The Linux System: Design Principles
 Linux is a multiuser, multitasking system with a full set of




UNIX-compatible tools.
Its file system adheres to traditional UNIX semantics, and it
fully implements the standard UNIX networking model.
Main design goals are speed, efficiency, flexibility and
standardization.
Linux is designed to be compliant with the relevant POSIX
documents; some Linux distributions have achieved official
POSIX certification.
The Linux programming interface adheres to the SVR4
UNIX semantics, rather than to BSD behavior.
1.1
The layers of a UNIX system
User
Interface
1.2
Components of a UNIX System
 Like
most UNIX implementations, Linux
composed of three main bodies of code:
is
 Standard Utilities Programs
 Standard Library
 Kernel
 Standard Utilities Programs perform individual
specialized management tasks.
 Shell
 Commands for the management of files and directories
 Filters
 Compilers
 Editors
 Commands for the administration of the system
…
1.3
The shell of a UNIX System
 The UNIX systems have a Graphical User Interface (Linux
uses KDE, GNOME …), but the programmers prefer to type
the commands.
 Shell: the user process which executes programs
(command interpreter)
 User types command
 Shell reads command (read from input) and translates it to the
operating system.
 Can run external programs (e.g. netscape) or internal shell
commands (e.g. cd)
 Various different shells available:
 Bourne shell (sh), C shell (csh), Korn shell (ksh), TC shell (tcsh),
Bourne Again shell (bash).
 The administrator of the system provides to the user a
default shell, but the user can change shell.
1.4
The shell of a UNIX System
 Example of commands
 $ cd /usr/src/linux
 $ more COPYING
 $ cp file1 file2
 $ head –20 file
 $ head 20 file
 $ ls *.c
 $ ls [abc]*
 $ sort < file1 > file2
 String multiple commands together in a shell script
 $ sort < file1 > file2; head –30 < file2 ; rm temp
 $ sort < file | head –30
 $ grep org CREDITS | sort | head –20 | tail –5 > file
1.5
The shell is more ...
 The shell is programmable, that is it possible to make the
shell scripts
 A shell script is a file containing a sequence of commands
addressed to the operating system that facilitates the
repeated execution of the included commands without their
having to be laboriously retyped each time they are
executed.
 If there is a distinct ordered list of operating system
commands that the user needs to execute repeatedly, for
example, immediately after every login or immediately
before every logout, then most operating systems have a
facility for recording the list of commands in a file, which can
then either be executed automatically upon login or logout,
or can be invoked by the user through the issuance of a
single command that results in the execution of the entire
contents of the batch file, which can contain as few as one
operating system command or as many as thousands.
1.6
The shell scripts
 Example:
 #!/bin/csh
clear
echo Menuset
stop=1
while ($stop>0) cat << ENDOFMENU
1: stampa data
2: stampa la directory corrente
3: esci
ENDOFMENU
echo -n Scegli il comando
set reply=$<
switch ($reply)
case 1 :
date
breaksw
case 2 :
pwd
breaksw
case 3 :
set stop=0
breaksw
default:
echo scelta sbagliata riprova
breaksw
endsw
end
exit 0
1.7
UNIX Utility Programs
A few of the more common UNIX utility programs required by POSIX
1.8
The Standard Library
 A system call is the method that the user process uses to ask for an




action from the O.S.
The programs perform the system calls by mean of trap.
trap instruction:
 changes from user mode to kernel mode
 controls the correctness of the call parameters
 execution done on behalf of the operating system
 returns to user mode
Since it is impossible to write a trap in C, it is provided a standard library
with a procedure for each system call. These procedures are written in
assembler and are called from C. For example a C program for
performing the system call read, it calls the procedure read ,of the
standard library.
So, the standard library defines a standard set of functions through
which applications interact with the kernel, and which implement much
of the operating-system functionality that does not need the full
privileges of kernel code.
POSIX establishes which are the procedures of the library that the
system has to provide, their parameters
and tasks.
1.9
The Kernel
 The kernel is responsible for maintaining the important abstractions of the
operating system.
 It provides the main functions of the abstract machine (system calls and
Interrupt and traps).
Approximate structure of generic UNIX kernel
1.10
The Kernel
 The Linux kernel uses a monolithic model. It does
not take the “new” client-server model (i.e., microkernel). The main reason for this choice is the
improvement of the performances.
 The kernel code is executed in kernel mode with full
access to all the physical resources of the computer.
 All kernel code and data are contained in a unique
address space.
 Although the kernel runs as a single process with a
single address space, Linux kernel uses the
modularity.
1.11
File System
 Linux files are organized by a hierarchy of labels,
commonly known as a directory structure. The files
referenced by these labels may be of three kinds:
 Regular files, which contains a sequence of bytes that
generally corresponds to code (programs) or data.
 Directory files, which are stored on disk in a special
format and form the backbone of the file system
 Special file, which correspond to peripherals such as
printers or disks.
 To the user, Linux file system appears as a
hierarchical
semantics.
directory
1.12
tree
obeying
UNIX
File System
 / is the root directory; reference point for all
directories.
pathname:
Every
file
 /home/user1/papers
1.13
has
a
unambiguous
Some directories found in UNIX systems
 /bin






Binaries which are absolutely essential to run Linux.
/boot All the files required for booting Linux on a system.
/dev All the devices have their corresponding files here.
/etc All the configuration files for the various software are
stored here. Don't play with this directory.
/home All users will have their 'My Documents' under this
directory. If your id is rossi, your 'My Documents' (called
home-directory) is /home/rossi.
/lib The libraries required by system-applications. (Just like
DLLs in Windows.)
/lost+found When a disk-check finds files which are
damaged or which are not linked to any directory, they are
recovered to this directory. Such damages are almost
always due to incorrect shutdown.
1.14
Some directories found in UNIX systems
 /mnt The directory where peripherals and other file






systems are mounted.
/opt The directory where optional software are installed.
/proc proc houses a pseudo-file system. Its contents really
do not exist anywhere on the disk, and are made available
only when you cd to this directory and look at some file.
/root The home-directory for the super-user: root.
/sbin The system-administration binaries exist here.
/tmp The directory where temporary files are created and
stored. All users can save files here.
/usr Everything related to users
/var Files whose contents vary frequently are in this
directory.
1.15
Some directories found in UNIX systems
 /usr Everything related to users
 /usr/bin







houses critical binaries of the users
/usr/include The header-files required by programs for compilation.
/usr/lib The libraries required by user-applications.
/usr/local Files peculiar to this particular machine.
/usr/sbin User-administration binaries.
/usr/share Information that can be shared by most users.
/usr/src The source-code for the Linux kernel.
/usr/X11R6 Files needed by the X Window system.
 /var Files whose contents vary frequently are in this
directory.
 /var/log
The log-files of the system.
 /var/spool Directories for mail, news, printing and other queued
work.
1.16
Mounting
 Separate file systems
 After mounting
Before mounting
After mounting
1.17