Unix - Shell Scripts

Day 4
Shell scripts of
Name
Ananthi Murugesan
presentation
• Company name

www.ananthim.wordpress.com

Author :- Ananthi Murugesan

2

Introduction
• A shell is a program constructed of shell commands
( $shell,$path,Ls,pwd,mkdir..)
• Shell is an environment for user interaction.But it is not a part
of kernel.
• Shell is just like as BAT files in MS-DOS.
• By default,Bash shell is default shell for Linux.



3

Shell script
can take
input from
user, file
and output
them on
screen.

4



Shell Scripting
• A script is defined as just a plain text file or ASCII file
• with a set of linux / unix commands.
• Flow of control
• IO facilities

• A shell script can be created using any text editor like
vim,emac,notepad++ etc.



5

Features of Shell Script
• Shells are CASE SENSITIVE.
• Shells allows interaction with kernel.
• Shells allow one to create functions and pass arguments to them.

• Shells provide help for each and every command using man or help.
• Helps in automation of tasks and thus time saving



6

• Shell script allow use of variables.
• Shell scripts are interpreted directly and are not compiled as cc++ codes.
• Shells provide many features including loop
constructs,arrays,variables,branches and functions.

•

shells provide logic with other utilities like pipelining,redirection etc.

• Shells allow file and directory management features.



7

Structure of a script
• #!/bin/bash –it defines that in which shell will be used to run the script.
• # comments –comments can be made by using # symbol in a script.
• Chmod +x script.sh – to tell the linux that file is executable.
• ./script.sh to execute the script.
To check current shell ,type following:
echo $SHELL.



8

When to Use Shell Script
Shell scripts can be used for a variety of tasks
•Customizing your work environment
•Every time login to see current date, welcome message etc
•Automating your daily tasks
•To take backup all your programs at the end of the day
•Automating repetitive tasks
•Producing sales report of every month etc
•Executing important system procedures
•Shutdown, formatting a disk, creating a file system on it, mounting
and un mounting the disk etc
•Performing same operation on many files
•Replacing printf with myprintf in all C programs present in a dir etc


9

When Not to Use Shell Scripting
When the task :
• is too complex such as writing entire billing system
•Require a high degree of efficiency

•Requires a variety of software tools



10

Shell Key words
• These are words whose meaning has already been explained to shell
• We cannot use as variable names
• Also called as reserved words
echo
if
until
trap read
wait set
fi
esac
eval unset
shift
do
continueulimit export
umask readonly for
return


else
case
while break exec
done
exit


11

Variables



In Linux (Shell), there are two types of variable:
(1) System variables - Created and maintained by
Linux itself. This type of variable defined in CAPITAL
LETTERS.



(2) User defined variables (UDV) - Created and
maintained by user. This type of variable defined in
lower letters.



12



BASH=/bin/bash Our shell name



HOME=/home/vivek Our home directory



LOGNAME=students Our logging name



PATH=/usr/bin:/sbin:/bin:/usr/sbin Our path settings



PS1=[u@h W]$ Our prompt settings



PWD=/home/students/Common Our current working directory



SHELL=/bin/bash Our shell name



USERNAME=vivek User name who is currently login to this PC

TERM =xterm name of the terminal on which you are working
We can see all the system variables and their values using
$set or env



13

how to use a variable

define a variable
DRIVE=drivedrivedrive
FUGA=fugafugafuga
num_of_my_awesome_defition=123456
 reference a variable
echo $DRIVE
echo "$DRIVE-$FUGA"
echo $num_of_my_awesome_definition >> $drove


Author :- Ananthi Murugasen

14

Variable – Tips
1. a variable is only string type
abc=1234 # quiv to abc="1234"
it's not a number
2. writing rules
standard definition
abc=hogehoge # it's ok
abc = fugafuga # ERROR! it's a function call
#$1='=', $2='fugafuga'; hint is white-space.
3.explicit value
def='#abc $abc @abc' # equiv: #abc $abc @abc
def="#abc $abc @abc'
# equiv: #abc fugafuga @abc


Author :- Ananthi Murugasen

15

4. implicit defined special values
$1 $2 $3 ...
it's params by a function call or run a script with params.
# f(){ echo $1; }
// int main(int ac, char** params) { }
$#
it's number of params
# f(){ echo $#; }
// int main(int ac, char** params) { }



16

echo & read commands
echo used to display messages on the screen
read used to accept values from the users, make
programming interactive

eg.
echo “Enter ur name “
read name
echo “Good Morning $name”



Command Line Arguments
Shell programs can accept values through
1. read [Interactive –used when there are more inputs]
2. From the command Line when u execute it[Non
interactive- used when only a few inputs are there]

For eg. sh1 20 30

Here 20 & 30 are the command Line
arguments.

Command Line args are stored in Positional parameters
$1 contains the first argument, $2 the second, $3 the third etc.
$0 contains the name of the file, $# stores the count of args
$* displays all the args



An example of Command Line args.
#! /bin/bash
echo “Program: $0”

echo “The number of args specified is $#”
echo “The args are $*”

sh sh1 prog1 prog2



exit : Script Termination
exit command used to prematurely terminate a program. It can
even take args.

eg.

grep “$1” $2 | exit 2
echo “Pattern found – Job over”



The logical operators && and ||
These operators can be used to control the execution of command
depending on the success/failure of the previous command

eg.
grep „director‟ emp1.lst && echo “Pattern found in file”
grep „manager‟ emp2.lst || echo “pattern not found”
or grep „director‟ emp.lst‟ && echo “Pattern found” || echo
“Pattern not found”



`for` statement and an array like variable

# it's like an array separated by the space char
values='aaa bbb ccc‘

# like the foreach in Java or for-in in JS
for value in $values
do
echo $value
done



22

seq` command
> seq 8
1
2
3
4
5
6
7
8



23

`...` <-- it's the back-quote pair, not a single-quote pair('...')
# if use the single-quote pair
> echo 'seq 512 64 768'
seq 512 64 768

# if use the back-quote pair
> echo `seq 512 64 768`
512 576 640 704 768 <-- it's like an array string
the back-quote pattern is like an eval() in JS.
it's run a text as a script code. be cautious!



24

`for` statement with `seq` command and ` pattern

it's give a loop with a numeric sequence.
for value in `seq 0 15`
do
echo $value
done


Author :- Ananthi Murgesan

25

`while` and ùntil` statements

● while <-- if true then loop
while true
do
echo '＼(ô^)／'
Done
● until <-- if false then loop
until false
do
echo '／(ô^)＼'
done



26

`test` command for comparison > test
test is used to check a condition and return true or false

Relational operators used by if

Operator

Meaning

-eq

Equal to

-ne

Not equal to

-gt

Greater than

-ge

Gfreater than or equal to

-lt

Less than

-le

Less than or equal to



27

test String comparison
test with Strings uses = and !=

String Tests used by test
Test

Exit Status

-n str1

true if str1 is not a null string

-z str1

true if str1 is a null string

s1 = s2true if s1 = s2
s1 != s2

true if s1 is not equal to s2

str1

true if str1 is assigned and not null


28

file

tests
File related Tests with test

Test

Exit Status

-e file

True if file exists

-f file

True if fie exists and is a regular file

-r file

True if file exists and is readable

-w file

True if file exists and is writable

-x file

True if file exists and is executable

-d file

True if file exists and is a directory

-s file

True if the file exists and has a size >0



`test` command for comparison > test
> test 3 -eq 4 <-- equiv (3 == 4)
> echo $?
1 <-- so false in shell-script
> test 3 -ne 4 <-- equiv (3 != 4)
> echo $?
0 <-- so true in shell-script
> test 3 -gt 4 <-- equiv (3 > 4)
> echo $?
1 <-- so false in shell-script



30

If statement
# if.sh
f(){
if test $1 -lt 0
then
echo 'negative'
elif test $1 -gt 0
then
echo 'positive'
else
echo 'zero'
fi
}


> source if.sh
>f0
zero
>f1
positive
> f -1
negative


31

Case Statement
# case.sh
f(){
case $1 in
0)
echo 'zero';;
1)
echo 'one';;
[2-9])
echo 'two-nine';;
default)
echo '????'
esac
}


> source case.sh
>f0
zerp
>f5
two-nine
> f -1
????


32

expr: Computation
Shell doesn’t have any compute features-depend on expr

expr 3 + 5
expr $x + $y
expr $x - $y
expr $x * $y
expr &x / $y
expr $x % $y

division gives only integers as expr

can handle only integers
x=5
x=`expr $x +1`
echo $x

it will give 6



sleep & wait
sleep 100

wait

the program sleeps for 100 seconds

wait for completion of all background processes

wait 138

wait for completion of the process 138



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