This document discusses user defined variables (UDV) in shell scripting and provides examples of defining, assigning values to, and printing variables. It also covers arithmetic operations using variables, conditional statements, loops, and other shell scripting concepts. Key points include: - Syntax for defining variables is variable_name=value - Variables are case-sensitive - The expr command is used for arithmetic operations - Conditional statements include if/then/else and case/esac - Looping statements include while, until, for, and nested loops - Examples are given for various shell scripting elements like variables, conditionals, loops, and arithmetic.

1. Dr. D. P.
Mishra
Digitally signed by Dr. D. P.
Mishra
DN: cn=Dr. D. P. Mishra, o=durg,
ou=BIT,
email=dpmishra@bitdurg.ac.in,
c=IN
Date: 2023.03.17 10:34:48 +05'30'

2. How to define User defined variables (UDV)
• Syntax: variable name=value
• NOTE: Here 'value' is assigned to given 'variablename'
For e.g.
$ no=10 # this is ok
$ 10=no # Error, NOT Ok
• Don't put spaces on either side of the equal sign
• $ no=10 is OK But here there will be problem for following
• $ no =10
• $ no= 10
• $ no = 10
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3. More on Variables.
• Variables are case-sensitive, just like filename in Linux. For e.g.
$ no=10
$ No=11
$ NO=20
$ nO=2
• Above all are different variable name, so to print value 20 we have to
use $ echo $NO - and Not any of the following
$ echo $no # will print 10 but not 20
$ echo $No # will print 11 but not 20
$ echo $nO # will print 2 but not 20
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4. More on variables ..
• How to Define variable x with value 10 and print it on screen ?
$ x=10
$ echo $x
How to Define variable xn with value Ram and print it on screen
$ xn=Ram
$ echo $xn
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5. More on Variables ...
• How to print sum of two numbers, let's say 6 and 3
$ echo 6 + 3 This will print 6 + 3, not the sum 9
• For math operations in shell use expr, syntax is : expr op1 operator op2
• Where, op1 and op2 are any Integer Number (Number without decimal
point) and operator can be
+ Addition
- Subtraction
/ Division
% Modular, to find remainder
• For e.g. 20 / 3 = 6 , to find remainder 20 % 3 = 2
• $ expr 6 + 3 Now It will print sum as 9 ,
• But $ expr 6+3 will not work because space is required between number
and operator
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6. More on Variables….
• How to define two variable x=20, y=5 and then to print division of x
and y (i.e. x/y)
$x=20
$y=5
$ expr x / y
• Modify above and store division of x and y to variable called z
$ x=20
$ y=5
$ z=`expr x / y`
$ echo $z
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7. Shell Arithmetic
• Use to perform arithmetic operations For e.g.
$ expr 1 + 3
$ expr 2 - 1
$ expr 10 / 2
$ expr 20 % 3 # remainder read as 20 mod 3 and remainder is 2)
$ expr 10 * 3 # Multiplication use * not * since its wild card)
$ echo `expr 6 + 3` For the last statement not the following points
• First, before expr keyword we used ` (back quote) sign not the (single quote i.e. ')
sign.
• Back quote is generally found on the key under tilde (~) on PC keyboards OR To the
above of TAB key.
• Second, expr is also end with ` i.e. back quote.
• Here expr 6 + 3 is evaluated to 9, then echo command prints 9 as sum
• If you use double quote or single quote, it will NOT work,
• For eg. $ echo "expr 6 + 3" # It will print expr 6 + 3 $ echo 'expr 6 + 3'
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8. if [condition]
then
statement1
else
statement2
fi
Command Description
&& Logical AND
-eq Equality check
-ne Inequality check
-lt Less Than
-le Less Than or Equal
-gt Greater Than
-ge Greater Than or Equal
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If else fi statement

9. Switch Case Statement
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10. Syntax
case <word> in
<first pattern>)
<statements>
;;
<second pattern>)
<statements>
;;
*)
<default statements>
;;
esac
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11. Example1
name="HowLinux"
case $name in
"Linux")
echo "Not quite right"
;;
"HowLinux")
echo "That's right!"
;;
esac
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12. Example2
echo “Enter a number”
read num
case $num in
[0-9])
echo “you have entered a single digit number”
;;
[1-9][1-9])
echo “you have entered a two-digit number”
;;
[1-9][1-9][1-9])
echo “you have entered a three-digit number”
;;
*)
echo “your entry does not match any of the conditions”
;;
esac
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13. Example3: case - esac
echo "Which color do you like best?"
echo "1 - Blue"
echo "2 - Red"
echo "3 - Yellow"
echo "4 - Green"
echo "5 - Orange"
read color;
case $color in
1) echo "Blue is a primary color.";;
2) echo "Red is a primary color.";;
3) echo "Yellow is a primary color.";;
4) echo "Green is a secondary color.";;
5) echo "Orange is a secondary color.";;
*) echo "This color is not available. Please choose a different one.";;
esac
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14. Example4 : Script to print capital of state
entered by user
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15. While Loop Syntax
while [ condition ]
do
command1
command2
done
• Here while, do, and done are built-in keywords.
• It will execute all the commands while the condition evaluates
to true.
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16. Nesting While loop
while command1 ; # this is loop1, the outer loop
do
Statement(s) to be executed if command1 is true
while command2 ; # this is loop2, the inner loop
do
Statement(s) to be executed if command2 is true
done
Statement(s) to be executed if command1 is true
done
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17. Example1 : While loop to display numbers
from 1 to 10.
number = 1
while [ $number –lt 11 ]
do echo $number
((number++))
done
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18. Example2
INPUT_STRING=hello
while [ "$INPUT_STRING" != "bye" ]
do
echo "Please type something in (bye to quit)"
read INPUT_STRING
echo "You typed: $INPUT_STRING"
done
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19. Example3
a=0
while [ "$a" -lt 10 ] # this is loop1
do
b="$a"
while [ "$b" -ge 0 ] # this is loop2
do
echo -n "$b "
b=`expr $b - 1`
done
echo
a=`expr $a + 1`
done
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20. #!/bin/sh
# This script finds the greatest common divisor (GCD) of two
numbers using the Euclidean algorithm
echo "Enter the first number: "
read num1
echo "Enter the second number: "
read num2
# Make sure num1 is greater than or equal to num2
if [ $num1 -lt $num2 ]
then
temp=$num1
num1=$num2
num2=$temp
fi
while [ $num2 -ne 0 ]
do
temp=$num2
num2=$((num1 % num2))
num1=$temp
done
echo "The greatest common divisor of the two numbers is
$num1."
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Example4: Shell script for finding the greatest common
divisor (GCD) of two numbers using the Euclidean
algorithm:

21. For Loop
• For loop is another type of looping statement to execute a set of
commands for a certain number of times.
for var in list
do
command 1
command 2
done
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22. Example1
for p_name in Ram Shyam Mahesh Rajesh
do
echo $p_name
done
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23. Example 2: Write a script to calculate factorial
of no
#!/bin/sh
# This script calculates the factorial of a given number
echo "Enter a number: "
read num
factorial=1
i=1
while [ $i -le $num ]
do
factorial=$((factorial * i))
i=$((i + 1))
done
echo "The factorial of $num is $factorial"
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