Unix System Architecture And Its Explanation

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Layered Architecture


Unix System  Architecture

  • Unix system follows a layered approach . It has 4 Layers
  • The inner most layer is the hardware layer.
  • In the second layer , the KERNEL is placed.
  • The utilities and other application programs form 3rd
  • Fourth layer is the one with which the user actually interacts.


  • Kernel is that part of OS which directly makes interface with the hardware system.

  Actions :

  • Provides mechanism for creating and deleting processes.
  • Provides processor scheduling, memory & I/0 management
  • Provides inter – process communication

  The Shell

  • A utility program that comes with the UNIX system

  Features of shell are  :

  • Interactive processing
  • Back ground processing
  • I/ O Redirection
  • Pipes
  • Shell Scripts
  • Shell Variables
  • Programming constructs

  Process Management

  • A process is a program in execution
  • Several processes can be executed simultaneously a UNIX system.
  • A process is generally created using the “fork ()”.system call
  • The process that invokes the “fork ()” system call is the parent process, and the newly created process is called the child process

  CPU Scheduling

  • UNIX uses round – ration scheduling to support its multi-user and time – sharing feature.
  • Round – robin fashion of scheduling is considered to be the oldest, simplest and widely used algorithm.
  • Every process is given a time slice.

       Memory Management  

  • Virtual memory
  • Swap area
  • Demand paging

  File Management

  • UNIX uses a hierarchical file system with ‘1’ as its root
  • Every non – leaf node of the tree is called as directory file
  • Every leaf node can either be a file, or an empty directory.

  Types of UNIX users

  • Broad classification of users

-  root - non-root

  • Group

-  Unix allows user ID’s to be grouped -  A single user ID can be member of multiple groups

  • Differentiating users with respect to file access

-OWNER -Group -others     Working with UNIX

  • User logs in with a valid user ID.
  • User logs out to terminate the login session.

  exec() Excel, Excel p, Excel e, Excel v, Excel vp – executes a file   Synopsis # include <Unistd. h> Extern char ** en viron; Int excel  (const char  * path , const char * arg,……………); Int excel p (const char  * file , const char * arg,……………); Int excel   e  (const char  * path , const char * avg,…, char * const envp [] ) ;   Return Value If any of the exec () functions returns , an error will have occurred. The return value is -1, and the global variable err no will be set to indicate the error.   Files /bin/sh   System () System  - executes a shell command   Synopsis # include <stdlib.h> Int system (const char  * command;)   Description System () executes a command specified  in command by calling /bin/sh – c command, and returns  after the command   has been completed. Using execution of the command , SIGCHLD will  be blocked and SIGINT and SIGQUIT will be ignored.   Return Value ‘- 1’ on error If  the value of command is NULL, sys n () returns non – Zero  if the shell is available and if not system () does not effect wait status of at other children .   PS: $ PS:

2561 Pts/0 00:00:00 Bash
2752 Pts/0 00:00:00 PS

  $ bash // opened bash for your own   $ Ps – l:

0 S 6524 2561 2559 0 75 0 - 1177 wait Pts/0 00:00:00 Bash
0 S 6524 2767 2561 0 76 0 - 1176 Wait Pts/0 00:00:00 Bash
0 S 6524 2785 2767 0 76 0 - 1084 - Pts/0 00:00:00 PS


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