UNIX

UNIX is, in short, a good operating system, for programmers.

-- Kernighan and Pike.
Pike

Education & Research Dept.

Infosys

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 Session Plan

 Day 1
 Introduction to Unix.
 General UNIX commands.
 Introduction to the on-line Tutor.

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 Session Plan (Contd.)

 Day 2
 The vi editor
 Regular expressions.
 Text processing utilities.
 Advanced commands.

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 Session Plan (Contd.)

 Day 3
 The Shell variables.
 The Korn shell.
 The Shell programming.

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 Session Plan (Contd.)

 Day 4
 Compression utilities.
 Communication.
 Software development tools.
 Project

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 Session Plan (Contd.)

 Day 5
 The Project.
 Submission by 2:00 pm.

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 Some Selected References
 Rebecca Thomas and Yates : A User
Guide to the Unix System, Mc. Graw
Hill, 1985
 Kernighan and Pike : The Unix
Programming Environment, PHI, 1990
 Stephen G. Kochan &
Patrick H. Wood : Unix Shell
Programming

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 Evaluation Strategy
 Project.
 Theory exam.
 Tutor usage.
 Assignments.
 Quizzes.

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 Introduction

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 What is UNIX ?
 A very popular multi-user,
multitasking, operating system.

 UNIX has become the operating

system of choice for engineering and
scientific workstations.

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 Background
-- how was it developed?
 1969 : Originally developed in Bell
laboratories by Ken Thompson and
Dennis Ritchie on a PDP-7 machine.

 1973 : Re-written in high-level language

C , thus making it machine-
independent.

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 Background
-- how was it developed?
 1977-1982
– UNIX is made widely available at minimal cost.
– Becomes popular for scientific applications.
 1983 System III.
 1984 System V, Release …
– …………………
 1992 4.4 BSD.
 1999 LINUX, A freeware flavor of UNIX
by Linus Trivolds is released.
– ………………….
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 Unix has so many flavors
 AIX.
 HP-UX.
 MINIX.
 SCO.
 SOLARIS. Two variations maintain
 SUN OS. popularity today :
 XENIX.  AT&T System V &
 LINUX.  Berkeley Standard Distribution.
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 Key features
what made UNIX different ?
 Portability as it was written in C.
– achieving machine independence.
 Background Processing.
 Hierarchical File System.
 Pipes and redirection tools.

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 Key features
-- what made UNIX different ?
 UNIX Shells.
 Security.
 Software development tools.
 Stable and reliable OS.
 Communication.

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 UNIX OS structure

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 Major Components of UNIX

 Kernel
 Shell
 Utilities
 User applications

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 Unix System Architecture

User
User
User
User
SHELL
SHELL
UNIX OTHER
COMMANDS APPLICATIONS

HARDWARE

DATABASE KERNEL
COMPILERS
PACKAGES

SHELL
SHELL

User
User User
User

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 The Login sequence
 Login id and password.
 The /etc/passwd file.
 The /etc/profile and .profile file.
 Shell prompt.
 Environment variables
– The $HOME, $PATH, $PS1, $TERM
and other variables.

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 The other system settings
 The $PATH variable.
 Aliases.
 The stty command.
 The [ctrl-c] , [ctrl-d] and other keys.

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 How your command is
interpreted ?
LOGIN

SHELL ASKS FOR A COMMAND

USER TYPES COMMAND

SHELL EXECUTES
UTILITY TO CARRY
OUT COMMAND
USER INTERACTS WITH
UTILITY
SHELL PROMPTS FOR
NEXT COMMAND
USER TYPES CONTROL-D
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LOGOUT
 UNIX file System

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 UNIX file system

 A hierarchical structure.
 Consistent treatment of file data.
 Treatment of peripheral devices as files.
 Information of files is stored in inode-
block.
 Unix keeps track of files using inode
numbers.
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 A UNIX File System

LEGEND

directory root
........
To other directories
file

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 Directory organization in UNIX
/ (root)

bin dev etc tmp lib usr

sh ls tty lp
passwd terminfo user1
user2

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 Directory organization in UNIX
 /dev
– Where special files are kept .
 /bin
– Executable system utilities, like sh, cp, rm .
 /etc
– System configuration files and databases.
 /lib
– Operating system and programming libraries .
 /tmp
– System scratch files (all users can write here)

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 Inodes and linking
A directory
FILENAME INODE Another directory
NUM
FILENAME INODE
file1 0221 NUM
file2 0412 name1 0221
file3 0981

• File1 and name1 are links

with same inode numbers
Inode
block #0412 #0221
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 The absolute path and relative paths
 The absolute path
– The entire pathname starting from root(/).
– Example : /home/trng01/file1.

 The relative path

– The path relative to your present working
directory.
– .. represents parent directory.
– . represents present working directory itself.
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 File and device independence

TAPES

DISK
FILES
UNIX
FILE PRINTERS
UNIX UTILITIES SYSTEM

TERMINALS

COMMUNICA-
TIONS LINES
PIPES
USER PROGRAMS
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 Processes in UNIX

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 Process in UNIX.
 A process is a program in execution.
 Each process is allocated a process-
identifier or PID.
 In general , each process is started by
another, known as parent process.
 When system starts, it runs a single
program, the process number one
called init ( PID = 1).

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 Process related commands
 The ps command.
– The info of all the processes in system.
 The background processing.
– fg command
 The who commnad
– The info of all the users logged into the system.
– whoami command.
 The kill command.
– kill –[signal] [PID]

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 UNIX command line structure
 A command is a program that tells UNIX
system to do something.
 It has the form : command [options]
[arguments]
 Option is generally preceded by hyphen(-)
– More than one option can be strung together.
– e.g. : ls -alR

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 Getting help
 The man command
– man [options] command_name.
 The – help
– command_name – help.
 The whatis command.
– whatis command_name.

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 I/O Redirection

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 I/O Redirection

 Processes have access to 3 opened files

0.standard input.
1.standard output.
2.standard error.
 These files can be redirected
independently.

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 I/O Redirection (Contd .)
Examples:
 $ ls > myfiles
 output from ls command goes to the file
"myfiles"
 $ mail sam < letter
 contents of file letter is mailed to user sam
 $ nroff < docl > docl.out 2> errors.
 standard input and output and error redirected
simultaneously

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 A simple demo
 How to create a simple file which has all
the filenames of current directory ?
 $ ls > temp
 Can I write into the other terminal using
the same command ?
 The /dev directory has terminal files.

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 PIPES -- a powerful feature.

 A data-connection between two or more

commands where in the output of one
command acts as an input to next
command.
 Usage :
– Command1 | Command2.
– Command1 | Command2 | Command3.
 Saves time and disk-space.
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 Pipes
process
A single process :
input output

The use of Pipe : Output 1 Input 2

pipe

Process1 Process
2

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 Pipe
Motivated example
 $ grep “pattern” file1 file2 > junk
$ wc -l junk

 $ grep “pattern” file1 file2 | wc -1

What’s the difference ?

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 Commands as hands – on

 File related commands

– cat , cp , rm , mv
 Directory commands
– pwd, mkdir, cd, ls, rmdir

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 The ln command :
 Creates a link for the file.
– ln [file1] [file2]
 The second column of ls –l is link-count.
 To delete a file with many links, you
need to delete all the links to that file.

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 File Security

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 File Security
 Permissions for a file
 read (r) , write (w) and execute (x)
 Classes of users
 Owner
 Group
 Others
 Nine bit field
 Composite permission for each file.

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 Setting file permissions

 Method of calculating file permissions

Owner Group Others

r w x r w x r w x
4 4 4
2 2 2
1 1 1

Digit Digit Digit

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 Setting file permissions
(Contd.)
 Set permission rwxr-xr-- on a <file>
Owner Group Others

r w x r w x r w x
4 4 4
2 - -
1 1 -

7 5 4

 $ chmod 754 <file>

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 Setting file permissions...

Symbolically setting file permissions

chmod [who] [+ - =] [permission] <file>
 who : u, g or o
 + adds , - removes and , = assigns
specified mode and removes others if
any for u/g/o
 permission : r or w or x

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 The umask command
 System wide default permission for a file
– rw-rw-rw (666)
 umask (User file creation mask)
– Sets default permissions for a newly created
file and directory.
 $ umask
022
 $ umask -S
– u=rwx,g=rx,o=rx
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 Default File Permissions

 Calculate system default file

permissions
6 6 6 - System wide default permissions
0 2 2 - Denial ‘mask’ set by UMASK
6 4 4 - Resultant permissions that will be set on all
files created (-rw-r—r--)

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 Default Directory Permissions
 System wide default permissions for a
directory: rwxrwxrwx (777)
 UMASK
 Default permissions for a directory:
7 7 7 - System wide default permissions
0 2 2 - Denial ‘mask’ set by UMASK
-----------
7 5 5 - Resultant permissions that will be
set on all files created (drwxr-xr-x)

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 Changing Owner and Group

 Change the owner of a file (or directory)

 $ chown newowner textfile
 Change group to which a file belongs:
 $ chgrp newgroup textfile

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 Unix On-line Tutor
for you !

Developed by E&R,
Infosys Technologies Ltd.

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 Today you have to do …
 Login
 Check home dir/ working dir
 Change Password
 Pathname and setting variables
 Simple commands and man help.
 Invoke Tutor : $ tut
 After the session, type exit to come out.
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 Regarding tutor…
 Recording the trails and logs of
– Topics covered.
– Time spent.

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 Exercise
Submission : Tomorrow
 Run and see the usage and meaning of
following commands and write down
with an example
cat, cd , chmod, comm, cp, date, diff, du,
echo, file, grep, head, kill, less, login, ln,
ls, mail, man, mesg, mkdir, more, mv,
passwd, ps, pwd, rm, rmdir, sort, stty,
tail, tee, tr, tty, wc, who, write

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 UNIX

End of day 1

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