Glossary¶

Short definitions of terms that appear throughout this knowledge base. Abbreviations are also explained when you hover over them on any page.

cgroup (control group): A Linux kernel feature that groups processes so their resource use can be limited, measured and controlled together. systemd places every service in its own cgroup.
cloud-init: A tool that configures a Linux system on first boot from metadata supplied by a cloud platform or a seed disk: users, SSH keys, hostname, packages and commands.
daemon: A program that runs in the background to provide a service, such as sshd or chronyd, usually started by the init system.
distribution (distro): A complete operating system built around the Linux kernel, with its own package manager, defaults and release cycle, such as Debian, AlmaLinux, Arch Linux or Gentoo.
file descriptor: A small integer a process uses to refer to an open file, socket or pipe. Standard input, output and error are 0, 1 and 2.
filesystem: The on-disk structure that organises data into files and directories, such as ext4, XFS or Btrfs. Also used for the single directory tree that all mounted filesystems form.
firewall: Rules that decide which network packets a host accepts, forwards or drops. On Linux they are enforced by netfilter and usually managed with nftables, firewalld or ufw.
init system: The first userspace program the kernel starts (process ID 1), and the service manager that brings the rest of the system up and down. Examples: systemd, OpenRC.
interface (network interface): A device through which a host sends and receives network traffic, such as enp1s0, eth0 or the loopback interface lo.
kernel: The core of the operating system: it manages memory, processes, devices, filesystems and networking, and provides the system calls programs use.
logical volume: A block device created by LVM from space in a volume group. It can be resized and moved without the constraints of disk partitions.
mount: To attach a filesystem to a directory (the mount point) so its contents appear in the directory tree.
netfilter: The packet-filtering framework inside the Linux kernel. nftables, iptables, firewalld and ufw are all ways of giving it rules.
package manager: The tool that installs, upgrades and removes software and resolves dependencies: apt (Debian, Ubuntu), dnf (AlmaLinux, Rocky Linux), pacman (Arch) and emerge (Gentoo).
root: The superuser account (user ID 0), which bypasses ordinary permission checks. Also the top of the directory tree, /.
route: An entry in the kernel's routing table that says where to send packets for a range of destination addresses: directly on an interface, or via a gateway.
runlevel: A named or numbered system state that determines which services run. OpenRC uses named runlevels such as boot and default; systemd uses targets instead.
service: A long-running program managed by the init system, or the definition (unit or script) that tells the init system how to run it.
socket: An endpoint for communication between processes, on the same host (Unix domain sockets) or across a network (TCP and UDP sockets).
sudo: A program that lets permitted users run commands as root or another user, with rules defined in /etc/sudoers and /etc/sudoers.d/.
systemd unit: A configuration file that describes something systemd manages: a service, socket, timer, mount, target and more.
volume group: An LVM pool of storage built from one or more physical volumes, from which logical volumes are allocated.