CELE key terms

The core component of an operating system that manages hardware resources, memory, and processes, and provides essential services to all other software running on the system.

A command-line interpreter that reads user commands and executes them, providing an interface to the operating system and allowing scripting and automation.

The first process started by the kernel during boot, responsible for initializing system services and managing processes; common examples include systemd and SysVinit.

A standard that defines the directory structure and directory contents in Linux systems, ensuring consistency across distributions for file locations like /etc, /var, and /usr.

A data structure on a filesystem that stores all metadata about a file except its name and actual data, including permissions, ownership, size, and pointers to data blocks.

A directory entry that points to the same inode as another file, meaning both names refer to the same data on disk; deleting one does not remove the data until all links are gone.

A special file that contains a reference to another file or directory by path, similar to a shortcut; if the target is removed, the link becomes broken.

A directory in the filesystem tree where an additional filesystem is attached and made accessible, such as /mnt or /media.

A configuration file that lists filesystems and their mount options, used by the system to automatically mount filesystems at boot or on demand.

A universally unique identifier assigned to a filesystem or partition, used in configuration files like /etc/fstab to reliably identify devices regardless of hardware changes.

A type of device that provides access to storage in fixed-size blocks, such as hard drives, SSDs, and partitions, typically found under /dev.

A logical division of a storage device that can be formatted with a filesystem and used independently, allowing multiple operating systems or separate data areas on one disk.

A method and data structure that an operating system uses to organize, store, and retrieve files on a storage device, such as ext4, XFS, or Btrfs.

A set of credentials and permissions that allows a person or service to log in and access resources on a Linux system, identified by a username and user ID (UID).

A collection of user accounts that simplifies permission management by allowing permissions to be assigned to multiple users at once; each group has a group ID (GID).

A numeric identifier assigned to each user account; the root user typically has UID 0, and system users often have UIDs below 1000.

A numeric identifier assigned to each group; used in file permissions and process credentials to determine group-based access rights.

A set of rights that control who can read, write, or execute a file or directory, defined for the owner, group, and others, and represented by a three-digit octal or symbolic notation.

A command used to change the access permissions of files and directories, using either symbolic (e.g., u+x) or octal (e.g., 755) notation.

A command used to change the owner and/or group of a file or directory; typically requires root privileges.

A command that allows a permitted user to execute a command as another user, typically the superuser, according to security policy defined in /etc/sudoers.

A command used to switch to another user account, often the root user, by providing the target user's password; it starts a new shell with that user's privileges.

The superuser account with UID 0, which has unrestricted access to all files, commands, and system resources; using it requires caution due to its power.

An instance of a running program, including its memory space, file descriptors, and execution state; each process is identified by a unique process ID (PID).

A unique numeric identifier assigned to each running process by the kernel, used to manage, monitor, and control processes.

A software interrupt sent to a process to notify it of an event or request an action, such as SIGTERM to request termination or SIGKILL to force termination.

A command used to send a signal to a process, typically to terminate it; by default it sends SIGTERM, but other signals can be specified.

A configuration file that describes a system resource or service managed by systemd, such as a service, mount point, or timer; units are defined in files with specific extensions like .service.

A command used to control the systemd system and service manager, allowing users to start, stop, enable, disable, and check the status of systemd units.

A legacy SysVinit concept representing a system state that defines which services are running; systemd uses targets instead, but runlevels are still partially supported for compatibility.

A numerical label assigned to each device on a network that uses the Internet Protocol for communication, identifying both the host and its network location.

A 32-bit number that divides an IP address into network and host portions, determining which part identifies the network and which part identifies the host.

A compact representation of an IP address and its associated subnet mask, written as address/prefix-length (e.g., 192.168.1.0/24), where the prefix length indicates the number of network bits.

The router on a local network that forwards traffic to destinations outside the local subnet; configured in the routing table to handle unknown destinations.

A data table stored in the kernel that lists the routes to various network destinations, used to determine where to send packets based on their destination IP address.

Secure Shell, a cryptographic network protocol and command suite for secure remote login and other secure network services over an unsecured network.

An SSH authentication method where the client proves its identity by signing a challenge with a private key, while the server verifies using the corresponding public key stored in authorized_keys.

The OpenSSH daemon that listens for SSH connections, handles authentication, and provides secure remote access to the system; configured via /etc/ssh/sshd_config.

A tool that automates the process of installing, upgrading, configuring, and removing software packages, resolving dependencies and managing repositories.

A centralized storage location from which software packages are retrieved and installed by a package manager; repositories contain metadata and package files.

A software package that another package requires to function correctly; package managers automatically resolve and install dependencies to ensure proper operation.

A dynamic named value that affects the behavior of processes on a Linux system, such as PATH, HOME, or USER, and can be inherited by child processes.

An environment variable that specifies a colon-separated list of directories the shell searches to find executable commands when a user types a command name.

The Bourne Again Shell, a widely used Unix shell and command language that provides scripting capabilities, command-line editing, and job control.

A time-based job scheduler in Unix-like systems that runs scheduled commands or scripts at specified intervals, defined in crontab files.

A file containing a list of commands to be run by cron at specified times, with each line representing a job and its schedule in five time fields.

A systemd unit that triggers another unit based on time, providing functionality similar to cron but with more advanced scheduling and logging capabilities.

A network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules, often implemented with iptables or firewalld.

A user-space utility program that allows a system administrator to configure the IP packet filter rules of the Linux kernel firewall, part of the Netfilter framework.

A dynamic firewall management tool that provides a simplified interface for managing firewall rules, using zones and services to define trust levels and allowed traffic.

Security-Enhanced Linux, a mandatory access control mechanism integrated into the Linux kernel that enforces security policies to confine processes and limit potential damage from compromised services.

A Linux security module that provides mandatory access control by confining programs to a limited set of resources, using profiles to define allowed actions.