Linux Architecture

Linux Architecture

The architecture of a Linux System consists of the following layers

Hardware layer 

Hardware consists of all peripheral devices (RAM/ HDD/ CPU etc)

Kernel

It is the core of Operating System, interacts directly with hardware, It controls computer hardware

Shell

An interface to kernel, Convert users informations to kernel standard format

Types of Shells in Linux

bash - Bourne-Again shell

Bash is a command processor that typically runs in a text window, where the user types commands that cause actions. Bash can also read and execute commands from a file, called a script. Like all Unix shells, When a user presses the tab key within an interactive command-shell, Bash automatically uses command line completion to match partly typed program names, filenames and variable names. The Bash command-line completion system is very flexible and customizable,

sh - Bourne shell

The Bourne shell (sh) is a shell, or command-line interpreter, for computer operating systems, Developed by Stephen Bourne at Bell Labs, It offers command line editing. It is easy to move cursor around and easy to edit text at command line.

csh  -  C shell

C shell is a Unix shell created by Bill Joy while he was a graduate student at University of California, Berkeley in the late 1970s. It was designed to allow users to write shell script programs using a syntax very similar to that of the C programming language.The C shell is a command processor typically run in a text window, allowing the user to type commands. The C shell can also read commands from a file, called a script. 

tcsh

TCSH is a Unix shell based on and compatible with the C shell (csh). It is essentially the C shell with programmable command-line completion, command-line editing, and a few other features. Unlike the other common shells, functions cannot be defined in a tcsh script and the user must use aliases instead (as in csh).

ksh - KornShell

KornShell (ksh) is a Unix shell which was developed by David Korn at Bell Labs in the early 1980s and announced at USENIX on July 14, 1983.  Ksh improves on the Bourne shell by adding floating-point arithmetic, job control, command aliasing and command completion.KornShell is backward-compatible with the Bourne shell and includes many features of the C shell.

Three types of users

Super User - It's username is root. root can access all directories in an os. It's UID is zero (0)

Normal Users - Normal users can access only limited directories. It's UID range is 500 - 60000

System Users - Users created automatically at the time of Operating system installation or at the time of software installation. It will be a non login users. It's UID range is 1 - 499