Understanding Linux: Kernel, Shell, Filesystem and Core Components Explained

1. Linux Kernel

The kernel is the core of the operating system, handling process, memory, device driver, file and network management, which directly impacts system performance and stability.

1.1 Memory Management

Linux uses virtual memory to map physical memory into 4 KB pages, providing mechanisms such as the slab allocator and page swapping. Source code resides in ./linux/mm.

1.2 Process Management

Processes are scheduled using time slices; the scheduler selects the next runnable process based on priority. Linux supports inter‑process communication via signals, pipes, shared memory, semaphores and sockets. System calls like fork, exec, kill and exit are exposed through the SCI layer.

1.3 Filesystem

Linux adopts a unified Virtual File System (VFS) that abstracts diverse filesystem implementations (ext2, FAT, NTFS, etc.) behind a common API. VFS sits above the buffer cache and device drivers, allowing user processes to access files without knowing the underlying filesystem type.

1.4 Device Drivers

Device drivers run in high‑privilege mode and provide abstract interfaces for hardware. Errors in drivers can crash the whole system.

1.5 Network Stack

The network subsystem implements BSD sockets and the full TCP/IP protocol suite, with architecture‑specific code under ./linux/arch.

2. Linux Shell

The shell is the user interface that interprets commands and passes them to the kernel. Common shells include:

Bourne Shell (sh)

BASH – GNU Bourne Again Shell (default on most distributions)

Korn Shell (ksh)

C Shell (csh)

3. Linux Filesystem

3.1 File Types

Regular files (text, binaries, scripts)

Directory files

Link files (hard and symbolic)

Device files (block and character devices under /dev)

FIFO (named pipe) files

Socket files

3.2 Directory Structure

Linux uses a single hierarchical tree starting at /. Key directories include /bin, /sbin, /etc, /home, /usr, /var, /tmp, /proc, /dev, and /mnt. Each serves a specific purpose such as binaries, system configuration, user data, temporary files, virtual process information, device nodes and mount points.

3.3 Disk Partitions

Partitions are identified as /dev/hd[a‑z][X] or /dev/sd[a‑z][X]. Types include Primary, Extended and Logical partitions. Example output of fdisk -l shows partition tables and sizes.

3.4 Filesystem Types and Features

Common Linux filesystems: ext2, ext3 (journaled), ext4, XFS, JFS, ReiserFS, Btrfs, etc. Each provides a superblock, inode table and data blocks. Inodes store metadata (permissions, timestamps, ownership) while data blocks hold file contents. The superblock records overall filesystem statistics.

3.5 VFS Representation in the Kernel

Key kernel structures: file – represents an opened file, holds flags and file offset. file_operations – function pointers for read, write, ioctl, etc. dentry – cached directory entry, maps paths to inodes. inode – stores metadata and points to inode_operations. super_block – contains filesystem‑wide information and the root dentry.

3.6 Mounting Filesystems

Mounting attaches a filesystem's root directory to a mount point in the existing tree. The basic syntax is: mount [-t fstype] [-o options] device mount_point Common options include ro, rw, user, nouser, codepage=, and iocharset=. Example to mount a Windows FAT partition: # mount -t vfat /dev/hda5 /mnt/winc Automatic mounting at boot is configured in /etc/fstab, where each line specifies device, mount point, filesystem type and options.

3.7 Links

Hard links share the same inode; they cannot cross filesystem boundaries. Symbolic (soft) links are special files that store a pathname to the target and can span filesystems. Commands:

ln source_file hard_link
ln -s source_file soft_link

3.8 Essential File‑Management Commands

Disk/space: fdisk, df, du Navigation: cd, pwd, mkdir, rmdir File operations: ls, cp, mv, rm Viewing content: cat, more, less, head, tail Permissions: chmod, chown, chgrp, umask Search: which, whereis, locate,

find

4. Linux Applications

Typical distributions ship with text editors, programming language runtimes, X Window system, office suites, internet tools and databases.

5. Kernel Parameter Tuning

Kernel parameters are exposed via the /proc filesystem. Adjusting values in /proc/sys (or via sysctl) can improve performance without recompiling the kernel.