Understanding Linux: Kernel, Memory, Processes, Filesystems, and More

Linux System Overview

Linux typically consists of four main parts: the kernel, the shell, the file system, and applications. Together they form the core operating‑system structure that lets users run programs, manage files, and interact with the system.

1. Linux Kernel

The kernel is the heart of the OS, handling process, memory, device driver, file, and network management. Its components include memory management, process management, device drivers, file systems, and network management.

System Call Interface (SCI)

The SCI layer provides mechanisms for user‑space to invoke kernel functions, with architecture‑specific implementations found in ./linux/kernel and ./linux/arch.

2. Memory Management

Linux uses virtual memory, dividing memory into 4 KB pages and employing mechanisms such as the slab allocator to manage these pages. When physical memory is exhausted, pages are swapped out to disk. Source code resides in ./linux/mm.

3. Process Management

Processes are execution entities; Linux achieves multitasking by time‑slicing CPU usage. The scheduler selects the next process based on priority. Inter‑process communication (IPC) mechanisms include signals, pipes, shared memory, semaphores, and sockets. Kernel APIs for creating, terminating, and synchronizing processes are provided via SCI.

4. File System

Unlike DOS, Linux organizes file systems into a single hierarchical tree, mounting each file system at a directory. The Virtual File System (VFS) abstracts hardware details, offering a uniform API for operations such as open, close, read, and write. Supported file systems include Ext2/Ext3, FAT, VFAT, NTFS, ISO9660, XFS, JFS, ReiserFS, and many others.

5. Device Drivers

Device drivers run in high‑privilege mode to control hardware. Errors in drivers can crash the system. Drivers provide abstract interfaces to the kernel, with implementations specific to the hardware (e.g., SCSI vs. IDE).

6. Network Stack

Linux supports BSD sockets and the full TCP/IP suite. The network subsystem consists of socket APIs, protocol layers, and device drivers that communicate with hardware.

7. Shell

The shell is the user interface for command execution. Common shells include Bourne Shell, Bash, Korn Shell, and C Shell.

8. Disk Partitions and Directory Structure

Partitions must be mounted to directories. The root directory (/) is the top of the hierarchy, with subdirectories such as /bin, /dev, /etc, /home, /usr, /var, etc., each serving specific purposes.

9. File Types and Commands

Regular files (text, binaries)

Directories

Links (hard and symbolic)

Device files (block and character)

Common commands to inspect files include ls -l, file, and stat.

10. Links

Hard links share the same inode, while symbolic links contain a pathname. Hard links cannot cross file‑system boundaries; symbolic links can.

11. VFS Representation in the Kernel

The kernel uses structures such as file, dentry, inode, and super_block to manage open files, directory entries, metadata, and file‑system specifics. Reference counting ensures proper resource cleanup.

12. Mounting Filesystems

Mounting attaches a file‑system’s root directory to a mount point. The mount command (e.g., mount -t vfat /dev/hda5 /mnt/winc) and the /etc/fstab file automate this process at boot.

13. Useful Commands

fdisk -l

– list partitions mkfs – format a partition mount / umount – mount/unmount ln – create hard or symbolic links