Introduction to UNIX System

1
Introduction to UNIX System
2
Introduction

• Developed at Bell Laboratories in the late 1960s
  by Dennis Ritchie and Ken Thompson
• Shell is simply a program that reads in the
  commands you type and converts them into a form
  that is more readily understandable by the UNIX
  system

3
Introduction (continue.)

• Shell includes some fundamental programming
  constructs that let you make decisions, loop, and
  store values in variables
• Bourne shell was written by Stephen Bourne in
  Bell laboratories
• Bourne shell is the standard shell

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UNIX System

• The UNIX system is logically divided into two
  pieces
• Kernel
• Utilities

5
UNIX System (continue.)

• Kernel is the heart of the UNIX system and
  resides in the computers memory

Utilities
UNIX system kernel
disks
Memory
6
UNIX System (continue.)

• Utility resides on the computers disk and are
  only brought into memory as requested.
• Virtually every command under UNIX is a utility
• Shell is a utility program loaded into memory for
  execution whenever you log into the system

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Logging in a UNIX system

• Terminal is connected to a UNIX system through
• Direct wire
• Modem
• LAN
• After you connect the UNIX system a login
  message appears

8
Logging in a UNIX system (continue.)
9
Logging in a UNIX system (continue.)

• init is the UNIX system
• init automatically starts up a getty program on
  each terminal port whenever the system is
  allowing users to log in
• After getty displays the message login and some
  types the usernames followed by RETURN, it starts
  up a program called login to finish the process
  of logging in. Then getty disappears
• /etc/passwd file has one line per user

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Logging in a UNIX system (continue.)
login Skan password
UNIX SYSTEM KERNEL
login
login
getty
getty
login
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Logging in a UNIX system (continue.)

• After login begins execution, Password message
  appears
• User types the password and hits RETURN
• The user name and the password will be checked
  against the corresponding entry in the file
  /etc/passwd

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Logging in a UNIX system (continue.)

• Every line has seven fields separated by .
• The fields are
• Login name
• Password (encrypted form).
• User ID
• Group ID
• User information which could be First and Last
  name, etc
• Home directory
• Program to start up when user logs in. Usually a
  shell program

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Logging in a UNIX system (continue.)

• Example

cat /etc/passwd rootx01Super-User//sbin/sh
daemonx11/ binx22/usr/bin sysx33
/ admx44Admin/var/adm lpx718Line
Printer Admin/usr/spool/lp uucpx55uucp
Admin/usr/lib/uucp nuucpx99uucp
Admin/var/spool/uucppublic/usr/lib/uucp/uucico l
istenx374Network Admin/usr/net/nls nobodyx
6000160001Nobody/ noaccessx6000260002No
Access User/ nobody4x6553465534SunOS 4.x
Nobody/ oracle10167DBA Account/export/home
/oracle/bin/csh webuser102102Web
User/export/home/webuser/bin/csh abuzneidx103
100Abdelshakour Abuzneid/home/abuzneid/sbin/csh

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Logging in a UNIX system (continue.)
login Skan password
UNIX SYSTEM KERNEL
/bin/sh
login
login Med password
/usr/lbin/ksh
/usr/data/bin /dat_entry
login
login slim password
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After Shell starts

• When shell starts up, it displays a command
  prompt
• in Bourne shell and Korn shell
• in C shell
• Shell goes to sleep after every command or
  program followed by RETUN until the program has
  finished
• This copied program is called a process

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After Shell starts (continue.)
shell
shell

ls
1.
2.
3.
4.
ls
shell
Foo rje
ls Foo rje
7.
8.
6.
5.
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Login cycle
init
init
init
getty
init
sh
init
login
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Login cycle (continue.)
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Responsibilities of Shell

• Program Execution
• Variable and File Name Substitution
• I/O Redirection
• Pipeline Hookup
• Environment Control
• Interpreted Programming Language

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Program Execution

• Format program-name arguments
• The shell scans the command line and determines
  the name of the program to be executed and what
  argument to pass to the program
• Multiple occurrences of white spaces characters
  are simple learned

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Program Execution (continue.)

• mv oldfile newfile
• echo Smile, you are in Bridgeport City
• Smile, you are in Bridgeport City

oldfile newfile
mv
arguments
Smile, you are in Bridgeport City
echo
arguments
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Program Execution (continue.)

• Shell has some built_in commands which execute
  them directly without searching the disk
• cd, pwd and echo are built_in commands

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Variables and File Name Substitution

• assign values to variables
• listls
• ls
• Carthage
• Damas
• list
• Carthage
• Damas
• file name substitution on the command line
• ?

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Variables and File Name Substitution (continue.)

• Examples

ls Documents Memos mail personal
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Variables and File Name Substitution (continue.)
echo Documents Memos mail personal
Documents Memos mail personal
arguments
echo
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Variables and File Name Substitution (continue.)
ls Documents mail Documents a.doc c.doc p1
p11 mail p1 p2
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Input/Output Redirection

• Examples

wc -l list 2 list wc -l 2
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Input/Output Redirection (continue.)
-l users
wc
arguments
wc
-l
arguments
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Input/Output Redirection (continue.)

• In the first command line, two arguments where
  passed to wc (word count) utility -l and users
• In the second command line, one argument is
  passed to wc utility -l. This gives the
  indication that the number of lines appearing on
  standard input is to count

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Pipeline Hookup

• Connects to commands
• Pipe characters
• Connects the standard output from the command
  preceding to the standard input of the one
  following the

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Pipeline Hookup (continue.)

• Example
• who wc -l
• Counts the number of users login to the system by
  connecting the standard output of who to the
  standard output for wc

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Environment Control

• Shell provides variables that customize the
  environment
• Examples

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Interpreted Programming Language

• The shell has its own built_in programming
  language
• This language is interpreted shell analyzes each
  statement in the language one line at a time and
  then execute it
• Different than high level programming language
  like C/C where the programming statements are
  typically compiled into a machine_executable form
  they are executed

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References

• UNIX SHELLS BY EXAMPLE BY ELLIE QUIGLEY
• UNIX FOR PROGRAMMERS AND USERS BY G. GLASS AND K
  ABLES
• UNIX SHELL PROGRAMMING BY S. KOCHAN AND P. WOOD