LFCS key terms

A configuration file that tells systemd how to manage one resource, such as a service, timer, mount, or socket. Units are stored in system directories and can be overridden locally.

The main command for controlling systemd: it starts, stops, enables, disables, and inspects units, and reports overall system state such as failed services or the default boot target.

The command that reads logs collected by systemd's logging service. It can filter by unit, boot session, priority, or time range, and follow new entries live.

Both the file and the command used to schedule recurring jobs for a user. Each line holds five time fields followed by the command to run, and jobs are executed by the cron daemon.

The bootloader used on most Linux systems. It presents a menu, loads the chosen kernel and initramfs into memory, and passes boot parameters; its configuration is generated by scripts in /etc/grub.d.

A small compressed filesystem loaded at boot that supplies the drivers and tools needed to find and mount the real root filesystem before switching to it.

A piece of kernel code that can be loaded or unloaded at runtime to add hardware drivers or features. lsmod lists loaded modules and modprobe loads or removes them with dependency handling.

A tool that installs, upgrades, and removes software from configured repositories while resolving dependencies, such as dnf on RHEL-family systems or apt on Debian-family systems.

A kernel security feature that labels files and processes and enforces policy rules about which process may access which resource, even when ordinary file permissions would allow it.

A shorthand for writing an IP address together with its network prefix length, such as 192.168.1.10/24, where the number after the slash counts the leading network bits.

The router address that a host sends packets to when the destination is not on a directly connected network. It is stored in the routing table as the default route.

The kernel's set of rules deciding where outgoing packets are sent. It can be displayed and edited with ip route, and each entry maps a destination to an interface or next hop.

A named kernel object representing a network device, such as eth0 or wlan0. Addresses and link state are configured with the ip command or by NetworkManager.

The command-line client for NetworkManager. It lists, creates, edits, and activates connection profiles, and is the usual way to configure networking on desktop-oriented distributions.

The service that converts hostnames into IP addresses. The servers a host queries are normally listed in /etc/resolv.conf, and lookups can be tested with tools such as getent or dig.

A matched public and private key used for encrypted, password-free SSH authentication. The public key is placed in the server's authorized_keys file while the private key stays on the client.

A named collection of firewall rules attached to particular interfaces or source addresses, letting an administrator change a host's trust level by switching zones instead of rewriting rule chains.

A 16-bit number identifying which service on a host should receive a network packet, allowing one address to serve many programs, for example 22 for SSH and 443 for HTTPS.

A storage device or logical volume that the kernel reads and writes in fixed-size blocks, appearing as a file under /dev such as /dev/sda or /dev/vg0/data.

The structure on a disk that records how it is divided into partitions. MBR is the older scheme with a four-primary-partition limit; GPT is modern and supports many more.

A virtual block device carved from a pool of physical storage called a volume group. Logical volumes can be grown or shrunk and moved across disks more easily than plain partitions.

The on-disk structure that organizes data into files and directories. Linux commonly uses ext4 and XFS, which are created with mkfs and managed with their own check and resize tools.

Attaching a filesystem to a directory so its contents become reachable in the directory tree. Mounts may be temporary or recorded in /etc/fstab to be repeated at boot.

The configuration file listing filesystems to mount automatically, with fields for device, mount point, type, options, dump flag, and filesystem check order.

A long identifier that uniquely marks a filesystem or partition. Configuration files use it instead of device names because names like /dev/sda can change when hardware is added or reordered.

A metadata record on a filesystem holding a file's owner, permissions, timestamps, size, and pointers to its data blocks. Filenames are kept separately in directory entries.

Disk space used as extra memory when RAM is under pressure or to hold a hibernation image. It may be a dedicated partition or a file, activated with swapon.

The shell operator | that sends the standard output of one command into the standard input of the next, allowing small tools to be chained into larger processing steps.

Shell syntax that changes where a command's input or output goes, such as > to write a file, >> to append, or 2> to capture error messages.

A command that searches input line by line for text matching a pattern, usually a regular expression, and prints the matching lines; options control case, inversion, and line numbers.

A command that walks a directory tree and acts on files matching tests such as name, size, type, or modification time, and can run another command on each match.

An archiving command that packs many files into a single stream or file and can compress it on the fly, typically used with options like -c, -x, -z, and -f.

A number from 0 to 255 that each command returns when it finishes, where 0 means success. The shell stores it in $? and scripts use it to decide what to do next.

A named value held in a process's environment that programs read for configuration. The shell can set one for a single command or export it to child processes.

The numeric identifier the kernel uses for a user account; root is normally 0. Human-readable usernames are mapped to UIDs in files such as /etc/passwd.

A root-readable file storing password hashes and aging information for local accounts, kept separate from /etc/passwd so ordinary users cannot read the hashes.

A program that runs a command as another user, usually root, after checking rules in /etc/sudoers. It logs the action and commonly asks for the caller's own password.

A bitmask that removes permission bits from newly created files and directories, so the default mode is the requested mode minus the masked bits.

A permission bit on an executable that makes it run with the file owner's user ID rather than the caller's, which is how programs like passwd can edit protected files.

Any group a user belongs to beyond the primary group listed in /etc/passwd. Membership grants extra file access and is set with commands like usermod -aG.