Operating_System_Lab_ClassOperating_System_2.pdf

How do you run a shell script from the command line?
To run a shell script from the command line, we need to
follow these steps:
•Make sure the script file has executable permissions using
the chmod command:
chmod +x myscript.sh
•Execute the script using its filename:
./myscript.sh

#!/bin/bash
#Creates a new variable with a value of "Hello World“
learningbash="Hello World"
echo $learningbash
The first line (/bin/bash) is used in every bash script. It instructs the operating system
to use a bash interpreter as a command interpreter.
Hello World is the most simple bash script to start with. We will create a new variable
called learningbash and print out the words Hello World. First, open a new shell script file with a
text editor of your choice

What is the difference between single quotes (‘) and double quotes (“) in shell
scripting?
Single quotes (‘) and double quotes (“) are used to enclose strings in shell scripting, but
they have different behaviors:
•Single quotes: Everything between single quotes is treated as a literal string. Variable
names and most special characters are not expanded.
•Double quotes: Variables and certain special characters within double quotes are
expanded. The contents are subject to variable substitution and command substitution.
#!/bin/bash
abcd=”Hello”
echo ‘$abcd’ # Output: $abcd
echo “$abcd” # Output: Hello

#!/bin/bash
#Creates a new variable
date=“Monday”
echo ‘$date’
echo “$date”
echo `date`

#!/bin/bash
echo "What is your age?"
read age
echo "Wow, you look younger than $age years old"
Get User Input
To take input from users, we’ll use the read bash command. First, create a
new bash shell file

How can you use command-line arguments in a shell script?
Command-line arguments are values provided to a script when it’s executed. They can be accessed within
the script using special variables like $1, $2, etc., where $1 represents the first argument, $2 represents the
second argument, and so on.
For Example: If our script name in `example.sh`
#!/bin/bash
echo “Script name: $0”
echo “First argument: $1”
echo “Second argument: $2”

Conditional Statements
The most popular and widely used conditional statement is if. Even though the if statement is
easy to write and understand, it can be used in advanced shell scripts as well.
#!/bin/bash
salary=1000
expenses=800
#Check if salary and expenses are equal
if [ $salary == $expenses ]
then
echo "Salary and expenses are equal" #Check if salary and
expenses are not equal
elif [ $salary != $expenses ]
then
echo "Salary and expenses are not equal"
fi

Check if a Number is Even or Odd
Odd and even numbers can be easily divided using the if statement and some simple math.
Create a file named evenoddnumbers.sh
#!/bin/bash
read -p "Enter a number and I will check if its odd or even " mynumber
if [ $((mynumber%2)) -eq 0 ]
then
echo "Your number is even"
else
echo "Your number is odd."
fi

#!/bin/bash
read -p "Please enter your choice: " response
## If the response given did not consist entirely of digits
if [[ ! $response =~ ^[0-9]*$ ]]
then
## If it was Quit or quit, exit
[[ $response =~ [Qq]uit ]] && exit
## If it wasn't quit or Quit but wasn't a number either,
## print an error message and quit.
echo "Please enter a number between 0 and 100 or "quit" to exit" && exit
Fi
## Process the other choices
if [ $response -le 59 ]
then
echo "F"
elif [ $response -le 69 ]
then
echo "D"
then
echo "C"
Then
echo "B"
then
echo "A"
elif [ $response -gt 100 ]
then
echo "Please enter a number between 0 and 100"
exit
fi

Loops
A loop is an essential tool in various programming languages. To put it simply,
a bash loop is a set of instructions that are repeated until a user-specified
condition is reached. Start by creating a loop bash program:
#!/bin/bash
n=0
while :
do
echo Countdown: $n
((n++))
done
This will work as a
countdown to infinity until
you press CTRL + C to stop
the script.

#!/bin/bash
for (( n=2; n<=10; n++ ))
do
echo "$n seconds"
done

Create an Array
A bash array is a data structure designed to store information in an indexed way. It is extra
useful if users need to store and retrieve thousands of pieces of data fast. What makes bash
arrays special is that unlike any other programming language, they can store different types
of elements. For example, you can use a bash array to store both strings and numbers.
#!/bin/bash
# Create an indexed array
IndexedArray=(egg burger milk)
#Iterate over the array to get all the values
for i in "${IndexedArray[@]}";do echo "$i";done

Generate Factorial of Number
#!/bin/bash
echo Enter the number you want to get factorial
read mynumber
factorial=1
for ((i=1;i<=mynumber;i++))
do
factorial=$(($factorial*$i))
done
echo $factorial

#!/bin/bash
echo "Enter a number:"
read number
i=2
if [ $number -lt 2 ]
then
echo "$number is not a prime number."
exit
fi
while [ $i -lt $number ]
do
if [ èxpr $number % $i` -eq 0 ]
then
exit
fi
i=èxpr $i + 1`
done
echo "$number is a prime number."
Shell Script to Check Prime Number
#!/bin/bash
echo "Enter a number:"
read number
i=2
if [ $number -lt 2 ]
then
exit
fi
max=ècho "sqrt($number)" | bc`
while [ $i -le $max ]
do
if [ èxpr $number % $i` -eq 0 ]
then
exit
fi
i=èxpr $i + 1`
done
echo "$number is a prime number."

#!/bin/bash
echo "Enter a number"
read num
reverse=0
while [ $num -gt 0 ]
do
remainder=$(( $num % 10 ))
reverse=$(( $reverse * 10 + $remainder ))
num=$(( $num / 10 ))
done
echo "Reversed number is : $reverse"
Script for reversing a number

# Program to print the
# given pattern
# Static input for N
N=5
# variable used for
# while loop
i=0
j=0
while [ $i -le èxpr $N - 1` ]
do
j=0
while [ $j -le èxpr $N - 1` ]
do
if [ èxpr $N - 1` -le èxpr $i + $j` ]
then
# Print the pattern
echo -ne "#"
else
# Print the spaces required
echo -ne " "
fi
j=èxpr $j + 1`
done
# For next line
echo
i=èxpr $i + 1`
done
Output:
#
##
###
####
#####

Functions
A bash function is a set of commands that can be reused numerous times
throughout a bash script.
#!/bin/bash
hello () {
echo 'Hello World!’
}
hello

Display String Length
#!/bin/bash
# Create a new string
mystring="lets count the length of this string"
i=${#mystring}
echo "Length: $i"
Extract String
#!/bin/bash
cut -d , -f 5 <<< "Website,Domain,DNS,SMTP,5005"
#!/bin/bash
expr substr "458449Hostinger4132" 7 9
Find and Replace String
#!/bin/bash
first="I drive a BMW and Volvo"
second="Audi"
echo "${first/BMW/"$second"}"
Concatenate Strings
#!/bin/bash
firststring="The secret is..."
secondstring="Bash"
thirdstring="$firststring$secondstring"
echo "$thirdstring"
#!/bin/bash
firststring="The secret is..."
firststring+="Bash"
echo "$firststring"

Read Files
#!/bin/bash
myvalue=`cat mysamplefile.txt`
echo "$myvalue"
Print Files With Line Count
#!/bin/bash
myfile='cars.txt’
i=1
while read lines;
do
echo "$i : $lines"
i=$((i+1))
done < $myfile
Delete Files
#!/bin/bash
myfile='cars.txt’
touch $myfile
if [ -f $myfile ]; then
rm cars.txt
echo "$myfile deleted"
fi
Test if File Exists
#!/bin/bash
MyFile=cars.txt
if [ -f "$MyFile" ]; then
echo "$MyFile exists."
else
echo "$MyFile does not exist.“
fi

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