How to use if-else statement in Linux?

Using If-Else Statements in Linux

In Linux, as in many other programming languages, the if-else statement is a fundamental control structure used to make decisions based on certain conditions. The if-else statement allows your program to execute different blocks of code depending on whether a particular condition is true or false.

The basic syntax for an if-else statement in Linux is as follows:

if [ condition ]
then
    # code to be executed if the condition is true
else
    # code to be executed if the condition is false
fi

Let's break down the syntax:

• if: This keyword initiates the conditional statement.
• [ condition ]: This is the condition that will be evaluated. The condition can be a comparison of variables, a test of file attributes, or any other valid expression that evaluates to either true or false.
• then: This keyword indicates the start of the code block to be executed if the condition is true.
• else: This keyword introduces the code block to be executed if the condition is false.
• fi: This keyword marks the end of the if-else statement.

Here's an example of using an if-else statement in Linux:

#!/bin/bash

# Prompt the user to enter a number
echo "Enter a number: "
read num

# Check if the number is positive
if [ $num -gt 0 ]
then
    echo "The number is positive."
else
    echo "The number is not positive."
fi

In this example, the user is prompted to enter a number. The program then checks if the number is positive (greater than 0) using the [ $num -gt 0 ] condition. If the condition is true, the program prints "The number is positive." If the condition is false, the program prints "The number is not positive."

You can also use the elif (else if) keyword to add additional conditions to your if-else statement:

#!/bin/bash

# Prompt the user to enter a number
echo "Enter a number: "
read num

# Check the number
if [ $num -gt 0 ]
then
    echo "The number is positive."
elif [ $num -lt 0 ]
then
    echo "The number is negative."
else
    echo "The number is zero."
fi

In this example, the program first checks if the number is positive. If it's not positive, it checks if the number is negative. If the number is neither positive nor negative, it must be zero.

To help visualize the flow of an if-else statement, here's a Mermaid diagram:

The diagram shows that the program first evaluates the condition. If the condition is true, the 'then' block is executed. If the condition is false, the 'else' block is executed. Finally, the program ends.

Using if-else statements is a fundamental skill in Linux shell scripting and programming. They allow you to create dynamic and responsive programs that can make decisions based on various conditions. By understanding and mastering the use of if-else statements, you can write more powerful and versatile Linux scripts and applications.