• Linux is an open-source, Unix-like operating system.
• It is known for stability, security, flexibility and performance.
• Linux is widely used on servers, desktop computers, embedded systems, supercomputers and mobile devices.
• It supports multitasking, multiple users, networking and a hierarchical file system.

• Linux was started by Linus Torvalds while he was a student at the University of Helsinki.
• It was inspired by Unix-like systems and was developed as a freely available operating system kernel.
• Over time, developers around the world contributed to Linux and many Linux distributions were created.

A Linux system can be understood as a layered structure consisting of hardware, kernel, shell and applications.

The Linux kernel is the core component of the Linux operating system. It manages the hardware and provides services required by programs.

Main Functions of the Kernel

• Process Management: controls processes and CPU scheduling.
• Memory Management: manages RAM and virtual memory.
• Device Management: communicates with devices through device drivers.
• File System Management: manages files, directories and storage.
• System Calls: provides a controlled interface through which applications request operating system services.
• Security: controls access to system resources.

A Linux distribution is a complete operating system built around the Linux kernel together with utilities, libraries, package-management tools and applications.

• Linux uses a hierarchical file-system structure.
• The topmost directory is called the root directory and is represented by /.
• Files, folders and devices are organized under the root directory.
• Linux commonly uses file systems such as ext4, XFS and Btrfs.

Linux controls access to files and directories using permissions.

Permissions are normally defined for three categories:

1. User / Owner
2. Group
3. Others

The chmod command is used to change file permissions.

Linux is a multiuser operating system. Different users can have separate accounts, passwords, files and permissions.

• A running program is called a process.
• Each process has a unique Process ID (PID).
• Linux can run foreground and background processes.
• Process-management commands help users view, monitor and stop processes.

Linux distributions use package managers to install, update and remove software.

• Debian and Ubuntu commonly use apt.
• Fedora commonly uses dnf.
• Red Hat based systems may use dnf or older yum tools.
• Arch Linux uses pacman.

• The shell is a command interpreter and interface between the user and the Linux kernel.
• Common shells include Bash, Zsh and Sh.
• A shell script is a file containing a sequence of commands that can be executed automatically.
• Shell scripts are commonly used to automate repetitive system tasks.

Linux provides strong networking support and is widely used for web servers, database servers, cloud systems and network services.

• User authentication through login accounts and passwords.
• Authorization through file ownership and read, write and execute permissions.
• Encryption for protecting data.
• Regular system updates and package management.
• Firewall and access-control tools can be used to secure network services.

1. Free and open source.
2. Stable and reliable.
3. Secure user and file-permission model.
4. Supports multitasking and multiple users.
5. Highly customizable.
6. Efficient use of hardware resources.
7. Strong networking support.
8. Many distributions are available for different purposes.

• Web and database servers
• Cloud computing
• Supercomputers
• Embedded and IoT systems
• Cybersecurity and penetration-testing environments
• Software-development systems
• Desktop computing
• Android and other Linux-kernel-based devices