apropos linux
Linux Glossary
134 Linux and command-line terms defined in plain English. Each entry has a short definition, an example you can run, and links to the full guides when you want to go deeper.
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A
- Access Control List (ACL) Extended permissions that grant specific users or groups access to a file beyond the basic owner, group, and other bits. Files & Permissions
- Alias A shell shortcut that replaces a short name with a longer command, such as
alias ll='ls -lah'. Shell & Command Line - AppArmor A path-based mandatory access control system that confines programs with per-application profiles, default on Ubuntu, Debian, and openSUSE. Security
- AppImage A single-file portable Linux app format: download, make executable, and run, with no installation or root access required. Packages & Distros
- APT The Advanced Package Tool, the package manager for Debian, Ubuntu, Linux Mint, and other distros that use .deb packages. Packages & Distros
- AUR (Arch User Repository) A community repository of build scripts (PKGBUILDs) for Arch Linux, used with helpers like yay or paru to install unofficial software. Packages & Distros
B
- Bash The GNU Bourne Again Shell, the default interactive shell and scripting language on most Linux distributions. Shell & Command Line
- Block Device A device such as a disk, partition, or USB drive that is read and written in fixed-size blocks, listed by
lsblk. Disks & Filesystems - Boot Process The sequence from power-on to login: firmware (UEFI/BIOS), bootloader, kernel, initramfs, init system, then services and login. Boot & Kernel
- Bootloader (GRUB) The program firmware runs to load the Linux kernel and initramfs. GRUB is the most common; systemd-boot is a simpler UEFI-only option. Boot & Kernel
- Btrfs A copy-on-write Linux filesystem with snapshots, subvolumes, transparent compression, and data checksums built in. Disks & Filesystems
C
- cgroups (Control Groups) A kernel feature that groups processes and limits, accounts for, and isolates their CPU, memory, I/O, and process counts. Processes & systemd
- chmod The command that changes a file's permission bits, using either octal numbers (
chmod 644) or symbolic notation (chmod u+x). Files & Permissions - chroot A command that runs a process with a different directory treated as
/, used for system rescue and as an early form of isolation. Containers & Virtualization - Container An isolated process (or group) that shares the host kernel but has its own filesystem, network, and process view, built on namespaces and cgroups. Containers & Virtualization
- Container Image A read-only, layered template containing an app and its filesystem, from which containers are started. Built from a Dockerfile. Containers & Virtualization
- Cron The classic Unix scheduler that runs commands at fixed times using five-field schedule expressions in a crontab file. Shell & Command Line
D
- Daemon A long-running background process with no terminal attached, usually started at boot to provide a service such as SSH, logging, or scheduling. Linux Basics
- Dependency Another package or library that a piece of software needs in order to install or run, resolved automatically by the package manager. Packages & Distros
- Desktop Environment (DE) The full graphical interface on a Linux desktop, including the panel, window manager, file manager, settings, and bundled apps. Linux Basics
- /dev The directory of device files that represent hardware and kernel devices, such as /dev/sda, /dev/null, and /dev/tty. Files & Permissions
- Display Manager The graphical login screen that authenticates users and starts their desktop session, such as GDM, SDDM, or LightDM. Linux Basics
- Distribution (Distro) A complete operating system built around the Linux kernel, bundling a package manager, userland tools, defaults, and a release model. Linux Basics
- DKMS A framework that automatically rebuilds out-of-tree kernel modules, like NVIDIA or ZFS drivers, whenever a new kernel is installed. Boot & Kernel
- dmesg The command that prints the kernel ring buffer: messages about hardware detection, drivers, and errors since boot. Boot & Kernel
- DNF The package manager for Fedora, RHEL, CentOS Stream, AlmaLinux, and Rocky Linux, which installs and updates .rpm packages. Packages & Distros
- DNS The system that translates hostnames like linuxdork.com into IP addresses, queried on Linux with tools like
dig. Networking - Docker The most widely used container platform, providing tools to build images, run containers, and manage networks and volumes. Containers & Virtualization
- Docker Compose A tool for defining multi-container apps in one YAML file and starting them together with
docker compose up. Containers & Virtualization - Dotfiles Configuration files in your home directory whose names start with a dot, like
.bashrcand.gitconfig, and are hidden from plainls. Shell & Command Line
E
- Environment Variable A named value such as
HOMEorPATHthat is passed from a process to its children and used to configure program behavior. Shell & Command Line - /etc The directory holding system-wide configuration files such as /etc/fstab, /etc/passwd, /etc/hosts, and service configs. Files & Permissions
- Exit Code The number from 0 to 255 a program returns when it finishes. Zero means success; anything else signals an error. Shell & Command Line
- ext4 The default journaling filesystem on many Linux distributions, valued for stability, speed, and mature repair tools. Disks & Filesystems
F
- Fail2ban A daemon that watches logs for repeated failed logins and temporarily bans the offending IP addresses with firewall rules. Security
- File Permissions (rwx) Read, write, and execute bits set separately for a file's owner, its group, and everyone else, shown as
-rwxr-xr--byls -l. Files & Permissions - Filesystem The on-disk format that organizes data into files and directories, such as ext4, XFS, Btrfs, or ZFS. Disks & Filesystems
- Filesystem Hierarchy Standard (FHS) The standard that defines what belongs in Linux top-level directories like /etc, /usr, /var, /home, and /tmp. Files & Permissions
- Firewall Rules that allow or block network traffic by address, port, and protocol. Linux firewalls are built on the kernel's netfilter framework. Networking
- Flatpak A cross-distribution app format that bundles desktop apps with their runtimes and runs them in a sandbox, distributed mainly via Flathub. Packages & Distros
- fsck The tool that checks a filesystem for inconsistencies and repairs them, typically run on an unmounted partition after a crash. Disks & Filesystems
- fstab The /etc/fstab file listing which filesystems to mount at boot, where, and with which options. Disks & Filesystems
G
- Glob (Wildcard) Shell patterns like
*,?, and[abc]that expand into matching file names before a command runs. Shell & Command Line - GNU The free software project that produced most of the core userland on Linux systems, including bash, coreutils, glibc, and GCC. Linux Basics
- GPG (GnuPG) GNU Privacy Guard, the OpenPGP tool used to encrypt files and to sign and verify software, packages, and Git commits. Security
H
- Hard Link An additional directory entry pointing at the same inode as an existing file, so one set of data has two equal names. Files & Permissions
- Heredoc Shell syntax (
<<EOF ... EOF) that feeds a multi-line block of text into a command's standard input. Shell & Command Line - Home Directory Each user's personal directory, usually
/home/usernameand abbreviated~, holding their files and per-user configuration. Files & Permissions
I
- Immutable Distro A distribution whose core system is read-only and updated as a whole image, with apps installed via Flatpak or containers. Packages & Distros
- Init System The first user-space process (PID 1) started by the kernel, responsible for booting the rest of the system and supervising services. Processes & systemd
- initramfs A small temporary root filesystem loaded into memory at boot, holding the drivers and tools needed to find and mount the real root. Boot & Kernel
- Inode The on-disk record that stores a file's metadata (owner, permissions, size, timestamps, block locations), everything except its name. Files & Permissions
- IP Address The numeric address identifying a network interface, such as 192.168.1.10 (IPv4) or 2001:db8::1 (IPv6). Networking
J
- Job Control Shell features for pausing, backgrounding, and resuming commands with Ctrl+Z,
bg,fg,jobs, and a trailing&. Shell & Command Line - journald (journalctl) systemd's logging service, which collects kernel, service, and application logs in a structured binary journal queried with
journalctl. Processes & systemd
K
- Kernel Command Line Options passed to the kernel by the bootloader at boot, such as
quiet,root=, ornomodeset, visible in /proc/cmdline. Boot & Kernel - Kernel Module A piece of kernel code, usually a driver, that can be loaded and unloaded at runtime without rebooting. Managed with
modprobe. Boot & Kernel - Kernel Panic A fatal kernel error from which Linux cannot safely recover, so it halts the system and prints diagnostic information. Boot & Kernel
L
- Linux Kernel The core program of a Linux system that manages hardware, memory, processes, and system calls between applications and the machine. Linux Basics
- Load Average Three numbers showing the average count of runnable or waiting processes over the last 1, 5, and 15 minutes. Processes & systemd
- Localhost (Loopback) The hostname that always refers to the current machine, mapped to 127.0.0.1 and ::1 on the virtual loopback interface
lo. Networking - LTS (Long-Term Support) A release that receives security and bug fixes for several years without major version changes, favored for servers and stable desktops. Linux Basics
- LUKS The standard Linux disk encryption format, managed with cryptsetup, that encrypts a whole partition behind one or more passphrases. Disks & Filesystems
- LVM (Logical Volume Manager) A storage layer that pools physical disks into volume groups and carves flexible, resizable logical volumes from them. Disks & Filesystems
M
N
- Namespace A kernel feature that gives a process its own isolated view of a system resource, such as PIDs, mounts, network, users, or hostname. Containers & Virtualization
- Network Interface A named network device, physical or virtual, such as enp3s0, wlan0, lo, wg0, or docker0, that sends and receives packets. Networking
- nftables The modern Linux packet filtering framework and
nftcommand, replacing iptables with one unified syntax for IPv4, IPv6, and more. Networking - Nice Value A process's CPU scheduling priority from -20 (highest) to 19 (lowest), set with
niceat launch orrenicewhile running. Processes & systemd
O
P
- Package Manager The tool that installs, updates, and removes software on Linux while resolving dependencies, such as apt, dnf, pacman, or zypper. Packages & Distros
- pacman The package manager for Arch Linux and its derivatives, known for speed and terse flags like
pacman -Syu. Packages & Distros - PAM Pluggable Authentication Modules, the framework Linux programs use to handle logins, passwords, 2FA, and session setup. Security
- Partition A defined section of a disk that the OS treats as a separate device, recorded in a GPT or MBR partition table. Disks & Filesystems
- PATH The environment variable listing, colon-separated, the directories the shell searches when you run a command by name. Shell & Command Line
- PID (Process ID) The unique number the kernel assigns to each running process, used to inspect, signal, or kill it. Processes & systemd
- Pipe The
|operator, which sends the standard output of one command directly into the standard input of the next. Shell & Command Line - Podman A daemonless, Docker-compatible container engine from Red Hat that runs containers rootless by default. Containers & Virtualization
- Port A number from 0 to 65535 that identifies a specific service on a host, like 22 for SSH, 80 for HTTP, and 443 for HTTPS. Networking
- POSIX An IEEE standard defining the shell, utilities, and system APIs a Unix-like OS should provide so scripts and programs stay portable. Linux Basics
- Principle of Least Privilege The security practice of giving every user, service, and program only the permissions it needs to do its job, and nothing more. Security
- /proc A virtual filesystem the kernel generates on the fly, exposing live information about processes and system state as readable files. Files & Permissions
- Process A running instance of a program, with its own memory, PID, owner, open files, and state, managed and scheduled by the kernel. Processes & systemd
R
- RAID Combining multiple disks for redundancy, speed, or both. On Linux it is usually done in software with mdadm, ZFS, or Btrfs. Disks & Filesystems
- Redirection Shell syntax such as
>,>>,<, and2>&1that connects a command's input or output to files instead of the terminal. Shell & Command Line - Regular Expression (Regex) A pattern language for matching text, used by grep, sed, awk, and most editors to search and transform lines. Shell & Command Line
- Repository (Repo) A signed collection of packages hosted by a distro or vendor that the package manager downloads software and updates from. Packages & Distros
- Reverse Proxy A server like nginx or Caddy that receives client requests and forwards them to backend apps, handling TLS, routing, and caching. Networking
- Rolling Release A distribution model with no major version upgrades; packages are updated continuously as upstream releases them. Linux Basics
- Root Directory (/) The top of the Linux directory tree, written as a single slash
/, under which every file and mounted disk appears. Files & Permissions - Root User The Linux superuser account with user ID 0, which bypasses normal permission checks and can change anything on the system. Linux Basics
S
- Secure Boot A UEFI feature that only runs bootloaders and kernels signed by trusted keys, blocking boot-level malware. Boot & Kernel
- SELinux A mandatory access control system that labels every file and process and enforces policy on what they may access, default on Fedora and RHEL. Security
- Shared Library A compiled
.sofile of code that many programs load at runtime instead of each including its own copy, such as libc.so.6. Packages & Distros - Shebang The
#!line at the top of a script that tells the kernel which interpreter should run it, such as#!/usr/bin/env bash. Shell & Command Line - Shell The program that reads the commands you type, expands variables and wildcards, and launches other programs. Bash, zsh, and fish are shells. Shell & Command Line
- Signal An asynchronous notification sent to a process, such as SIGTERM (please exit), SIGKILL (exit now), or SIGHUP (reload/hang up). Processes & systemd
- Snap Canonical's containerized package format for apps and services, installed from the Snap Store and default on Ubuntu. Packages & Distros
- SSH (Secure Shell) An encrypted protocol for logging into and running commands on remote machines, also used for file transfer and tunneling. Networking
- SSH Key A public/private key pair used to log in over SSH without a password. The public key goes on the server; the private key stays with you. Networking
- stdin, stdout, stderr The three streams every process starts with: standard input (fd 0), standard output (fd 1), and standard error (fd 2). Shell & Command Line
- Sticky Bit A directory permission bit that lets users delete only their own files, even when everyone has write access, as on /tmp. Files & Permissions
- sudo The command that lets permitted users run individual commands as root (or another user), with authentication and logging. Security
- SUID and SGID Special permission bits that make a program run with its owner's (SUID) or group's (SGID) privileges instead of the caller's. Files & Permissions
- Swap Disk space (or compressed RAM with zram) the kernel uses to page out inactive memory when physical RAM runs short. Disks & Filesystems
- Symbolic Link (Symlink) A special file that points to another path by name, like a shortcut. Created with
ln -s target linkname. Files & Permissions - /sys (sysfs) A virtual filesystem that exposes the kernel's view of devices, drivers, and buses as a structured directory tree. Files & Permissions
- sysctl The interface for reading and changing kernel runtime parameters, such as networking and memory settings, under /proc/sys. Boot & Kernel
- System Call (Syscall) The interface programs use to ask the kernel to do privileged work, such as opening files, allocating memory, or creating processes. Boot & Kernel
- systemd The init system and service manager used by most Linux distributions, controlling boot, services, logging, timers, and more. Processes & systemd
- systemd Timer A systemd unit that activates a service on a schedule or after a delay, a modern alternative to cron jobs. Processes & systemd
- systemd Unit File An INI-style file describing something systemd manages, such as a service, timer, mount, or socket, with its dependencies and settings. Processes & systemd
T
- Terminal Emulator A graphical app that emulates a hardware text terminal, giving you a window in which a shell runs. Examples: GNOME Terminal, Konsole, Alacritty. Shell & Command Line
- Terminal Multiplexer A program like tmux or screen that runs multiple shells in one terminal and keeps them alive after you disconnect. Shell & Command Line
- TLS Certificate A signed file that proves a server's identity and enables encrypted HTTPS connections. Let's Encrypt issues them for free. Security
- /tmp The world-writable directory for temporary files, often backed by RAM (tmpfs) and cleared on reboot. Files & Permissions
- TTY A terminal device in Linux, either a text virtual console reached with Ctrl+Alt+F-keys or a pseudo-terminal used by terminal emulators and SSH. Linux Basics
U
- udev The device manager that reacts to hardware events, creates /dev nodes, loads drivers, and applies custom rules for naming and permissions. Boot & Kernel
- UEFI Modern PC firmware that replaced legacy BIOS, booting from .efi files on a FAT-formatted EFI System Partition (ESP). Boot & Kernel
- UFW Uncomplicated Firewall, a simple front end for Linux firewall rules, default on Ubuntu, using commands like
ufw allow 443/tcp. Networking - umask A per-process mask that removes permission bits from newly created files and directories. A umask of 022 yields 644 files and 755 directories. Files & Permissions
- Users and Groups Linux identifies every account by a numeric user ID (UID) and assigns it to groups (GIDs), which together decide file and process access. Files & Permissions
- /usr The directory containing installed programs, libraries, and shared data. Package-managed software goes in /usr; locally built software in /usr/local. Files & Permissions
- UUID (Filesystem) A unique ID written into a filesystem when it is created, used to identify disks reliably even if device names like /dev/sdb change. Disks & Filesystems
V
- /var The directory for variable data that changes while the system runs: logs, caches, spools, databases, and package state. Files & Permissions
- Virtual Machine (VM) A fully emulated computer with its own kernel running on a hypervisor. On Linux this is typically KVM with QEMU and libvirt. Containers & Virtualization
W
- Wayland The modern Linux display protocol replacing X11, in which the compositor draws the screen and isolates each app's windows and input. Linux Basics
- WireGuard A fast, modern VPN protocol built into the Linux kernel, configured with short key-based config files. Networking
- WSL Windows Subsystem for Linux, which runs real Linux distributions inside a lightweight VM on Windows with tight integration. Containers & Virtualization
X
Z
- ZFS A combined filesystem and volume manager with end-to-end checksums, snapshots, compression, and software RAID, popular on NAS and servers. Disks & Filesystems
- Zombie Process A process that has exited but whose parent has not yet collected its exit status, leaving an entry marked
Zor<defunct>. Processes & systemd