Unit-5: Operating system lecture notes (Case study)

Unit-5: Operating system lecture notes (Case study) in Operating System uploaded at P.R.Engineering college

Unit-5: Operating system lecture notes (Case study) in Operating System at P.R.Engineering college uploaded by P.R.Engineering college Staff in lecturenotes.net

Summary / document preview text:

UNIT V
CASE STUDY
LINUX interfaces and Shell.
Interfaces to Linux
A Linux system can be regarded as a kind of pyramid, as illustrated in Fig. 
10-1. at the bottom is the hardware, consisting of the CPU, memory, disks, a monitor and 
keyboard, and other devices. Running on the bare hardware is the operating system. Its 
function is to control the hardware and provide a system call interface to all the 
programs. These system calls allow user programs to create and manage processes, 
files, and other resources.
Programs make system calls by putting thearguments in registersand issuing trap instructions 
to switch from user mode to kernel mode. Since there is no way to write a trap 
instruction in C, a library is provided, with one procedure per system call. These 
procedures are written in assembly language, but can be called from C. Each one first puts 
its arguments in the proper place, then executes the trap instruction. Thus to execute 
the read system call, a C program can call the read library procedure. As an aside, it is 
the library interface, and not the system call interface, that is specified by POSIX. In 
other words, POSIX tells which library procedures a conformant system must supply, 
what their parameters are, what they must do, and what results they must return. It 
does not even mention the actual system calls. UNIT V
Operating Systems Page 2
In addition to the operating system and system call library, all versions of Linux 
supply a large number of standard programs, some of which are specified by the 
POSIX 1003.2 standard, and some of which differ between Linux versions. 
These include the command processor (shell), compilers, editors, text processing 
programs, and file manipulation utilities. It is these programs that a user at the 
keyboard invokes. Thus we can speak of three different interfaces to Linux: the true 
system call interface, the library interface, and the interface formed by the set of 
standard utility programs. 
Most personal computer distributions of Linux have replaced this keyboard oriented user 
interface with a mouse-oriented graphical user interface, without changing the operating 
system itself at all. It is precisely this flexibility that makes Linux so popular and has 
allowed it to survive numerous changes in the underlying technology so well. 
The GUI creates a desktop environment, a familiar metaphor with windows, icons, 
folders, toolbars, and drag-and-drop capabilities. A full desktop environment contains a 
window manager, which controls the placement and appearance of windows, as well 
as various applications, and provides a consistent graphical interface. GUis on Linux 
are supported by the X Windowing System, or commonly Xll or just X, which defines 
communication and display protocols for manipulating windows on bitmap displays for 
UNIX and UNIX-like systems. The X server is the main component which controls 
devices such as keyboards, mouse, screen and is responsible for redirecting input to or 
accepting output from client programs. The actual GUI environment is typically built 
on top of a low-level library, xlib, which contains the functionality to interact with the X 
server. The graphical interface extends the basic functionality of Xll by emitting the 
window view, providing buttons, menus, icons, and other options. 
When working on Linux systems through a graphical interface, users may use mouse 
clicks to run applications or open flies, drag and drop to copy files from one location 
to another, and so on. In addition, users may invoke a terminal emulator program, or 
xterm, which provides them with the basic command-line interface to the operating 
system. 
The Shell
Although Linux systems have a graphical user interface, most programmers and 
sophisticated users still prefer a command-line interface, called the shell. 
The shell command-line interface is much faster to use, more powerful, easily 
extensible, and does not give the user RSI from having to use a mouse all the time. 
Below we will briefly describe the bash shell (bash). It is heavily based on the original 
UNIX shell, Bourne shell, and in fact its name is an acronym for Bourne Again Shell. 
Many other shells are also in use (ksh, csh, etc.), but, bash is the default shell in most 
Linux systems.  UNIT V
Operating Systems Page 3
When the shell starts up, it initializes itself, then types a prompt character, often a 
percent or dollar sign, on the screen and waits for the user to type a command line. 
When the user types a command line, the shell extracts the first word from it, assumes 
it is the name of a program to be run, searches for this program, and if it finds it, runs the 
program. The shell then suspends itself until the program terminates, at which time it tries to 
read the next command. What is important here is simply the observation that the shell is 
an ordinary user program. All it needs is the ability to read from the keyboard and write 
tothe monitor and the power to execute other programs. 
Commands may take arguments, which are passed to the called program as 
character strings. For example, the command line 
cpsrc dest 
Invokes the cp program with two arguments, src and dest. This program interprets the first 
one to be the name of an existing file. It makes a copy of this file and calls the copy dest. 
Not all arguments are file names. In head -20 file the first argument, -20, tells head to print 
the first 20 lines of file, instead of the default number of lines, 10. Arguments thatcontrol the 
operation of a command or specify an optional value are called flags, and by convention are 
indicated with a dash. The dash is required to avoid ambiguity, because the command head 
20 file is perfectly legal, and tells head to first print the initial 10 lines of a file called 20, 
and then print the initial 10 lines of a second file called file. Most Linux commands
accept multiple flags and arguments. 
To make it easy to specify multiple file names, the shell accepts magic characters, sometimes 
called wild cards. An asterisk, for example, matches all possible strings, so 
Is *.C 
tellsls to list all the files whose name ends in .c. If files named x.c, y.c, and z.c all exist, the 
above command is equivalent to typing 
Is x.cy.cz.c
Another wild card is the question mark, which matches any one character. A list of 
characters insidesquare brackets selects any of them, so
Is [ape]* 
Listsall files beginning with «a", "p", or "e". 
A program like the shell does not have to open the terminal in order to read from it 
or write to it. Instead, when it starts up, it automatically has access to a file called 
standard input (for reading), a file called standard output (for writing normal output), 
and a file called standard error (for writing error messages). Normally, all three default  UNIT V
Operating Systems Page 4
to the terminal, so that reads from standard input come from the keyboard and writes to 
standard output or standard error go to the screen. Many Linux programs read from 
standard input and write to standard output as the default. For example, 
sort
invokes the sort program, which reads lines from the terminal (until the user types a 
CTRL-D, to indicate end of file), sorts them alphabetically, and writes the result to the 
screen. It is also possible to redirect standard input and standard output, as that is often 
useful. The syntax for redirecting standard input uses a less than sign ( <) followed by 
the input file name. Similarly, standard output is redirected using a greater than sign 
(> ). It is permitted to redirect bothin the same command. For example, the command 
sort<in>OUt
causes sort to take its input from the file in and write its output to the file out. Since 
standard error has not been redirected, any error messages go to the screen. A program 
that reads its input from standard input, does some processing on it, and writes its 
output to standard output is called a filter. 
Consider the following command line consisting of three separate commands: 
sort<in>temp; head -30 <temp>foo 
Here all the lines containing the string "ter" in all the files ending in .t are written to 
standard output, where they are sorted. The first 20 of these are selected out by head,  UNIT V
Operating Systems Page 5
which passes then to tail, which writes the last five (i.e., lines 16 to 20 in the sorted list) 
to foo. This is an example of how Linux provides basic building blocks (numerous 
filters), each of which does one job, along with a mechanism for them to be put 
together in almost limitless ways. 
Linux is a general-purpose multiprogramming system. A single user can run several 
programs at once, each as a separate process. The shell syntax for running a process in the 
background is to follow its command with an ampersand. Thus 
we -I <a>b &
runs the word-count program, we, to count the number of lines (-1 flag) m 1ts input, a, 
writing the result to b, but does it in the background. As soon as the command has 
been typed, the shell types the prompt and is ready to accept and handle the next 
command. Pipelines can also be put in the background, for example, by 
sort<X I head &
Multiple pipelines can run in the background simultaneously. It is possible to put a list of 
shell commands in a file and then start a shell with this file as standard input. The (second) 
shell just processes them in order, the same as it would with commands typed on the 
keyboard. Files containing shell commands are called shell scripts. Shell scripts may 
assign values to shell variables and then read them later. They may also have
parameters, and use if, for, while, and case constructs. Thus a shell script is really a program 
written in shell language. The Berkeley C shell is an alternative shell tha

Title:
Unit-5: Operating system lecture notes (Case study)

Subject:
Operating System

Institute:
P.R.Engineering college

Unit-5: Operating system lecture notes (Case study)

Unit-5: Operating system lecture notes (Case study) in Operating System uploaded at P.R.Engineering college

Summary / document preview text:

Modal

Are you sure to delete this information ?