Table of Contents
What the Linux Kernel Is
The Linux kernel is the core program of a Linux system. It is:
- A single, always-running program that starts early in the boot process.
- Responsible for talking to hardware.
- Responsible for managing memory, processes, and permissions.
- Sitting between applications (your programs) and the hardware (CPU, RAM, disks, network cards, etc.).
Applications almost never talk directly to hardware. Instead, they make system calls (through libraries like glibc), and the kernel does the “dangerous” low-level work safely.
In short: Linux = Kernel, while a Linux distribution = kernel + tools + libraries + applications.
Main Responsibilities of the Linux Kernel
1. Process and Task Management
The kernel manages all running programs, called processes.
Key tasks:
- Creating and destroying processes
When you run a program, the kernel sets up a new process for it. - Scheduling
Multiple processes appear to run “at the same time” even on a single CPU.
The kernel’s scheduler decides: - Which process runs next.
- For how long.
- On which CPU core (on multi-core systems).
- Multitasking
Linux is a preemptive multitasking system: - The kernel can interrupt a running process.
- It can switch to another process to keep things responsive.
- Process isolation
Processes are kept separate: - One process generally cannot read or write another’s memory.
- This protects stability and security.
2. Memory Management
The kernel manages all system RAM and swap.
Key ideas:
- Virtual memory
Each process sees its own virtual address space, as if it had the whole memory to itself.
The kernel maps these virtual addresses to real physical memory. - Paging and swapping
If RAM is full, the kernel may move rarely used memory pages to a swap area on disk, freeing RAM for active tasks. - Protection
The kernel prevents processes from: - Reading memory that doesn’t belong to them.
- Writing to read-only areas (like some code sections).
This makes the system more stable and secure.
3. Filesystems and Storage
The kernel handles reading and writing data to storage devices like HDDs, SSDs, and USB drives.
Key roles:
- Unified filesystem tree
Linux presents storage as a single directory tree starting at/.
Different disks or partitions are mounted into this tree. - Filesystem drivers
The kernel supports many filesystems (e.g., EXT4, XFS, Btrfs, FAT, NTFS via modules).
Each has a driver in the kernel that knows how to: - Read and write files.
- Organize directories.
- Track permissions and metadata.
- Caching and buffering
The kernel keeps frequently accessed data in memory caches to speed up file operations.
4. Device Management and Drivers
The kernel talks to hardware using device drivers.
Examples of devices:
- Storage: SSD, HDD, USB drives
- Network: Ethernet cards, Wi-Fi adapters
- Input: keyboard, mouse, touchpad
- Display: GPUs, framebuffers
- Others: sound cards, webcams, sensors
Key concepts:
- Drivers in the kernel
Each device type usually has a specific driver, built into or loaded by the kernel. - Device files
In Linux, many devices are represented as special files under/dev(for example/dev/sdafor disks,/dev/null, etc.).
Programs read/write these device files; the kernel routes this to the hardware. - Abstraction
Applications don’t need to know how a Wi-Fi card or SSD works internally; they just use standard APIs, and the kernel (with the driver) does the low-level work.
5. Networking
The kernel includes a powerful network stack.
Responsibilities:
- Handling network protocols:
- IP, TCP, UDP, ICMP, and many others.
- Managing:
- Network interfaces (Ethernet, Wi-Fi, virtual interfaces).
- Routing (choosing where packets go).
- Firewalls and packet filtering (using systems like
iptables/nftables).
When your browser accesses a website:
- It asks the kernel to open a network connection.
- The kernel prepares packets, sends them through the network card.
- When data is received, the kernel delivers it back to the browser.
The heavy lifting is done inside the kernel.
6. Security and Permissions
The kernel enforces many basic security mechanisms:
- User and group IDs
Every process runs as some user and group (e.g.,root,alice,www-data). - File permissions
The kernel checks read/write/execute (r/w/x) rights whenever a process tries to access a file. - System calls control
The kernel controls what operations are allowed: - Bind to restricted network ports
- Load kernel modules
- Access raw devices
- Change system time, and more
Advanced mechanisms (like SELinux and AppArmor) build extra policies on top of this, but the kernel is always at the core of enforcement.
Monolithic Kernel and Modules
Linux is a monolithic kernel:
- Most core services (scheduling, memory, filesystem, drivers) run in kernel space.
- User applications run in user space.
Within this design, Linux supports loadable kernel modules:
- Drivers and some features can be compiled as modules.
- Modules can be:
- Loaded at runtime:
modprobe <module> - Unloaded when no longer needed:
rmmod <module>
Benefits:
- You don’t need everything built into the kernel at all times.
- You can add support for new hardware without rebooting (in many cases).
- Distributions can ship a generic kernel plus many optional modules.
Kernel Space vs User Space
Linux logically splits the system into:
- Kernel space
- Where the kernel runs.
- Has full access to hardware and memory.
- Crashes here can bring down the whole system.
- User space
- Where normal programs and most system services run.
- Restricted access; must ask the kernel for privileged operations using system calls.
This separation:
- Protects the kernel from buggy user programs.
- Protects user programs from each other.
- Helps overall system stability.
How Programs Talk to the Kernel
Programs interact with the kernel using system calls (syscalls).
Examples of common syscalls:
open()– open a fileread()/write()– read/write datafork()/execve()– create new processes / start a new programsocket()– create a network communication endpointchmod()/chown()– change permissions and ownership
Typically:
- Your program calls a library function (e.g.,
printf,fopen). - The library eventually issues one or more syscalls.
- The CPU switches from user space to kernel space to handle the syscalls.
- The kernel does the requested work and returns.
From a user’s perspective, you almost never call syscalls directly when using the command line or GUI applications—they are hidden behind higher-level tools.
Kernel Versions and Releases
The Linux kernel is developed in the open, with frequent releases.
Basic version format:
X.Y.ZX– major versionY– minor versionZ– patch level (bug fixes, minor changes)
Distributions usually:
- Choose a particular kernel version (or family of versions).
- Backport important security and stability fixes.
- Sometimes provide multiple kernel packages (e.g., “generic”, “low-latency”, or “LTS”).
You can check your running kernel version with:
uname -rWhy the Kernel Matters for You
Even as a beginner, the kernel affects you in several ways:
- Hardware support
Whether your Wi-Fi, graphics card, or sound works often depends on kernel drivers. - Performance
The scheduler, filesystem drivers, and network stack influence how fast things feel. - Stability and security
Kernel bugs can cause crashes or security issues; updates often fix these. - Advanced features
Concepts like containers, cgroups, namespaces, and advanced filesystems all rely heavily on kernel capabilities (even if you don’t use them yet).
Summary
- The Linux kernel is the central part of a Linux system that:
- Manages processes and memory.
- Handles filesystems and devices.
- Provides networking.
- Enforces security rules.
- It runs in kernel space, separate from user space where your applications run.
- It uses drivers, often as loadable modules, to support hardware.
- Programs use system calls to request services from the kernel.
- Kernel version and configuration influence hardware support, performance, and security.
Understanding at a high level what the kernel does helps you make sense of many Linux behaviors you’ll see as you go deeper into the system.