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Understanding the Linux Kernel
The Linux kernel is the central program of a Linux system. It sits between your hardware and all other software, including the graphical desktop and command line tools. Applications never talk directly to your physical devices. Instead, they ask the kernel to do it for them. Without the kernel, the rest of the system cannot run.
The kernel as the core of the system
When you power on a Linux machine, the kernel is among the first pieces of software that gets loaded into memory. Once it is running, it stays there as long as the system is on. All user programs, from web browsers to shells, depend on the kernel to create processes, access storage, and communicate over networks.
You can think of the kernel as the operating system core, while the rest of the tools, libraries, and user interfaces form the surrounding environment that makes the system convenient for humans to use.
The kernel is always running while your system is on. If the kernel crashes, the whole system crashes.
What the kernel does
The Linux kernel performs a set of essential tasks that are common to all modern operating systems. These tasks are not optional. If they failed, the system would quickly become unusable or corrupt its data.
Process and memory management
The kernel manages processes. A process is a running program. When you start a program, the kernel creates a process for it, gives it a unique identifier, and decides when and how long it can use the CPU.
Modern computers can run many processes at once. The kernel carefully switches the CPU between processes so that they appear to run in parallel. It also makes sure that one process cannot directly read or overwrite the memory of another process.
The kernel also manages the machine's memory. It tracks which parts of RAM are in use, which are free, and which parts contain code and data that can be moved to disk when necessary. This allows the system to run more programs than would fit in physical memory at one time, by using techniques such as virtual memory and swapping.
Talking to hardware through drivers
The kernel is responsible for controlling hardware devices. This control happens through code called device drivers. Each driver knows how to talk to a specific type of hardware, such as a network card, a graphics adapter, a sound card, or a particular storage controller.
Applications do not embed hardware-specific code. Instead, they use generic system calls to ask the kernel for actions like "send this data over the network" or "write this block of data to disk." The kernel then passes these requests to the appropriate driver, which performs the low level operations the device requires.
Because of this design, you can often change hardware without needing to change applications. As long as there is a suitable driver in the kernel, your programs can continue to work in the same way.
Files as a universal interface
The kernel presents many things as files, not only data on disk, but also some devices and kernel information. This is part of the "everything is a file" idea common in Unix-like systems.
From the kernel's point of view, reading a text document and reading from a serial port can both look like reading bytes from a file. The kernel hides the differences between these operations and provides a unified way for user programs to access them. This design is central to how Linux systems are structured and affects tools and directories that are discussed elsewhere in the course.
System calls: how programs ask the kernel for help
User programs cannot directly perform privileged actions like writing to hardware registers or configuring the network. Instead, they invoke system calls, often shortened to syscalls. A system call is a controlled request from a user program to the kernel.
For example, when a program wants to open a file, it typically uses a C library function such as open(). That function then calls into the kernel using a system call. The kernel checks permissions, accesses the underlying filesystem, and either returns a handle for the opened file or an error.
The same pattern exists for creating new processes, communicating over the network, or asking the kernel for the current time. The kernel is the final authority on what is allowed and how it happens.
All access to hardware and protected resources from user programs must go through the kernel using system calls.
In simple terms, if a program wants to do anything beyond basic calculations in its own memory, it needs the kernel's cooperation.
Kernel space and user space
One of the fundamental design ideas in Linux is the separation between kernel space and user space. These terms refer to different levels of privilege and protection within the system.
Kernel space is where the kernel code and its data structures live. Code running in kernel space has full access to hardware and memory. It is not restricted by the same protections that apply to ordinary programs.
User space is where regular applications run. Programs in user space cannot directly access hardware or arbitrary memory locations. They must use system calls to ask the kernel to act on their behalf.
This separation improves stability and security. A bug in a user space application might crash that application, but it usually does not bring down the whole system. A serious bug in kernel space, by contrast, can cause a system-wide crash or data corruption, because the kernel controls everything.
The Linux kernel as a monolithic kernel
The Linux kernel is usually described as a monolithic kernel. This means that the core functionality and most drivers run within a single large kernel program in kernel space.
In a monolithic design, the scheduler, memory manager, filesystem code, and many device drivers are all part of one kernel image. They can call each other directly and share data structures. This can provide high performance, but it also means that driver code has a high level of privilege.
Linux tries to reduce some of the risks of this design by allowing parts of the kernel to be compiled as separate modules that can be loaded and unloaded at runtime. These are called kernel modules. Although modules still run in kernel space and have full privileges, their modular structure makes it easier to add or remove support for certain hardware or features without rebuilding or rebooting the entire kernel.
The details of kernel modules and how to work with them are discussed in a later part of the course. At this stage, it is enough to understand that the Linux kernel is one large program, with many built-in parts and optional pieces that can be loaded as needed.
Kernel versions and releases
The Linux kernel is under continuous development. New versions are released regularly, with bug fixes, new drivers, performance improvements, and new features. Each version has a version number with multiple components, such as 6.6.12.
A version number can be understood as having the form:
$$
\text{major}.\text{minor}.\text{patch}
$$
For example, in 6.6.12, 6 is the major version, 6 is the minor version, and 12 is the patch level.
Different Linux distributions choose specific kernel versions to include. Some focus on long term support kernels that receive mainly security and stability updates. Others prefer more recent kernels to gain access to the latest hardware support and features.
From the point of view of a beginner user, the exact minor and patch number often matters less than whether the kernel is maintained and supported by the distribution. Later sections of the course on kernel management will go deeper into how to view, update, and customize the kernel on your system.
How the kernel fits into a full Linux system
The Linux kernel by itself does not provide a complete environment for users. It can manage hardware and run processes, but it does not include basic tools like a shell, compilers, or the graphical desktop. These parts come from other projects that surround the kernel.
In a typical Linux distribution, the kernel works together with libraries and user space tools to form a complete operating system. The kernel offers system calls and low level services. Libraries wrap those calls into more convenient functions. User space tools use those functions to implement shells, file utilities, and graphical environments.
This separation has an important consequence. You can have different Linux distributions that feel very distinct on the surface, while all of them depend on the same or similar Linux kernels underneath. Desktop environments, package managers, and default applications may change, but the way the kernel manages processes, memory, and hardware remains similar across systems.
Understanding the kernel as this central, shared component will help you later when you explore distributions, system administration, and performance tuning. Many tools you will use are, in the end, friendly interfaces to kernel functions that are always present at the heart of your Linux system.