LTF12 key terms

The central part of an operating system that stays in memory, schedules processes, allocates memory, and talks to hardware through drivers on behalf of every running program.

A program that reads commands typed by a user or supplied in a script, interprets them, and asks the kernel to run the requested programs, returning output and an exit status.

A documented convention that defines which directories hold configuration, binaries, logs, and user data, so software can find files at predictable paths on any compliant system.

The top of the directory tree, written as a single slash, from which every other directory and file on a mounted filesystem descends; it is not the root user's home directory.

An account with unrestricted privileges that can read or modify any file, change ownership, and alter system configuration; routine work should avoid it because mistakes are not blocked.

A command that lets a permitted user run a single program with another user's privileges, usually root's, after authenticating, while recording who ran what.

Three sets of read, write, and execute flags that control access for a file's owner, its group, and everyone else, as displayed by tools such as ls -l.

A per-process mask that removes selected permission bits from newly created files and directories, determining the default access rights a user's new objects receive.

A filesystem record holding a file's metadata, including owner, permissions, size, timestamps, and data block locations, but not its name, which lives in a directory entry.

A directory entry that gives an additional name to an existing inode, so two paths refer to the same file data and the data survives until the last name is removed.

A separate file whose contents are a path pointing to another file or directory; it can cross filesystems and stops working if the target is removed.

A directory where an additional filesystem is attached to the main directory tree, making that filesystem's contents appear beneath the chosen path for as long as it stays mounted.

A running instance of a program with its own memory space, identifier, credentials, and state, which the scheduler switches on and off the processor.

A positive integer the kernel assigns to each running process so that users and programs can signal, monitor, or trace that specific process by number.

A short asynchronous notice the kernel or a user delivers to a process, such as a request to terminate or reload, which interrupts it and triggers default or handled behavior.

A background process that runs without a controlling terminal, usually started at boot to provide a long-lived service such as logging, scheduling, or web serving.

A configuration file describing something the init system manages, such as a service, socket, timer, or mount, including how and when it should start and stop.

A named state that groups units to define which services are active after startup, replacing older numbered runlevels; common examples are multi-user and graphical targets.

A tool that installs, upgrades, verifies, and removes software from trusted repositories while tracking dependencies so that libraries a program needs are present.

A server or directory holding packaged software plus an index of its contents, which package managers query to find, download, and verify available versions.

A package or library that another piece of software requires in order to run correctly; package managers resolve these relationships automatically during installation or removal.

A scheduling service that runs commands at times described in entries of five time fields plus a command, commonly used for recurring maintenance and batch jobs.

A named value held by a process and inherited by the programs it starts, commonly used to pass configuration such as search paths and locale settings.

An environment variable listing directories, separated by colons, that the shell searches in order when resolving the name of a command the user wants to execute.

The three default data channels every process inherits: input for reading, output for normal results, and error for diagnostics, each redirectable to files or other programs.

A shell operator that connects the standard output of one command to the standard input of the next, letting small tools be combined into a processing chain.

Shell syntax that changes where a command's input or output streams go, writing to a file, appending instead of overwriting, or discarding output entirely.

A small integer a finished process returns to its parent, where zero conventionally means success and nonzero values indicate a particular kind of failure.

A numeric label assigned to a network interface so packets can be routed to and from it; version four uses 32 bits and version six uses 128.

A notation stating how many leading bits of an address identify the network, so a host can tell which destinations are local and which must go to a router.

The router address a host sends packets to when the destination is not on any directly connected network, allowing traffic to leave the local subnet.

A hierarchical naming system that translates human-readable hostnames into IP addresses by querying resolvers and caching answers, so users need not remember numeric addresses.

A cryptographic protocol and tool suite for logging into and running commands on a remote machine, providing encrypted authentication and optional tunnelling of other traffic.

A set of rules that inspects network traffic and permits or blocks packets based on addresses, ports, and protocol, applied by the kernel or a dedicated appliance.

A software-defined computer, including virtual processor, memory, and disk, that a hypervisor runs as a process on a physical host, isolated from other guests.

An isolated group of processes on a single kernel, packaged with its own filesystem view and resource limits, sharing the host kernel rather than emulating hardware.

A file where programs and services append timestamped records of events, used to diagnose failures; on modern systems these are often collected by a central logging daemon.

A text file that sets a program's options at startup, usually kept under the system configuration directory, so behavior can change without recompiling or editing the program itself.

Small software started by firmware that locates a kernel, loads it into memory with the needed parameters, and transfers control to it, sometimes after offering a menu.