How to use if statements to process inputs?

Using If Statements to Process Inputs in Shell

In the world of shell scripting, if statements are a powerful tool for making decisions based on the input or conditions encountered during the script's execution. These conditional statements allow you to control the flow of your script, enabling it to take different actions depending on the evaluation of specific criteria.

Understanding If Statements

The basic structure of an if statement in shell scripting is as follows:

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

The [ condition ] part is where you specify the condition you want to evaluate. This condition can be a comparison of variables, a check for the existence of a file, or any other logical expression that can be evaluated as true or false.

Here's an example of an if statement that checks if the value of a variable x is greater than 10:

x=15
if [ $x -gt 10 ]
then
    echo "The value of x is greater than 10."
else
    echo "The value of x is less than or equal to 10."
fi

In this example, the condition [ $x -gt 10 ] checks if the value of the variable x is greater than 10 (-gt is the "greater than" operator). If the condition is true, the statements within the then block are executed; otherwise, the statements within the else block are executed.

Processing User Input

One of the common use cases for if statements in shell scripting is to process user input. This allows your script to adapt its behavior based on the user's responses or choices.

Here's an example of a script that prompts the user for their name and then greets them accordingly:

echo "What is your name?"
read name

if [ -z "$name" ]
then
    echo "You didn't enter a name."
else
    echo "Hello, $name!"
fi

In this example, the read command is used to capture the user's input and store it in the name variable. The if statement then checks if the name variable is empty (-z is the "is empty" operator). If the user didn't enter a name, the script prints a message indicating that. Otherwise, it greets the user by name.

Nested If Statements

Sometimes, you may need to perform more complex decision-making in your script. In such cases, you can use nested if statements, where an if statement is placed within another if statement.

Here's an example of a script that checks the user's age and provides different responses based on their age:

echo "How old are you?"
read age

if [ $age -lt 18 ]
then
    echo "You are a minor."
else
    if [ $age -ge 18 ] && [ $age -lt 65 ]
    then
        echo "You are an adult."
    else
        echo "You are a senior citizen."
    fi
fi

In this example, the outer if statement checks if the user's age is less than 18. If it is, the script prints a message indicating that the user is a minor. If the user's age is 18 or greater, the inner if statement checks if the age is less than 65. If it is, the script prints a message indicating that the user is an adult. If the user's age is 65 or greater, the script prints a message indicating that the user is a senior citizen.

Visualizing the Concept with a Mermaid Diagram

Here's a Mermaid diagram that illustrates the flow of an if statement in shell scripting:

This diagram shows the basic structure of an if statement, where the condition is evaluated, and the script then executes the appropriate block of code based on the result.

In conclusion, if statements are a fundamental part of shell scripting, allowing you to make decisions and control the flow of your script based on various conditions. By understanding how to use if statements to process user input, you can create more robust and adaptable shell scripts that can handle a wide range of scenarios.