Linux is a free and open-source operating system (OS) based on Unix. It is widely used on servers, desktops, and embedded systems. Linux is known for its stability, security, and flexibility, and it supports a variety of software applications.

The main components of Linux include:

• Kernel: The core part of the OS that manages system resources and hardware.

• Shell: A command-line interface that allows users to interact with the kernel.

• File System: Organizes and stores files on storage devices.

• User Interface: Can be either command-line or graphical (GUI).

• System Libraries and Utilities: Provide essential services and tools for the system to function.

The Linux kernel is the core component of the Linux operating system. It manages hardware resources, such as CPU, memory, and devices, and provides a set of system calls that applications use to interact with the hardware. The kernel is responsible for tasks like process management, memory management, and device management.

A Linux distribution (distro) is a packaged version of the Linux operating system that includes the Linux kernel, system libraries, applications, and a package management system. Examples of popular Linux distributions include Ubuntu, Fedora, CentOS, Debian, and Arch Linux.

Software on Linux is typically installed using a package manager that comes with your distribution. Common package managers include apt (Debian/Ubuntu), yum or dnf (Fedora/CentOS), and pacman (Arch Linux). Software can also be installed by compiling source code or using containers like Docker.

The Linux terminal is a command-line interface (CLI) where users can enter commands to perform tasks such as navigating the file system, running programs, and managing processes. The terminal provides powerful tools for interacting with the OS, and it is often preferred by advanced users for its efficiency and flexibility.

Linux file permissions determine who can read, write, or execute a file or directory. Each file has three sets of permissions:

• Owner permissions: Apply to the file's owner.

• Group permissions: Apply to a group of users.

• Others permissions: Apply to all other users. Permissions are represented as a combination of r(read), w(write), and x(execute), or as a numeric mode (e.g., 755).

In Linux, a file is a single entity that contains data, which could be text, images, or executable code. A directory is a special type of file that contains references to other files or directories, effectively organizing the file system in a hierarchical structure.

You can navigate the Linux file system using terminal commands:

• pwd: Prints the current working directory.

• cd: Changes the current directory.

• ls: Lists the contents of a directory.

• cp: Copies files or directories.

• mv: Moves or renames files or directories.

• rm: Removes files or directories.

The root user is the superuser in Linux, with unrestricted access to all commands and files on the system. The root user can perform administrative tasks, such as installing software, managing user accounts, and modifying system configurations. It's important to use the root account with caution, as actions performed as root can significantly impact the system.

• ls: Lists files and directories.

• cd: Changes the directory.

• cp: Copies files or directories.

• mv: Moves or renames files.

• rm: Deletes files or directories.

• chmod: Changes file permissions.

• chown: Changes file ownership.

• ps: Lists running processes.

• kill: Terminates a process.

• grep: Searches for patterns within files.

• find: Searches for files and directories.

A package manager is a tool that automates the process of installing, upgrading, configuring, and removing software packages. It handles dependencies and ensures that all necessary components are installed. Examples include apt for Debian-based distributions, yum and dnf for Red Hat-based distributions, and pacman for Arch Linux.

A shell script is a text file containing a series of commands that are executed by the shell (command-line interpreter). Shell scripts are used to automate tasks, such as system administration, file management, and software installation. They typically have a .sh extension and can be run in the terminal.

You can check system information using various commands:

• uname -a: Displays kernel and OS information.

• df -h: Shows disk usage.

• free -m: Displays memory usage.

• top: Monitors system processes in real-time.

• lscpu: Provides CPU architecture information.

• lsblk: Lists block devices (e.g., drives).

A daemon is a background process that runs continuously and typically provides system or network services. Daemons often start during system boot and run silently in the background, waiting for specific events or requests to trigger them. Common examples include httpd (Apache web server), sshd (SSH server), and cron (task scheduler).

A cron job is a scheduled task in Linux that is executed automatically at specified intervals. It is managed by the cron daemon. Cron jobs are defined in the crontab file and can be used for tasks like running backups, cleaning up files, and updating software.

• Hard link: A direct reference to the data on the disk. Multiple hard links to the same file point to the same inode (data block). Deleting one hard link does not affect the others.

• Soft link (symlink): A pointer to another file or directory. If the original file is deleted, the symlink becomes a broken link and does not contain the original data.

Linux uses commands to manage users and groups:

• useradd: Adds a new user.

• usermod: Modifies a user account.

• userdel: Deletes a user.

• passwd: Changes a user's password.

• groupadd: Adds a new group.

• groupdel: Deletes a group.

• gpasswd: Manages group membership.

The /etc directory contains system-wide configuration files and shell scripts used to boot and initialize the system. Important files in /etc include passwd (user account information), shadow (secure user password information), fstab (file system mount points), and hosts (IP address to hostname mappings).

apt-get is the original command-line tool for handling packages in Debian- based distributions. apt is a newer, more user-friendly command-line tool that combines features from apt-get, apt-cache, and other apt-* tools into a single command. apt provides a more convenient and concise way to manage packages, but both tools ultimately achieve the same results.