Types Of Unix :
There are many different versions of UNIX, although they share common similarities. The most popular varieties of UNIX are Sun Solaris, GNU/Linux, and MacOS X.
Unix Operating System :
Unix is a common operating system(suite of programs which make the computer work) made up of three parts :
1. Kernel :
If we think of Unix operating system in terms of layers, the Kernel is the lowest layer. The Kernel of UNIX is the hub of operating system.It interfaces directly with the computer hardware and is responsible for allocating and managing the resources available to programs. It allocates processor time and memory to each program and determines when each program will run.The kernel performs many low-level and system-level functions.
2. Shell :
The shell acts as an interface between the user and the kernel.The instructions to the kernel are complex and highly technical. To protect the kernel from the short comings of user, a shell is built around the kernel. Whenever a user run a command, the shell interprets the command and passes the command to the kernel.
Three types of shell are standard in Unix –
Bourne shell: Most widely used shell and is a program name with sh. It prompts with $ symbol.
Korn shell: Called by the name ksh.
C shell: It is called by csh.
3. Programs :
The commands are themselves program: when they terminate, the shell gives the user another prompt(~> on our systems).One of the main features of UNIX is that it includes a variety of small programs to meet various needs. Typically, each of these programs does one thing and does it well. This modular design allows the functionality of small programs to be mixed and matched. Typically these programs operate on top of the shell, but they may also interface directly with the kernel.
Filename Completion :
By typing part of the name of a command, filename or directory and pressing the [Tab] key, the tcsh shell will complete the rest of the name automatically. If the shell finds more than one name beginning with those letters you have typed, it will beep, prompting you to type a few more letters before pressing the tab key again.
The shell keeps a list of the commands you have typed in. If you need to repeat a command, use the cursor keys to scroll up and down the list or type history for a list of previous commands.