: A pattern that matches many file names at once: * (anything), ? (one character), [abc] (one of these), like *.log. Learn more: Wildcards & your first pipes