UNIT 1 Theoretical Concepts of UNIX Operating System
Objectives
After going through this unit you will be able to:
BASIC FEATURES OF UNIX OPERATING SYSTEM
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UNIX System Architecture
As do most computer system, UNIX consists of two separable parts: the kernel and System programs. We can view the UNIX operating system as being layered as shown in figure1
Everything below the system call interface and above the physical hardware is the Kernel.
The Kernel provides the file system, CPU Scheduling, memory management and other operating system functions through system calls.
Programs such as shell(Sh) and editors (vi) shown in the top layer interact with the Kernel by involving a well defined set of system calls. The system call instructs the Kernel to do various operations of the calling programs and exchange data between the Kernel and the program.
System call for UNIX can be roughly grouped into three categories: file manipulation, process control and information manipulation. Another category can be considered for device manipulation, but, since devices in UNIX are treated as (special) files, the same system call support both files and devices.
1.3 FILE STRUCTURE
A file in UNIX is sequence of bytes. Different programs expect various levels of structures, but the kernel does not impose any structure on files and no meaning is attached to its contents. The meaning of the bytes depends solely on the programs that interpret the file.
This is not true if just disc files but of peripheral devices as well, Magnetic tapes, mail messages, character types on the keyboard, line printer output, data flowing in pipes - each of these is just a sequence of bytes as far as the system and the programs in it are concerned.
Files are organized in tree-structured directories. Directories are themselves files that contain information on how to find other files. A path name to file is a text string that identifies a file by specifying a path through the directory structure to the file. Syntactically it contains of individual file name elements separated by the slash character, for example in /usr/hello/data, the first slash indicates the root of the directory tree, called the root directory. The next element, usr, is a subdirectory of the root, hello is a subdirectory of usr and data is a file or directory in the directory hello.
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