Unix System Architecture And Its Explanation

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 3^rd
• Fourth layer is the one with which the user actually interacts.

  Kernel

• 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:

PID	TTY	TIME	CMD
2561	Pts/0	00:00:00	Bash
2752	Pts/0	00:00:00	PS

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

F	S	UID	PID	PPIS	0	PRI	NT	ADDR	S7	WCHAN	TTY	TIME	CMD
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