ARCHITECTURE OF UNIX SYSTEM

 1.SHELL:

THE SHELL READS YOUR COMMANDS AND INTERPRETS THEM AS REQUESTS TO EXECYE A PROGRAM OR PROGRAMS,WHICH IT THEN ARRANGES TO HAVE CARRIED OUT.BECAUSE THE SHELL PLAYS THIS ROLE,IT IS CALLED A COMMAND INTERPRETER.BESIDES BEING A COMMAND INTERPRETER,THE SHELL IS ALSO A PROGRAMMING LANGUAGE.AS A PROGRAMMING LANGUAGE,IT PERMITS YOU TO CONTROL HOW AND WHEN COMMANDS ARE CARRIED OUT.SHELL ACTS AS AN INTERFACE BETWEEN USER AND THE KERNEL.

2.KERNEL:

THE KERNEL IS THE PART OF THE OPERATING SYSTEM THAT INTERACTS DIRECTLY WITH THE HARDWARE OF A COMPUTER,THROUGH DEVICE DRIVERS THAT ARE BUILT INTO THE KERNEL.IT PROVIDES SET OF SERVICES THAT CAN BE USED BY PROGRAMS,INSULATING THESE PROGRAMS FROM THE UNDERLYING HARDWARE.THE MAJOR FUNCTIONS OF THE KERNEL ARE TO MANAGE COMPUTER MEMORY,TO CONTROL ACCESS TO THE COMPUTER,TO MAINTAIN FILE SYSTEM,TO COMPUTER MEMORY,TO CONTROL ACCESS TO THE COMPUTER,TO MAINTAIN FILE SYSTEM,TO HANDLE INTERRUPTS(SIGNALS TO TERMINATE EXECUTION),TO HANDLE ERRORS,TO PERFORM INPUT AND OUTPUT SERVICES(WHICH ALLOW COMPUTER TO INTERACT WITH TERMINALS,STORAGE DEVICES,AND PRINTERS),AND TO ALLOCATE THE RESOURCES OF THE COMPUTER(SUCH AS THE CPU OR INPUT/OUTPUT DEVICES)AMONG USERS.PROGRAMS INTERACT WITH THE KERNAL THROUGH APPROXIMATELY 100 SYSTEM CALLS.SYSTEM CALLS TELL THE KERNEL TO CARRY OUT VARIOUS TASKS FOR THE PROGRAM,SUCH AS OPENING A FILE,WRITING TO A FILE,OBTAINING INFORMATION ABOUT A FILE ,EXECUTING A PROGRAM,TERMINATING A PROCESS,CHANGING THE PRIORITY OF A PROCESS,AND GETTING THE TIME OF DAY.

THE UNIX FILE SYSTEM:

A FILE IS THE BASIC STRUCTURE USED TO STORE INFORMATION ON THE UNIX SYSTEM.TECHNICALLY, A FILE IS A SEQUENCE OF BYTES THAT IS STORED SOMEWHERE ON A STORAGE DEVICE,SUCH AS A DISK.THE UNIX FILE SYSTEM PROVIDES A LOGICAL METHOD FOR ORGANIZING,STORING,RETRIEVING,MANIPULATING,AND INFORMATION.A FILE CAN STORE MANUSCRIPTS AND OTHER WORD PROCESSING DOCUMENTS,INSTRUCTIONS OR PROGRAMS FOR THE COMPUTER ITSELF,AN ORGANIZED DATABASE OF BUSSINESS INFORMATION,A BITMAP DESCRIPTION OF A SCREEN IMAGE ,OR ANY OTHER KIND OF INFORMATION STORED AS SEQUENCE OF BYTES ON A COMPUTER.FILES ARE ORGANIZED INTO A HIERSRCHICAL FILE SYSTEM,WITH FILES GROUPED TOGETHER INTO DIRECTORIES.WITHIN UNIX THERE ARE THREE DIFFERENT TYPES OF FILES.

1.REGULAR FILES

2.DIRETORY FILES

3.SPECIAL FILES

3.ORDINARY FILES:

AS A USER ,THE INFORMATION THAT YOU WORK WITH WILL BE STORED AS AN ORDINARY FILE.ORDINARY FILES ARE AGATES OF CHARACTES THAT ARE TREATED AS AUNIT BY THE UNIX SYSTEM.AN ORDINARY FILE CAN BE CREATED ,CHANGED,OR DELETED AS YOU WISH.

4.DIRECTORY FILES:

DIRECTORY IS A FILE THAT HOLDS OTHER FILES AND CONTAIN INFORMATION ABOUT THE LOCATION AND ATTRIBUTES OF THESE OTHER FILES.FOR EXAMPLE, A DIRECTORY INCLUDES A LIST OF ALL THE FILES AND SUBDIRECTORIES THAT IT CONTAINS,AS WELL AS THEIR ADDRESSES,CHARATERISTICS,FILE TYPES(WHETER THEY ARE ORDINARY FILES,SYMBOLIC LINKS,DIRECTORIES,OR A SPECIAL FILES),AND OTHER ATTRIBUTES.

5.SPECIAL FILES:

SPECIAL FILES CONSTITUTE AN UNUSUAL FEATURE OF THE UNIX FILE SYSTEM.A SPECIAL FILE REPRESENTS A PHYSICAL DEVICE.IT MAY BE A TERMINAL,A COMMUNICATIONS DEVICE,OR A STORAGE UNIT SUCH AS DISK DRIVE.FROM THE USER'S PERSPECTIVE,THE UNIX SYSTEM TREATS SPECIAL FILE JUST AS IT DOES ORDINARY FILES;THAT IS,YOU CAN READ OR WRITE TO DEVICES EXACTLY THE WAY YOU READ AND WRITE TO ORDINARY FILES.YOU CAN TAKE THE CHARATERS TYPED AT YOUR KEYBOARD AND WRITE THEM TO AN ORDINARY FILE OR A TERMINAL SCREEN IN THE SAME WAY.THE UNIX SYSTEM TAKES THESE READ AND WRITE COMMANDS AND CAUSES THEM TO ACTIVATE THE HARDWARE CONNECTED TO THE DEVICE.