Overview:Unix System Organization

The function of Unix is manned in three levels. On the outer crust reside the application programs and the other utilities, which speak our language. At the heart of Unix on the other hand is the kernel which interacts with actual hardware in machine language. The streamlining of these two modes of communication is done by the middle layer called shell.

The shell or the command line interpreter as it is called is the mediator, which interprets the commands that we give and they conveys them to the kernel which ultimately executes them. You can imagine kernel as monarch who is in overall control of everything, whereas the shell as its emissary.

The kernel has various functions. It manages files, carries out all the data transfer between the file system and the hardware, and also manages memory. The onus of scheduling of various programs running in memory or allocation of CPU time to all running programs lies with the kernel. It also handles any interrupts issued, as it is the entity that has direct dealing with the hardware.

The kernel program is usually stored in the file called Unix whereas the shell program is in a file called sh . For each user working with Unix at any time different shell programs are running. Thus at a particular point in time there may be several shells running in memory but only one kernel This is because at any instance Unix is capable of executing one program as the other program wait for their turn. And  since it is the kernel which executes the program one kernel is sufficient . However different terminal are trying to seek kernel’s attention. And since the user interacts with the kernel through the shell different shells are necessary.

Types Of Shells:

Different people implemented the interpreter function of the shell in different ways.This gave rise to various type of shells , the most prominent of which are outlined below,

Bourne Shell

Steve Bourne

Among all Steve Bourne’s creation know after him the Bourne shell is the most popular. Probably that’s why it is bundled with every Unix system. Or perhaps it is the other way around Because it was bundled with every system it became popular. Whatever the cause and the effect, the fact remains that this is the shell used by many Unix users. This will also be the shell we shall use by many Unix users.

C Shell

Bill Joy

This shell is a hit with those who are seriously into Unix programming. It was created by Bill Joy then pursuing his graduation at the University of California at Berkeley.

It has two advantage over the Bourne shell,

1) It allows aliasing of commands. That is you can decide what name you want to call a command by. This proves very useful when lengthy commands which are used time and again are renamed by you. Instead of typing the entire command you can simply use the short alias at the command line.

2) If you want to save even more typing work , C shell has a command history feature . This is the second benefit that comes with C shell. Previously typed commands can be recalled, since the C shell keeps track of all commands issued at the command line. This feature is similar to the one provided by the program DOSKEY in MS-DOS environment.

Korn Shell

If there was any doubt about the cause-effect relationship of the population of Bourne Shell and its inclusion in every package, this adds fuel to it. The not so widely used Korn Shell is very powerful and is a superset of Bourne Shell. It offers a lot more capabilities and is decidedly Korn of AT & T’s Bell Labs.