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:

$ bash // opened bash for your own

$ Ps – l: