Unix notes

UNIX Overview

The UNIX operating system was created in 1969 at Bell Labs, intended to unify system software and resolve issues present in earlier systems like MULTICS.

Kernel

The core program of the operating system that manages system resources and the execution of processes.

Multi-user

An ability of an operating system that allows multiple users to operate on it simultaneously, with individual authenticated sessions.

Process

An instance of a task being executed, which operates in a private address space.

Shell

An interactive interface that allows users to communicate with the operating system, interpreting user commands.

GNU

Refers to the Free Software Foundation's project aiming to develop a free UNIX-like operating system and software.

Linux

An operating system derived from UNIX, providing similar functionalities with an emphasis on openness and flexibility.

Boot-Strapping

The process that a computer undergoes to prepare itself for operation and provide a user login prompt.

Environment Variable

Variables that are set in a shell to define certain aspects of the user's session, such as HOME or PATH.

File Hierarchy Standard

A standard that defines the directory structure for UNIX-like systems, allowing for a consistent organization of files.

Access-Control Level (ACL)

A system that specifies permissions for users and groups, determining who can access or modify resources in UNIX.

X-Windows System

A graphical user interface system for UNIX, allowing for the display of graphical applications.

Pipe

A mechanism in a shell that allows the output of one command to be used as the input to another command.

Redirection

The process of changing the standard input/output of commands in the shell, directing them to files or other commands.

Free Software

Software that can be freely used, modified, and distributed under certain conditions, often associated with the GNU project.

User Mode vs Kernel Mode

User mode provides a limited execution context for applications, while kernel mode has full access to system resources and capabilities.

File System

The method and structure used by the operating system to manage files on storage devices.

Virtual Memory

A memory management capability that allows a computer to use more memory than is physically available.

Daemon

A background process that runs independently of user interaction, often handling system or application services.

Core Dump

A captured memory state of a program at the time of an error which can be analyzed to determine the cause of the failure.

Segmentation Fault

An error that occurs when a program attempts to access a memory segment that it is not allowed to, causing a crash.

System Call

An interface for user applications to request services from the operating system's kernel.

Runlevel

A state of init in a UNIX/Linux operating system that determines which services are operating.

Script

A file containing a series of shell commands that can be executed as a program.

Common Shell Command

Command-line instructions like ls, pwd, or cp that are used to interact with the file system or other components.

Executable File

A file that is capable of being executed or run as a program on a computer.

Shell Scripting

A way to automate tasks using a script written for the shell, which contains a series of commands.

System Administrator

A person responsible for managing and maintaining computer systems and networks.

Root User

The superuser account in UNIX/Linux systems with full administrative access to all files and commands.

Mounting

The process of making a file system accessible at a certain point in the directory tree.

Virtual File System (VFS)

An abstraction layer in UNIX that allows programs to access different file systems in a standard way.

Kernel Panic

An action taken by an operating system upon detecting an internal fatal error that it cannot safely recover from.

Inter-Process Communication (IPC)

A mechanism that allows processes to communicate and synchronize their actions.

Repository

A centralized place where software packages are stored and can be retrieved, commonly used in Linux distributions.

Hardware Abstraction Layer (HAL)

A layer of programming that allows a computer's operating system to interact with hardware devices.

What is a Virtual Machine (VM)?

A software emulation of hardware that allows multiple operating systems to run on a single physical machine.

What does CLI stand for?

Command Line Interface, a way to interact with a computer program by typing commands.

What is a Package Manager?

A tool that automates the installation, upgrade, configuration, and removal of software packages.

What is SSH?

Secure Shell, a protocol for securely accessing and managing networked devices over an unsecured network.

What is a Firewall?

A network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules.

What is a File Descriptor?

An abstract indicator used to access a file or other input/output resource, such as a socket.

What is a Cron Job?

A time-based job scheduler in UNIX-like operating systems that allows users to run scripts or commands at specified intervals.

What is a Symbolic Link?

A type of file that is a reference to another file or directory, used for creating shortcuts.

What is an Environment?

A set of variables set in a shell that influence the behavior of processes, affecting how they run.

What is SELinux?

Security-Enhanced Linux, a Linux kernel security module that provides a mechanism for supporting access control security policies.