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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 755
724
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
is a complete waste of valuable hacking time.
To extract all the lines containing
the string ‘‘ard’’ from the file
f
, the Linux programmer merely types
grep ard f
The opposite approach is to have the programmer first select the
grep
program
(with no arguments), and then have
grep
announce itself by saying: ‘‘Hi, I’m
grep
,
I look for patterns in files. Please enter your pattern.’’ After getting the pattern,
grep
prompts for a file name. Then it asks if there are any more file names.
Final-
ly, it summarizes what it is going to do and asks if that is correct. While this kind
of user interface may be suitable for rank novices, it drives skilled programmers up
the wall. What they want is a servant, not a nanny.
10.2.2 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 op-
erating system.
Its function is to control the hardware and provide a system call in-
terface to all the programs. These system calls allow user programs to create and
manage processes, files, and other resources.
Users
Standard utility programs
(shell, editors, compliers etc)
Standard library
(open, close, read, write, fork, etc)
Linux operating system
(process management, memory management,
the file system, I/O, etc)
Hardware
(CPU, memory, disks, terminals, etc)
User
interface
Library
interface
System
call
interface
User
mode
Kernel mode
Figure 10-1.
The layers in a Linux system.
Programs make system calls by putting the arguments in registers (or some-
times, on the stack), and issuing trap instructions to switch from user mode to ker-
nel mode. Since there is no way to write a trap instruction in C, a library is pro-
vided, with one procedure per system call. These procedures are written in assem-
bly language but can be called from C.
Each one first puts its arguments in the
Page 756
SEC. 10.2
OVERVIEW OF LINUX
725
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.
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 of the common personal computer distributions of Linux have replaced
this keyboard-oriented user interface with a mouse-oriented graphical user inter-
face, without changing the operating system itself at all.
It is precisely this flexi-
bility that makes Linux so popular and has allowed it to survive numerous changes
in the underlying technology so well.
The GUI for Linux is similar to the first GUIs developed for UNIX systems in
the 1970s, and popularized by Macintosh and later Windows for PC platforms. The
GUI creates a desktop environment, a familiar metaphor with windows, icons,
folders, toolbars, and drag-and-drop capabilities. A full desktop environment con-
tains a window manager, which controls the placement and appearance of win-
dows, as well as various applications, and provides a consistent graphical interface.
Popular desktop environments for Linux include GNOME (GNU Network Object
Model Environment) and KDE (K Desktop Environment).
GUIs on Linux are supported by the X Windowing System, or commonly X11
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 the keyboard, mouse, and screen
and is responsible for redirecting input to or accepting output from client pro-
grams. 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 X11 by enriching the window view,
providing buttons, menus, icons, and other options. The X server can be started
manually, from a command line, but is typically started during the boot process by
a display manager, which displays the graphical login screen for the user.
When working on Linux systems through a graphical interface, users may use
mouse clicks to run applications or open files, 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. Its description is given in the following section.
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