Linux Terms: The Concepts Behind the Commands

The vocabulary of the machine, ordered the way you actually learn it on a real server.

How to Read This Index

Commands are what you type; terms are what you have to picture in your head before the commands make sense. This is the mental-model half of the knowledge base. We've grouped the concepts in roughly the order they pile up on you when you first take a Linux box seriously: the big picture, then CPUs and processes, then memory, disks, the network, users, and so on out to containers and the shell.

Read top-to-bottom if you're starting out, and entire sections will click into place at once. Skim if you already know the basics and just want to fill specific gaps. Every entry links to its own page, where the term is explained in plain language with the gotchas it tends to carry. The full alphabetical list at the bottom is for jumping straight to a term you already have a name for.

1. The Big Picture — How Linux Works

Start here. Everything else in the index sits inside one of these ideas.

Linux — the kernel that runs more servers than anything else on Earth
Unix — the 1970s ancestor whose ideas Linux still follows
Operating System — the layer between your program and the hardware
Kernel — the part of the OS that talks to hardware and shares it out
Shell — your text conversation with the kernel
Terminal — the window the shell prints into
tty — the old name for that window, still used everywhere
Command Line Interface — the input style the whole admin world runs on
GUI — the graphical alternative servers usually skip
Server — a machine whose job is to answer requests
Sysadmin — the person whose job is to keep it answering
DevOps — what happens when developers and admins stop being separate teams

2. CPUs and Processes — How Work Runs

The kernel's main trick is sharing a few cores between many programs. These are the words for that.

CPU — the chip that actually does the work
Process — a running program, with its own memory and PID
PID — the number the kernel uses to refer to a process
Daemon — a process that runs in the background, on purpose
Service — a daemon wearing a name tag, managed by an init system
Load Average — the three numbers that summarize how busy the box is
IOWait — CPU time spent doing nothing while waiting for disk
IRQ — the hardware's way of tapping the CPU on the shoulder
DMA — hardware moving data without bothering the CPU at all

3. Memory — Where Programs Live

Memory is the second resource the kernel rations. When something goes wrong here it usually goes wrong loudly.

RAM — the fast, volatile workspace every running program lives in
Swap — disk pretending to be RAM when RAM runs out
Page Cache — the kernel's automatic disk cache in RAM, and why "used" memory isn't what you think
ECC — the extra bits that catch a flipped memory bit before it corrupts your data

4. Disks and Filesystems

The longest section, because storage has the most moving parts and the most flavors.

HDD — the spinning platter that stores most of the world's data
SSD — flash storage without moving parts, the speed upgrade
NVMe — the protocol that lets SSDs run at the speed flash deserves
SATA — the interface that connects most drives to most servers
S.M.A.R.T. — the early warning system built into every disk
Block Device — anything you can read and write in fixed-size chunks
Device — anything the kernel exposes as a file under /dev
Device Mapper — the kernel layer that turns block devices into other block devices
Partition — a slice of a disk, treated as its own device
Mounting — attaching a filesystem to a point in the directory tree
Filesystem — the layer between your data and the raw disk
Directory — a folder, but actually just a special kind of file
File — a named blob of bytes the kernel keeps track of
Inode — the kernel's record card for a file, separate from its name
Hard Link — a second name pointing at the same inode
Symlink — a file whose contents are the path of another file
ext2 — the original Linux filesystem
ext3 — ext2 with a journal, so crashes don't shred it
ext4 — the modern default on most distros
XFS — the high-throughput default on RHEL
Btrfs — copy-on-write with snapshots, powerful and sharp-edged
ZFS — the filesystem that swallowed the volume manager too
FAT / VFAT — the ancient FAT family that still lives on USB sticks
NTFS — Windows's filesystem, readable on Linux
NFS — mount a remote directory as if it were local
CIFS — same idea, but the Windows version
LVM — turn disks into a pool, carve out volumes you can resize later
RAID — combine disks for redundancy, speed, or both

5. The Network

Servers exist to be talked to. This is the vocabulary for the talking.

Protocol — an agreed-upon way for two programs to exchange bytes
OSI Model — the seven-layer cake everyone draws once and then forgets
TCP — reliable, ordered byte streams, the workhorse of the internet
UDP — fire-and-forget packets, used where speed beats reliability
ICMP — the network's own diagnostics, including ping
NAT — sharing one public IP between many private ones
DHCP — handing out IP addresses on boot
DNS — turning names into addresses
NIC — the chip that puts your machine on the wire
NTP — keeping clocks in sync across the network
Firewall — the rules deciding which packets are allowed through
SSH — your encrypted shell to a remote box
SSH Key — the keypair that lets you log in without a password
SCP — copy files over SSH
SFTP — interactive file transfer over SSH
FTP — the unencrypted ancestor, please retire it
SSL / TLS — the encryption layer behind HTTPS
Certificate — the file that proves you are who you say you are
HTTP — the protocol of the web
HTTPS — HTTP with TLS wrapped around it
Web Server — the program that answers HTTP requests
SNMP — the old-school protocol for monitoring network gear
RDP — Microsoft's graphical remote-desktop protocol
VNC — the cross-platform graphical remote-desktop protocol
libpcap — the library that lets tcpdump and friends sniff packets

6. Email and Identity

Sitting between "the network" and "running a service" — the protocols and authentication pieces you meet the first time you run a mail server or set up logins.

SMTP — how mail servers hand mail to each other
IMAP — how clients read mail that stays on the server
POP3 — how clients download mail and (usually) delete it
MTA — the program in the middle, like Postfix or Exim
DKIM — signing your outbound mail so the recipient can verify it
DMARC — telling the world what to do with mail that fails the checks
SPF — listing the IPs allowed to send mail for your domain
LDAP — the directory protocol behind a lot of corporate logins
Kerberos — single sign-on with cryptographic tickets
KDC — the server that issues those tickets
NSS — the glue that tells glibc where to look up users and hosts

7. Users and Permissions

Who can do what. The whole Unix security model starts here.

User — an account with a name and a UID
UID — the integer the kernel actually checks
Group — a bag of users you can hand permissions to
GID — the integer behind a group
Root — UID 0, the account that can do anything
Superuser — another name for root
Permission — the read/write/execute bits on every file
SELinux — mandatory access control, much stricter than the bits

8. Boot, Init, and Logs

How the machine wakes up, brings up its services, and writes down what happened.

Bootloader — the tiny program that loads the kernel
GRUB — the bootloader you'll meet on most Linux boxes
Init System — the first userspace process and everything it manages
systemd — the modern, dominant init system
SysVinit — the older one, still on a few distros
Upstart — Ubuntu's now-retired middle step
Runlevel — the SysV-era way of saying "how booted are we"
inetd — the old trick of one daemon spawning others on demand
Cron — the time-based scheduler that's older than you
Syslog — the universal logging protocol
Logging — the general practice of writing down what happened
Monitoring — watching the machine so it tells you before users do
Man Page — the manual that ships with everything

9. Distributions and Their Tooling

Same kernel, very different personalities. The choice colors everything else you do.

Distro — short for distribution, a packaging of Linux
Debian — the universal community distribution
Ubuntu — the polished, Debian-based default for many
Fedora — Red Hat's fast-moving upstream
CentOS — the long-running RHEL clone, now Stream
Arch — minimalist and rolling-release
Gentoo — compile-everything-yourself
Slackware — the oldest still around
BSD — the Unix cousin you'll meet on FreeBSD or OpenBSD
POSIX — the standard that says what "Unix-like" means
Package — a single installable unit of software
Repository — the place packages come from
Dependency — what a package needs to function

10. Containers, Virtualization, and the Cloud

Layers on top of Linux that pretend to be whole machines.

Virtualization — running a whole OS inside another
Virtual Machine — the guest OS that virtualization creates
Hypervisor — the layer doing the pretending
KVM — the kernel feature that makes virtualization fast on Linux
QEMU — the userspace half, paired with KVM almost everywhere
Container — like a VM, but sharing the kernel
cgroup — the kernel knobs that limit a container's resources
Docker — the tool that made containers mainstream
Kubernetes — the orchestrator that runs many containers across many hosts
Pod — Kubernetes's smallest unit, one or more containers together
Data Center — the building full of the machines all this runs on

11. Data, Files, and Stacks

The terms that show up when you start running applications instead of just operating systems.

Database — organized storage for structured data
Key-Value Store — the simplest possible database shape
JSON — the data format your APIs are probably speaking
API — the contract between two programs
CMS — content-management software, the WordPress shape
LAMP — the classic Linux + Apache + MySQL + PHP stack
CUPS — the Linux printing system, included because it's everywhere

12. The Shell's Plumbing

The small concepts you need to read shell pipelines and scripts.

Pipe — connecting one program's output to another's input
stdin — where a program reads its input
stdout — where it writes its output

13. Time and Identifiers

The boring-but-essential glue.

Timezone — the world's clocks, offset and labeled
UTC — the one true time, before timezones are applied
UUID — a 128-bit identifier that's unique enough to never collide

Full Alphabetical List

Everything in the knowledge base, A to Z. If you already know the term, jump straight in.

API — application programming interface, the contract between two programs
Arch — minimalist rolling-release distribution
block device — storage you read and write in fixed-size chunks
bootloader — the tiny program that loads the kernel
BSD — the Unix family parallel to Linux
btrfs — modern copy-on-write filesystem with snapshots
CentOS — community RHEL rebuild, now Stream
certificate — the file that proves a server's identity
cgroup — kernel feature that limits and isolates resource use
CIFS — Windows network file sharing protocol
CMS — content management system
command line interface — typing commands to a shell
container — isolated process group sharing the host kernel
CPU — the chip that does the work
cron — time-based job scheduler
CUPS — the Linux printing system
daemon — a background process, by design
database — organized storage for structured data
data center — building full of servers and the gear to run them
Debian — the universal community distribution
dependency — what one package needs from another
device — anything the kernel exposes under /dev
device mapper — the kernel layer behind LVM, encryption, and snapshots
DevOps — the merging of development and operations
DHCP — automatic IP address assignment
directory — a folder, technically a special file
distro — short for distribution
DKIM — cryptographic signing of outbound email
DMA — hardware reading and writing memory without the CPU
DMARC — the email policy that ties SPF and DKIM together
DNS — turning names into IP addresses
Docker — the tool that made containers mainstream
ECC — error-correcting memory that catches a flipped bit before it corrupts data
ext2 — the original Linux filesystem
ext3 — ext2 plus a journal
ext4 — the modern default Linux filesystem
FAT — the ancient filesystem that lives on every USB stick
Fedora — Red Hat's upstream distribution
file — a named blob of bytes
filesystem — the layer between your data and the raw disk
firewall — the rules controlling which packets pass
FTP — the unencrypted file transfer protocol you should retire
Gentoo — the compile-everything-yourself distribution
gid — group identifier
group — a named collection of users
GRUB — the most common Linux bootloader
GUI — graphical user interface
hard link — a second name pointing at the same inode
HDD — hard disk drive, the spinning platter that stores most of the world's data
HTTP — the protocol of the web
HTTPS — HTTP wrapped in TLS
hypervisor — the software that runs virtual machines
ICMP — the network's own diagnostics, including ping
IMAP — server-side email access protocol
inetd — the old daemon that spawned other daemons on demand
init system — the first userspace process and what it supervises
inode — the kernel's per-file record card
IOWait — CPU time spent idle, waiting for I/O
IRQ — hardware interrupt request to the CPU
JSON — the data interchange format of modern APIs
KDC — Kerberos key distribution center
Kerberos — single sign-on with cryptographic tickets
kernel — the core of the operating system
key-value store — the simplest database shape
Kubernetes — the orchestrator for containerized workloads
KVM — Linux's built-in virtualization engine
LAMP — Linux + Apache + MySQL + PHP web stack
LDAP — directory protocol behind corporate logins
libpcap — the packet capture library
Linux — the kernel running most servers in the world
load average — the three numbers summarizing system busyness
logging — writing down what the system did
LVM — Logical Volume Manager
man page — built-in documentation
monitoring — watching the system so it warns you in time
mounting — attaching a filesystem to the directory tree
MTA — mail transfer agent
NAT — sharing one public IP across many private ones
NFS — Unix-style network file sharing
NIC — the network interface card
NSS — the name service switch in glibc
NTFS — Windows's filesystem, readable on Linux
NTP — network time protocol
NVMe — the flash-native storage protocol that replaced SATA's bottleneck
operating system — the layer between programs and hardware
OSI model — the seven-layer model of networking
package — a single installable unit of software
page cache — the kernel's automatic disk cache in RAM
partition — a slice of a disk treated as its own device
permission — the read/write/execute bits on a file
PID — process identifier
pipe — connecting one program's output to another's input
Pod — Kubernetes's smallest deployable unit
POP3 — the download-and-delete email protocol
POSIX — the standard defining Unix-like behavior
process — a running program with its own memory and PID
protocol — an agreed-upon way for programs to exchange bytes
QEMU — the userspace half of Linux virtualization
RAID — combining disks for redundancy or speed
RAM — the fast, volatile memory programs run in
RDP — Microsoft's remote desktop protocol
repository — the place packages are fetched from
root — the superuser account, uid 0
runlevel — the SysV-era system mode
SATA — the serial interface connecting most drives to most servers
SCP — secure copy over ssh
SELinux — mandatory access control for Linux
server — a machine whose job is to answer requests
service — a daemon managed by an init system
SFTP — secure file transfer over ssh
shell — the text interface to the operating system
signal — the smallest possible message in Unix, just a number
Slackware — the oldest surviving distribution
S.M.A.R.T. — the early warning system built into every disk
SMTP — protocol for sending mail between servers
SNMP — the old standard for monitoring network gear
SPF — declares which IPs may send mail for your domain
SSH — encrypted shell access to a remote machine
SSH key — keypair for passwordless ssh login
SSD — solid state drive, flash storage without moving parts
SSL — the predecessor of TLS, the encryption behind HTTPS
stdin — standard input
stdout — standard output
superuser — another name for root
swap — disk space used as overflow RAM
symlink — a file whose contents are another file's path
sysadmin — the human who keeps the system running
syslog — the universal logging protocol
systemd — the modern Linux init and service manager
SysVinit — the classic init system that systemd replaced
TCP — reliable ordered byte streams
terminal — the window the shell prints into
timezone — the world's clocks, offset and labeled
TLS — the modern encryption layer behind HTTPS
TTY — the old name for a terminal device
Ubuntu — the polished Debian-based distribution
UDP — fire-and-forget packets, no reliability guarantees
uid — user identifier
Unix — the family of OSes Linux descends from
Upstart — Ubuntu's now-retired init system
user — a regular account on the system
UTC — coordinated universal time
UUID — a 128-bit identifier unique enough never to collide
VFAT — the FAT variant with long filenames
virtual machine — a guest OS running inside virtualization
virtualization — running a whole OS inside another
VNC — cross-platform graphical remote desktop
web server — the program serving HTTP requests
XFS — the high-throughput default filesystem on RHEL
ZFS — the filesystem that swallowed the volume manager

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