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Control Statements

Control statements are used to direct the flow of execution in a script. The primary control statements in UNIX/Linux are:

  1. if statement
  2. case statement
  3. while loop
  4. for loop
  5. until loop
  6. break
  7. continue
  8. exit

1. if Statement

The if statement evaluates a condition and executes code blocks based on whether the condition is true or false.

Types of if Statements

  1. Simple if
  2. if-else
  3. Nested if
  4. Ladder if

Simple if

The simple if statement executes a block of code only if the condition is true.

Syntax:

if [ condition ]
then
    action
fi

If the condition evaluates to true, the action will be executed.

Example (Q1):
Write a script to read a name from the user and display a special message if the name is "pranita."

#! /bin/bash

read -p "Enter Your Name:" name

if [ $name = "pranita" ]
then
    echo "Hello pranita Very Good Evening"
fi

echo "How are you"

Execution:

durgasoft@durgasoft:~/scripts$ ./test.sh
Enter Your Name:durga
How are you

durgasoft@durgasoft:~/scripts$ ./test.sh
Enter Your Name:pranita
Hello pranita Very Good Evening
How are you

Note:

  • x=10 → Assignment
  • x = 10 → Comparison (incorrect syntax, spaces matter in shell scripting)

if-else

The if-else statement executes one block of code if the condition is true, and another block of code if the condition is false.

Syntax:

if [ condition ]
then
    action if condition is true
else
    action if condition is false
fi

Nested if

The nested if allows embedding an if statement inside another if statement.

Syntax:

if [ condition ]
then
    # Outer condition is true
    if [ condition ]
    then
        # Inner condition is true
        action
    else
        # Inner condition is false
        action
    fi
else
    # Outer condition is false
    action
fi

Ladder if

The ladder if is used for evaluating multiple conditions sequentially.

Syntax:

if [ condition ]
then
    action-1
elif [ condition ]
then
    action-2
elif [ condition ]
then
    action-3
else
    action for all other cases
fi

Q2 Write a Script to Find the Greater of 2 Numbers

#! /bin/bash  
read -p "Enter First Number:" a 
read -p "Enter Second Number:" b  

if [ $a -gt $b ] 
then
    echo "Greater Number is:$a"
else
    echo "Greater Number is:$b"
fi

Example Output:

durgasoft@durgasoft:~/scripts$ test.sh 
Enter First Number:10 
Enter Second Number:20 
Greater Number is:20 

durgasoft@durgasoft:~/scripts$ test.sh 
Enter First Number:20 
Enter Second Number:10 
Greater Number is:20 

durgasoft@durgasoft:~/scripts$ test.sh 
Enter First Number:-10 
Enter Second Number:-20 
Greater Number is:-10

Q3 Write a Script to Check Whether Numbers Are Equal or Not

If the Numbers Are Not Equal, Print the Greater Number.

#! /bin/bash  
read -p "Enter First Number:" a 
read -p "Enter Second Number:" b  

if [ $a -eq $b ] 
then
    echo "Both Numbers are equal"
elif [ $a -gt $b ] 
then
    echo "First Number is greater than Second Number"
else
    echo "First Number is less than Second Number"
fi

Write a Script to Find the Greater of 3 Numbers

1st Way:

#! /bin/bash  
read -p "Enter First Number:" a 
read -p "Enter Second Number:" b 
read -p "Enter Third Number:" c  

if [ $a -gt $b ] 
then
    if [ $a -gt $c ] 
    then
        echo "The Greater Number:$a"
    else
        echo "The Greater Number:$c"
    fi
elif [ $b -gt $c ] 
then
    echo "The Greater Number:$b"
else
    echo "The Greater Number:$c"
fi

2nd Way:

#! /bin/bash  
read -p "Enter First Number:" a 
read -p "Enter Second Number:" b 
read -p "Enter Third Number:" c  

if [ $a -gt $b -a $a -gt $c ] 
then
    echo "The Greater Number: $a"
elif [ $b -gt $c ] 
then
    echo "The Greater Number: $b"
else
    echo "The Greater Number: $c"
fi

WAS: Check Whether a Student is Pass or Fail Based on 3 Subject Marks

If Any Subject Marks Are Less Than 35, the Status is Fail.

1st Way:

#! /bin/bash  
read -p "Enter First Subject Marks:" a 
read -p "Enter Second Subject Marks:" b 
read -p "Enter Third Subject Marks:" c  

if [ $a -lt 35 ] 
then
    echo "Student Failed"
elif [ $b -lt 35 ] 
then
    echo "Student Failed"
elif [ $c -lt 35 ] 
then
    echo "Student Failed"
else
    echo "Result is Pass"
fi

2nd Way:

#! /bin/bash  
read -p "Enter First Subject Marks:" a 
read -p "Enter Second Subject Marks:" b 
read -p "Enter Third Subject Marks:" c  

if [ $a -ge 35 -a $b -ge 35 -a $c -ge 35 ] 
then
    echo "Result is Pass"
else
    echo "Result is Fail"
fi

The Funniest Example with if-else

#! /bin/bash  
read -p "Enter Your Favourite Brand:" brand  

if [ $brand = "KF" ] 
then
    echo "It is Children's Brand"
elif [ $brand = "KO" ] 
then
    echo "It is not that much Kick"
elif [ $brand = "RC" ] 
then
    echo "It is too light"
elif [ $brand = "FO" ] 
then
    echo "Buy One Get One FREE"
else
    echo "Other Brands are not recommended"
fi

Example Output:

durgasoft@durgasoft:~/scripts$ ./test.sh 
Enter Your Favourite Brand:KF 
It is Children's Brand

durgasoft@durgasoft:~/scripts$ ./test.sh 
Enter Your Favourite Brand:KO 
It is not that much Kick

durgasoft@durgasoft:~/scripts$ ./test.sh 
Enter Your Favourite Brand:RC 
It is too light

durgasoft@durgasoft:~/scripts$ ./test.sh 
Enter Your Favourite Brand:FO 
Buy One Get One FREE

durgasoft@durgasoft:~/scripts$ ./test.sh 
Enter Your Favourite Brand:KALYANI 
Other Brands are not recommended