Conditional Statement And Loops In Shell Script

Conditional Statement

-> Conditional Statement is used to perform a certain task based on some conditions.

IF-ELSE

#!/bin/bash

read -p "Enter Your Age - " age

if [ $age -lt 18 ]
then
        echo "You can not vote"
else
        echo "You can vote"
fi

Output -

ELIF

#!/bin/bash

read -p "Enter Your Marks - " marks

if [ $marks -gt 40 ] && [ $marks -lt 60 ]
then
        echo "You got 2nd Division"
elif [ $marks -ge 60 ]
then
        echo "You got 1st Division"
else
         echo "You are fail"
fi

Output -

CASE

-> CASE is similar to the switch case in other programming language.

Example -

#!/bin/bash

echo "Select One Option - "

echo "a = To see the current date"
echo "b = To see the list of files in current directory"

read choice

case $choice in
        a) date;; # We have to use ;; at the last of the command
        b) ls;;
        *) echo "Please Enter Valid Input"
esac  # We have to end the case with esac

Output -

We can use multiple commands as -

#!/bin/bash

echo "Select One Option - "

echo "a = To see the current date"
echo "b = To see the list of files in current directory"

read choice

case $choice in
        a)
                echo "Hello , We are going to print date"
                date
                echo "Date is printed"
                ;;
        b) ls;;
        *) echo "Please Enter Valid Input"
esac

Output -

Logical Operators

-> We are going to see about &&, || and ! operator.

&&

-> It is a logical AND operator. Here all the conditions must have to be true then only the result will be true else it will be false.

#!/bin/bash

read -p "Enter your age " age
read -p "Enter your country  " country

if [[ $age -ge 18 ]] && [[ $country == "India" ]] # In case of string we use "==" and in case of numeric we use "-eq"
then
        echo "You can vote in India"
else
        echo "You can not vote in India"
fi

Output -

||

-> It is a logical OR operator. Here if any of the conditions is true then the result will be true. If all conditions are false then only the result will be false.

#!/bin/bash

read -p "Enter your age " age
read -p "Enter your country  " country

if [[ $age -ge 18 ]] || [[ $country == "India" ]] # In case of string we use "==" and in case of numeric we use "-eq"
then
        echo "You can vote in India"
else
        echo "You can not vote in India"
fi

Output -

Loops

1. For Loop - If we have to perform repetitive tasks then we can use the loop.

    #!/bin/bash
   
    for i in 1 2 3 4 5
    do
            echo "print $i"
    done
   

Output -

-> Other way to use For Loop

#!/bin/bash

for i in {1..5}
do
        echo "print $i"
done

Output -

Read the values from a File using For Loop -

Example- filename = name.txt

Bhaskar
Mehta
Raju

#!/bin/bash

file=/home/ubuntu/test.txt

for i in $(cat $file)
do
        echo "Name is $i"
done

Output -

Read the Values From Array Using For Loop

#!/bin/bash

myArray=(1 2 45 65 "Bhaskar")

arrayLength=${#myArray[*]}
echo "Length of the Array is $arrayLength"

for (( i=0; i<$arrayLength; i++ ))
do
        echo "Value is - ${myArray[i]}"
done

Output -

While Loop

-> It will run till the condition is true

#!/bin/bash

count=0
sum=10
while [[ $sum -ge $count ]]
do
        echo "Count is - $count"
        let count++
done

Output -

Until Loop

-> It is just the opposite of While Loop. i.e. It will run till the condition is false

#!/bin/bash

count=10
sum=5

until [[ $count -lt $sum ]]
do
        echo "Count is $count"
        let count--
done

Output -

Infinite Loop Using While Loop

-> If we have to run the commands infinite times then we can use this.

#!/bin/bash

while true
do
        echo "Hello"
        sleep 2s # Every 2s it will print Hello
done

Output -

Infinite Loop Using For Loop

#!/bin/bash

for (( ;; ))
do
        echo "Hello"
        sleep 2s # Every 2s it will print Hello
done