LINUX System : Lecture 2(English-Only Lecture)

Bong-Soo Sohn

Assistant ProfessorSchool of Computer Science and Engineering

Chung-Ang UniversityAcknowledgement : (i) wikipedia.org , (ii) http://www.doc.ic.ac.uk/~wjk/UnixIntro

Operating Systems

OS acts as an intermediary between user and HW

Resource Manager Allocates and manages system resources (CPU, Memory, Disks, Ne

tworks, … ) Efficient and fair use of the resources

Interface OS allows users and application programs to access system resour

ces Controls the execution of user programs accurately

Efficiently, Fairly, Safely(Accurately), Abstractly

Computer System Structure

HW – provides basic computing resources CPU, memory, disk, …

OS Kernel direct control of underlying HW System call

Programming interface to the services provided by the OS

The one program running at all times on the computer is the kernel. Everything else is either a system program (shipped with OS) or an application program

Application programs Define the ways in which the system resources

are used to solve user problems executed thru shell/GUI. Ex) word processor, video games, etc

Operating Systems Functionality

Process Management CPU scheduling : batch vs time-sharing

Memory Management Memory allocation : determine what is in memory when? Virtual memory : a technique that gives an application program the impressio

n that it has large contiguous working memory, while in fact it is physically fragmented and may even overflow on to disk storage.

Disk and file system Device driver : interface for communicating with HW devices Networking : supports network protocols. (ex) TCP/IP Security : authentication (login), authorization (file permission), … UI : GUI (Graphical User Interface), CLI (Command Line Interface)

OS comes with application programs that provide an interface for managing OS.

Storage Hierarchy Register, Cache, Main memory, Secondary Storage

(disks), Magnetic tapes

Why hierarchical structure? Speed Cost Volatility

Caching Copying information into faster storage system. Faster storage checked first to determine if information

is there Cache<->main memory, main memory<->disk Important principle, performed at many levels in

computer

OS Structures Multiprogramming

a technique used to utilize maximum CPU time by running multiple programs simultanously

Single user cannot keep CPU and I/O devices busy at all times. Multiprogramming organizes jobs (code and data) so CPU always has one to

execute One job is selected and run via job scheduler When a running job has to wait for I/O, OS switches to another job

Multitasking (time-sharing) CPU switches jobs so frequently that users can interact with each job while it

is running. Creates interactive computing Response time can be controlled If processes don’t fit in memory, swapping moves them in and out to run

Operating Systems

MS Windows Most popular on desktop computers Gaining popularity in servers.

UNIX/LINUX Most popular in server environments LINUX are gaining popularity on desktop

PC.

UNIX System has been popular for more than two decades

Portability Written mostly in C + a few in assembly language Kernel size is small

Multi-user, multi-tasking environment Stability Powerful networking capabilities Free, open source code(LINUX)

Suitable for using as a server

UNIX History Created by Ken Thompson et al. ( a researcher at AT&T Bell Lab) UNICS(1969) (shortened to UNIX later) : first version Released Fifth version of UNIX

kernel was rewritten in C, which was a big step forwards in terms of system portability (1973)

LINUX Developed by Linus Torvalds, a finnish undergrad student, in 1991 Free open source UNIX OS for PC Open source code nature means any one can add features and correct defici

encies. Turned in to collaboration of hundreds of volunteer developers around the w

orld. Several different distributions (e.g. Redhat, Debian, … etc) Distribution comprises a prepackaged kernel, system utilities, GUI, applicati

on programs

LINUX

Kernel Device driver support for various HW device Advanced processor and memory management Support different types of file system

Shells / GUI : two forms of command input Textual command line shells (e.g. C shell, … ) Graphical user interface (e.g. GNOME window manager)

System utilities Virtually every UNIX system utilities were ported to LINUX Include daemons

server program running as a background process Application programs

Come with standard application programs e.g. vi, emacs, xv, gcc/g++, xfig, latex, …

Cygwin

Login, Password When connecting a UNIX system (locally or remotely)

login : will

password :

After successful login, you will get shell prompt where you can give command input.

$

To logout, type exit

In GUI, it is trivial to login/logout Password change

$ passwd Avoid dictionary words

Understanding /etc/passwd

/etc/passwd Stores essential information required during login i.e. user acco

unt information It contains one entry per line for each user (or user account) of t

he system. All fields are separated by a colon (:) symbol. Total seven fields as follows

Username(1), password(2)-x means passwords are stored in /etc/shadow, user ID(3), group ID(4), user ID info(5), home directory(6), command/shell(7)

UNIX File System

Three types of UNIX files Ordinary files

Contain text, data, program Cannot contain other files or directories Filename is not divided into name and extension officially Up to 256 characters long

Directory file A file that has a list of other files and directories

Special file : I/O device Devices : for easy access to HW device, a device is dealt with as a file. e.

g.) READ/WRITE for printer, network socket, … Links

A pointer to another file hard link : direct pointer ex) $ln filename linkname soft(symbolic) link : indirect pointer ex) $ln –s filename linkname

Directory in UNIX

< hierarchical tree structures >

Directory Contains a list of files or directories and their properties/location

s

tree structure a parent may have many childs, and a child can have only one par

ent

Path absolute path : /home/bongbong/a.txt relative path : usr/bin/xv

Directory Home directory, eg) cd ~bongbong Current directory : . , parent directory : ..

File Access

Example : /usr/bin/xv1. Read root(/) directory2. Find the location of “usr” from “/”3. Read “usr” and find the location of “bin”4. Read “bin” and find the location of “xv”

Commands for Files and Directories

pwd : prints [current] working directory cd : change directory mkdir , rmdir : create/remove a directory cp, mv, rm : copy, move, remove chmod : change permission of a file cat , more : prints text files man : manual for a command ls : list files

File properties ls –l

type is a single character which is either 'd' (directory), '-' (ordinary file), 'l' (symbolic link), 'b' (block-oriented device) or 'c' (character-oriented device).

permissions is a set of characters describing access rights. There are 9 permission characters, describing 3 access types given to 3 user categories. The three access types are read ('r'), write ('w') and execute ('x'), and the three users categories are the user who owns the file, users in the group that the file belongs to and other users (the general public). An 'r', 'w' or 'x' character means the corresponding permission is present; a '-' means it is absent.

links refers to the number of filesystem links pointing to the file/directory owner is usually the user who created the file or directory.

group denotes a collection of users who are allowed to access the file according to the group access rights specified in the permissions field.

size is the length of a file, or the number of bytes used by the operating system to store the list of files in a directory.

date is the date when the file or directory was last modified (written to). The -u option display the time when the file was last accessed (read).

name is the name of the file or directory.

File Permission

Link a pointer to another file

Hard link to a file is indistinguishable from the file itself ex) $ln filename linkname Soft link (symbolic link) provides indirect pointer or shortcut to a fi

le ex) $ln –s filename linkname

$ ln -s hello.txt bye.txt $ ls -l bye.txt

lrwxrwxrwx   1 will finance 13 bye.txt -> hello.txt $

Soft link may point to a non-existing file

Wildcard : Specifying multiple files

UNIX shell processes this

‘?’ matches any one character ‘*’ matches any of zero or more characters Characters enclosed in square brackets ('[' and ']') will match any filename that has o

ne of those characters in that position A list of comma separated strings enclosed in curly braces ("{" and "}") will be expand

ed as a Cartesian product with the surrounding characters

he* matches any filename beginning with 'he'. [m-z]*[a-l] matches any filename that begins with a letter from 'm'

to 'z' and ends in a letter from 'a' to 'l'. {/usr,}{/bin,/lib}/file expands to /usr/bin/file /usr/lib/file /bin/fil

e and /lib/file.