Unix Presentation

UNIX

Author – Gayathri Rudrareddy (228433)

The Unix Operating System

•An operating system (OS) is a set of programs that act as a link between the computer and the user.

•It controls both the

–hardware (things you can touch—such as keyboards, displays, and disk drives)

–software (application programs that you run, such as a word processor).

•Some computers have a single-user OS, which means only one person can use the computer at a time. They can also do only one job at a time.

•UNIX is a multiuser, multitasking operating system. It can let many people use the computer at the same time and let each user run several jobs at once.

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Kernel

• Manages memory and allocates it to each process

• Schedules work done by the CPU

• Organizes transfer of data from one part of machine to another

• Accepts instructions from shell and carries them out

Shell

• Users communicate with the kernel through a program known as the shell.

• interprets the user commands/requests

• calls programs from memory

• executes them one at a time or in a series (pipe)

Applications

• specific capabilities added on to the operating system

Kernel( Heart of Unix Operating System)

Shell Interpreter

Tools and Applications

Unix Structure

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File System

Contains data, text or program instructions Stores ordinary &

special files.

Usually represent devices used by the system.eg., /dev/cd0

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All data in UNIX is organized into files.

All files are organized into directories.

These directories are organized into a tree-like structure called the filesystem.

File Structure

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Path names

Absolute Path name

* A file is identified by the path name from the root

e.g., /usr/trg/c/test.c

where

• test.c is an ordinary file

• usr, trg, c are directories

• trg is a sub-directory under usr

Relative path name

* UNIX keeps track of the user's current directory

* If a "/" does not precede a file name then the name interpretation

begins with the current directory

e.g.,

If current directory is /usr/trg

then the file could be just referenced as c/test.c

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Starting and Ending a Session

login : User can type his name and password to identify himself

Enter username at login: prompt. (Unix is case sensitive.)

Enter password at password: prompt.

passwd : To change your password

logout : exits or logs-out from the current user and executes the file .logout

before coming out

e.g., $ logout

or

$ exit

or

$ <ctrl-d>

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Command line structure

General Structure:

Command Name [Options] [Arguments]

E.g.,

Command Name

Tells the shell what to do

(in this case list files) Options

Control how the command will work

(in this case ask for a long listing)

Arguments

on which the command works

(in this case the directory

usr/trg )

ls -l /usr/trg

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Basic UNIX commands

• mkdir – creates a new directory ( ―-p‖ - creates missing intermediate directories)

- eg., mkdir /usr/test

• rmdir – removes directory ( ―-p‖ – removes all directories)

- directory must be empty in order to delete it.

- eg., rmdir /usr/test

• cd – change working directory (―cd ..‖ – one level back, ―cd ~‖ - home,

―cd /‖ - root, ―cd .‖ – current directory)

• pwd – print working directory

• touch - creates a zero-length file.

- If a file with the name already exists the last modification time is updated with the

current time.

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List command

ls – list all files in a directory

options: -a lists files including hidden files

-x multi-column output

-F to identify directories (/) and executables (*)

-d force listing of directories

-t sorts files by modification time

-r reverse sort order

-l long listing (lists mode, link info, owner, size, last modification)

eg.,

drwxr-xr-x 2 qdaemon GG_CRMCU 256 Jan 11 12:04 temp

-rwxrwxrwx 1 qdaemon qdaemon 886362 Jan 12 16:49 sqlnet.log

$ ls -l

-rwxr-xr-x 2 ankit staff 1024 jan 12 10:35 test

Permission bits

file/dir link count

user name

user group

character count

last modified time

filename 10

File Permissions rwx r-x r-x

owner(U) owner’s others(O)

group(G)

r ---- read (4)

w ---- write (2)

x ---- execute (1)

Access Mode Ordinary Directory

Read Examination of

contents

Listing of files in the

directory

Write Changing of the

contents

Creating new files in

the directory

Execute Execution of the file

as a command

Searching the

directory

• chmod - change the file or directory access permissions (mode)

eg., chmod 777 /usr/test/script.sh

• chgrp - change the group of the file (can be done only by the member of the group)

• chown - change the owner of a file

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Basic commands (contd.)

echo prints the string

cat concatenate (list) output, displays a file and creates a file if file not present

more displays file contents one screen at a time

cp copies a file

rm removes a file

mv moves a file to a different directory or renames a file

cut to cut a part of the string

grep searches a pattern in a file

find searches a files in a directory

sort sorts the input received

wc displays no. of lines (-l), words (-w) and characters (-m)

head display first 10 (or #) lines of file

tail display last 10 (or #) lines of file

date displays current date and time

df information about space on file systems

ps shows current status of active processes

who lists all users currently on system

who am i reports information on command user

clear clears screen

cal displays calendar

bc calculator mode (^D to leave calc mode)

expr to perform arithmetic operations

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stdin, stdout, and stderr

• Each shell (and in fact all programs) automatically open three ―files‖ when they start up

– Standard input (stdin): Usually from the keyboard – FILE ID 0

– Standard output (stdout): Usually to the terminal – FILE ID 1

– Standard error (stderr): Usually to the terminal – FILE ID 2

Redirecting stdin

• Instead of reading from the terminal, you can tell a program to read from another file using the <

operator

Examples:

– Mail [email protected] < message

Redirecting stderr

• Instead of writing errors to the terminal, you can tell a program to write them to another file using the:

Examples (suppose a file that does not exist)

home$cat IPL_team.list

cat: cannot open IPL_team.list

home$cat IPL_team.list 2> error_file.txt

Home$cat error_file.txt

cat: cannot open IPL_team.list

Redirecting stdout

• Instead of writing to the terminal, you can tell a program to print its output to another file using the >

operator

Examples: man ls > ls_help.txt

cat file1 >> file2 13

Pipes and filters

• Pipe (|): a way to send the output of one command to the input of another

• Filter: a program that takes input and transforms it in some way

e.g., awk, sed, grep

1) cat file | wc

2) $ cat basket.txt

Layer1 = cloth

Layer2 = strawberries

$ cat basket.txt | awk -F= '{print $1}'

Layer1

Layer2

3) $ cat apple.txt

worm

seed

$ cat apple.txt | sed -e "s/e/WWW/"

worm

sWWWed

4) $ grep seed apple.txt

seed

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Program & Process

• Program is an executable file that resides on the disk.

• Process is an executing instance of a program.

• A Unix process is identified by a unique non-negative integer called the process ID.

• Check process status using the “ps” command.

– After a successful login, the shell program is run.

– /home/userid$ ps

PID TTY TIME CMD

23110 pts/3 0:00 ps

18666 pts/3 0:00 ksh

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Foreground and Background Process

• A program run using the ampersand operator “&” creates a background process.

E.g.: /home/userid$ back &

• otherwise it creates a foreground process.

E.g.: /home/userid$ back

• Only 1 foreground process for each session. Multiple background processes.

• Where are background processes used?

• All system daemons, long user processes, etc.

e.g. printer-daemon process or mailer-daemon process.

• These processes are always running in background.

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How to stop a process?

• Foreground processes can generally be stopped by pressing CONTROL C (^C).

• Background processes can be stopped using the kill command.

• Usage: kill SIGNAL <process id list>

• kill -9 <process id list> (-9 means no blocked) Or kill <process id list>.

• If a foreground process is not stopping by ^C, you can open another session and use the kill command.

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UNIX Shell

The shell is your interface with the Unix system.

Is an ―interpreter‖ of commands

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Shell Scripting

Creating a simple shell script

A shell script is a plain text file that contains series of commands that the shell can execute.

• Any commands you enter in response to a shell prompt

A utility

A compiled program

Another shell script

• Control flow commands

Run a shell script

Enter the script filename on the command line

The shell interprets and execute the commands sequentially

Why shell script?

Simply and quickly initiate a complex series of tasks or a repetitive procedure.

Execution of script – 3 Simple steps

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1. Use any editor to write shell script.

2. After writing shell script set execute permission for your script.

Syntax:

chmod <permission> <your-script-name>

Example:

$chmod +x your-script-name

$chmod 755 your-script-name

3. Execute your script

Syntax:

sh <your-script-name>

./<your-script-name>

Run the script with different shell other than your interactive shell

Ex: home>sh test

Put special characters on the first line of a shell script

The firsts two character of a script are #! (shebang)

Then followed by the absolute pathname of the program that should execute the script

Ex:

home> cat test

#!/bin/ksh

# This line will not run since it is commented out...

echo 'This is Test!„

Pound sign (#) start a comment line until the end of that line as second line

in previous example, except

#! In the first line.

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Environmental Variables

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Special variables used by the system -- you can use these, but may not be able to change

them.

The special variables use uppercase names, or punctuation characters.

Some variables are set by the login process and inherited by the shell (e.g. $USER), while

others are used only by the shell.

e.g., .profile file in UNIX korn shell is executed whenever a user logs in. It is used to set

environment variables and shell options.

Example :

log_path=/usr/test/logs

export log_path

User Defined Variables

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1. To define user defined variable (UDV) use following syntax

Syntax: variable_name=value

Example:

To define variable called 'vehicle' having value Bus

vehicle1=Bus

echo “The value of vehicle1 is : $vehicle1.”

The value of vehicle1 is : Bus.

To define variable called n having value 10

n=10

echo “The value of n is $n.”

The value of n is 10.

Note: Syntax for referencing UDV is $variable_name

User Defined Variables (contd.,)

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1. Rules for naming variable name

• Variable name must begin with Alphanumeric character or

underscore character (_), followed by one or more Alphanumeric

character ( eg. HOME, SYSTEM_VERSION )

• Don't put spaces on either side of the equal sign when assigning

value to variable. ( number=10 )

• Variables are case-sensitive. (no, No, NO, nO are all different

variable names)

• You can define NULL variable as follows

• vech= (or) vech=“”

• Do not use ?, * etc. for naming variables.

Wild Cards

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1. * - Matches any string or group of characters.

• ls ut*.c – will show all files having extension .c but file name must

begin with “ut”.

2. ? - Matches any single character.

• ls fo? - will show all files whose names are 3 character long and

file name begin with “fo”

3. […] - Matches any one of the enclosed characters

• ls [abc]* - will show all files beginning with letters a,b,c

4. [..-..] – Matches range of character

ls [a-d]* - will show all files beginning with letters a,b,c,d

ls [^a-c]* - will show all files beginning with letters other than a,b,c

Shell script

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1. Exit Status of command or shell script?

echo $? ( zero return value indicates success, else failure )

2. Read statement

• Use to get input (data from user) from keyboard and store (data) to

variable.

Syntax:

read variable1

3. To run two command with one command line.

Syntax:

command1;command2

4. Continuing Lines

Type a backslash, \, at the end of line

to continue command on the next line.

echo –n “Please enter your name: “

read NAME

echo “Welcome! $NAME !”

date;who

name=Raghu

echo “Value of name is: \

$name “

Shell Arithmetic

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1. Use to perform arithmetic operations.

2. Syntax:

expr op1 math-operator op2

Examples:

$expr 1 + 3

$expr 2 - 1

$expr 10 / 2

$expr 20 % 3

$expr 10 \* 3

$echo `expr 6 + 3`

Redirection operators

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1. > - To output result (output of command or shell script) to file

• Note: If file already exists, it will be overwritten else new file is

created

• $ls tmp > ls.out 2>errors

2. >> - To output result to END of file

• Note: If file exists , it will be opened and new information/data

will be written to END of file, without losing previous

information/data

• If file does not exist, then new file is created

3. < - To take input from file instead of key-board

• $sort < /home/emp.txt > results

Special command symbols

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; command separator

& run the command in the background

&& run the following command only if previous command completes successfully

|| run the following command only if previous command not complete successfully

\ escape the following character (take it literally)

‘ ‘ don‘t allow any special meaning to characters within single quotes

" " allow variable and command substitution inside double quotes (does not disable $ and \)

` ` take the output of command and substitute it into the command line (Works inside

double-quotes)

Arguments & Command Substitution

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Arguments can be passed to a script from the command line

-- Positional parameters are used to receive their values from within the script

EXAMPLE

At the command line:

$ scriptname arg1 arg2 arg3 …

In a script:

echo $1 $2 $3 Positional parameters

echo $* All the positional parameters

echo $# The number of positional parameters

Command Substitution allows to assign the output of a UNIX command to a

variable or use the output of a command in a string

-- back quotes are used.

EXAMPLE

variable_name=`ls`

echo $variable_name

Shell scripting - Test

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• test expression OR [ expression ] is used to see if an expression is true or false.

Comparing Numbers

Test Usage

$x –eq $y Returns true if x equal y

$x –ne $y Returns true if x does not equal y

$x –gt $y Returns true if x is greater than y

$x –ge $y Returns true if x is greater than or equal to y

$x –lt $y Returns true if x is less than y

$x –le $y Returns true if x is less than or equal to y

Testing Numbers

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# Number test

echo “***Equal Comparison*** “

X=5

Y=10

if [ $X –eq $Y ]then

echo “X = Y.”

else

echo “X != Y.”

fi

# Number test

echo “***Less than Comparison*** “

X=5

Y=10

if test $X –lt $Y

then

echo “X < Y.”

else

echo “X > Y.”

fi

Comparing Strings

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Test Usage

“$s1” = “$s2” Returns true if s1 equal s2

“$s1” != “$s2” Returns true if s1 does not equal s2

$s1 Returns true if s1 is not null

$s1 -z Returns true if length of s1 is zero

$s1 -n Returns true if length of s1 is not zero

#String test

echo “***Null Comparison*** “

String=“Unix Class”

if (test “$String”) then

echo “We have non-null string”

else

echo “We have null string”

fi

#String test

echo “***Length Check*** “

String=“Unix Class”

if (test –z “$String”) then

echo “Length of string is zero”

else

echo “Length of string is NOT zero”

fi

Testing Files

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Test Usage

-d filename Returns true if filename exists and is a directory

-e filename Returns true if filename exists

-f filename Returns true if filename exists and is a regular file

-r filename Returns true if filename exists with read permission

-s filename Returns true if filename exists and is not empty (Len>0)

-w filename Returns true if filename exists with write permission

-x filename Returns true if filename exists with execute permission

Control flows

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1. „For‟ Loop

2. „While‟ Loop

3. „Until‟ Loop

4. For each and every loop

• First, the variable used in loop condition must be initialized, then

execution of the loop begins.

• A test (condition) is made at the beginning of each iteration.

• The body of loop ends with a statement that modifies the value of

the test (condition) variable.

For Loop

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Syntax:

for { variable name } in { list }

do

command1

command2

…

last command

done

Example

#print welcome 5 times

for i in 1 2 3 4 5

do

echo "Welcome $i times“

done

# Example to look at files in a directory and print out file names

for filename in *

do

echo “$filename”

done

Looping over files

--- FOR Loops can also be nested

While Loop

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Syntax:

while [ condition ]

do

command1

command2 …

done

Note:

Loop is executed as long as

given condition is true

Example:

#print multiplication table for given number

if [ $# -eq 0 ] then

echo "Syntax : $0 <positive number>"

exit 1

fi

n=$1

i=1

while [ $i -le 10 ]

do

echo "$n * $i = `expr $i \* $n`"

i=`expr $i + 1`

done

Until Loop

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Syntax:

Until [ condition ]

do

command1

command2 …

done

Note:

Similar to while, but the

loop is executed as long as

given condition is false

Example:

#print multiplication table for given number

if [ $# -eq 0 ] then

echo "Syntax : $0 <positive number>"

exit 1

fi

n=$1

i=1

until [ $i -eq 11 ]

do

echo "$n * $i = `expr $i \* $n`"

i=`expr $i + 1`

done

Break and Continue

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• Break and Continue commands take no arguments.

• When the shell encounters a break command, it immediately exits the body

of the enclosing loop ( while, until, or for) command.

• When the shell encounters a continue command, it immediately

discontinues the current iteration of the loop. If the loop condition permits,

other iterations may occur; otherwise the loop is exited.

Conditional Statements – IF loop

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Syntax:

if [ expression ]

then

block of statements

[elif command]

[else command]

fi

Example

#Testing equality

if [ $a –eq $b ] then

echo “Both are qual”

else

echo “Both are not equal”

fi

The if construct is followed by a command

If an expression is to be tested, it is enclosed in square brackets

The then keyword is placed after the closing parenthesis

An if must end with a fi

Case Statement

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Syntax:

case $variable-name in

pattern1) command1... ..

command2;;

pattern2) command1 ... ..

command2;;

patternN) command1 ... ..

command2;;

*) command1... ..

command2;;

esac

FRUIT="kiwi"

case "$FRUIT" in

"apple")

echo "Apple pie is quite tasty." ;;

"banana")

echo "I like banana nut bread." ;;

"kiwi")

echo "New Zealand is famous for kiwi." ;;

esac

Functions

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Functions allow you to define a section of shell code and give it a name.

Syntax:

function_name() {

block of code

}

-------------------------------------------------

Example:

lister() {

echo Your present working directory is `pwd`

echo Your files are:

ls

}

Crontabs

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• Crontab file contains the schedule of cron entries to be run and at specified

times.

• The jobs which are scheduled are called as cron jobs.

• Location - /var/spool/cron/ --- only root user has access to the directory.

crontab -e Edit your crontab file, or create one if it doesn‗t already exist.

crontab -l Display your crontab file.

crontab -r Remove your crontab file.

crontab -v Display the last time you edited your crontab file.

(option available only on few systems.)

Crontab Restrictions

-- User can execute crontab if name appears in the file /usr/lib/cron/cron.allow.

-- If allow file does not exist, then if username does not appear in the file

/usr/lib/cron/cron.deny.

-- If only cron.deny exists and is empty, all users can use crontab.

-- If neither file exists, only the root user can use crontab.

Crontab syntax

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