Companion to the Linux cheat sheet and the Linux series starting at A short history of Unix and Linux.
AppArmor
Path-based mandatory access control; default LSM on many Debian services. See Linux security primitives.
APT
High-level Debian tool that resolves dependencies and installs .deb packages from signed repositories. See Package management on Debian.
Capability
Fine-grained slice of traditional root power (for example binding low ports). See Linux security primitives.
cgroup
Kernel control group limiting CPU, memory, I/O, or pids for a process tree. See Linux security primitives.
D-Bus
Message bus for desktop and system service integration on Linux. See Processes and inter-process communication.
Distribution
Curated operating system built around the Linux kernel with packages and policy. See Linux distributions and Debian.
DRM/KMS
Kernel graphics subsystem for mode setting and buffer management. See Graphical desktops on Linux.
ext4
Common journaling filesystem on Debian installs. See Linux filesystems.
Firmware
Microcode loaded by drivers, often packaged in Debian non-free-firmware. See Hardware and devices in Linux.
FHS
Filesystem Hierarchy Standard describing conventional paths such as /etc and /var. See Linux filesystems.
GPL
GNU General Public License; Linux kernel uses GPLv2. See A short history of Unix and Linux.
GRUB
Bootloader loading the kernel and initramfs on typical Debian amd64 systems. See Booting Linux and systemd.
initramfs
Early userspace archive for drivers and crypto before mounting real root. See Booting Linux and systemd.
Inode
Filesystem metadata record; directory names point to inode numbers. See Linux filesystems.
iproute2
Modern ip, ss, and routing tools for the Linux network stack. See Networking on Linux.
journald
systemd component storing structured service logs. See Booting Linux and systemd.
Kernel
Privileged core implementing syscalls, drivers, and scheduling. See The Linux kernel.
Namespace
Kernel isolation of PID, mount, network, or other views. See Linux security primitives.
netfilter
Kernel framework for packet filtering and NAT; configured with nftables. See Networking on Linux.
PAM
Pluggable Authentication Modules stacking login and sudo checks. See Users, groups and permissions in Linux.
POSIX
Portable API and shell standards shared by Unix-like systems. See A short history of Unix and Linux.
PTY
Pseudo-terminal pair connecting terminal emulators to shells. See Terminals and shells in Linux.
seccomp
Syscall filter restricting which kernel entry points a process may use. See Linux security primitives.
Shell
Program interpreting commands on a TTY; distinct from the terminal emulator. See Terminals and shells in Linux.
systemd
PID 1 and service manager on current Debian; units, targets, timers. See Booting Linux and systemd.
System call
Controlled entry from user space into kernel services. See The Linux kernel.
TTY
Teletypewriter line discipline; console or serial character device. See Terminals and shells in Linux.
udev
Userspace manager for /dev nodes, permissions, and persistent symlinks. See Hardware and devices in Linux.
umask
Mask subtracted from default permission bits on file creation. See Users, groups and permissions in Linux.
Wayland
Display protocol where compositors own the screen; default for many GNOME sessions. See Graphical desktops on Linux.
WSL2
Windows subsystem running a real Linux kernel in a utility VM. See Linux on Windows, WSL, containers, and VMs.