UNIX Advanced System Programming-shell

its the Architecture of UNIX operating systems.

( applications ( library routines ( systemcalls (( kernel )) systemcalls ) shell ) applications )

In a broad sense, an operating system is the kernel and all the other software that makes a
computer useful and gives the computer its personality

When we login into a UNIX operation system we enter our login name and our password. The system looks uo our login name and our passwaord, usually the file :

etc/paswd

In the password file our entry is like that: Its have 7 entries like (Example) :

sar:x:205:105:Stephen Rago:/home/sar:/bin/ksh

When we entering the operation system after the login then we see a shell. A shell is a command-line interpreter that reads user input and commands and executes them.

There Common shells used on UNIX systems:

Bourne-shell, C shell, Korn Shell, Tenex C shell, Bourne-again shell

Also Khazar Operation system have own system shell writed by Voctl
(check Khazar if u reading this)(https://github.com/Voctl/Khazar)

Linux uses Bourne-again shell for its default shell. In fact, /bin/sh is a link to /bin/bash.
The default shell in FreeBSD and MacOS X is the TENEX C shell, but they use the Bourne shell for their administrative shell scripts beacuse C shells programming language is notoriusly difficult to use.