In this lesson, we will delve into an essential aspect of shell scripting — Control Structures and Logical Operators. Control structures such as if, elif, and else allow you to make decisions within your scripts, enabling them to respond intelligently based on different conditions. Additionally, logical operators like && (AND) and || (OR) help you combine multiple conditions to create more complex logic.

Let's get started!

The if/elif/else control structure allows you to perform different actions based on various conditions. Here’s the syntax:

Bash
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1if [ condition1 ]
2then
3    # Commands to execute if condition1 is true
4elif [ condition2 ]
5then
6    # Commands to execute if condition1 is false and condition2 is true
7else
8    # Commands to execute if both condition1 and condition2 are false
9fi

Each condition must be enclosed in []. After the condition, the then keyword specifies the code to execute if the condition evaluates to 0 (true).

All control structures must start with an if statement. The elif and else statements are optional. Each elif condition is checked until one of the conditions evaluates to true. If none of the if or elif conditions are true, the else statement is executed.

The control structure must end with fi.

Let's take a look at an example to determine if a number is positive, negative, or neither.

Bash
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1#!/bin/bash
2# Script to demonstrate if-else control structure
3number=5
4
5if [ $number -gt 0 ]
6then
7   echo "The number is positive"  # Prints: "The number is positive"
8elif [ $number -lt 0 ]
9then
10   echo "The number is negative"  # This won't execute
11else
12   echo "The number is zero"  # This won't execute
13fi

• number=5 initializes the variable number with the value 5.
• The if statement checks if number is greater than 0.
• If true, it prints "The number is positive".
• If the first condition is false, the elif statement checks if number is less than 0.
• If the elif condition is also false, the else statement executes, printing "The number is zero".

Logical operators allow you to combine multiple conditions in your control structures. The && (AND) operator ensures both conditions must be true, while the || (OR) operator requires at least one condition to be true.

Let's examine how they work:

The && (AND) operator is used to combine multiple conditions in a control structure such that all the conditions must be true for the block of commands to execute. For example:

Bash
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1#!/bin/bash
2# Variables
3num1=10
4num2=20
5str1="Hello"
6str2="World"
7
8# Using && (AND) Operator
9if [ $num1 -lt $num2 ] && [ "$str1" != "$str2" ]
10then
11    echo "num1 is less than num2 AND str1 is not equal to str2"  # Prints: "num1 is less than num2 AND str1 is not equal to str2"
12else
13    echo "Condition is false"  # This won't execute
14fi

• num1 is 10, num2 is 20, str1 is "Hello", and str2 is "World".
• The if statement checks if num1 is less than num2 AND str1 is not equal to str2.
• Both conditions are true, so the script prints: "num1 is less than num2 AND str1 is not equal to str2".

The || (OR) operator is used to combine multiple conditions in a control structure such that at least one of the conditions must be true for the block of commands to execute. For example:

Bash
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1#!/bin/bash
2
3# Variables
4num1=10
5num2=20
6
7# Using || (OR) Operator
8if [ $num1 -gt 5 ] || [ $num2 -lt 5 ]
9then
10    echo "num1 is greater than 5 OR num2 is less than 5"  # Prints: "num1 is greater than 5 OR num2 is less than 5"
11else
12    echo "Neither condition is true"  # This won't execute
13fi

• The if statement checks if num1 is greater than 5 OR num2 is less than 5.
• Since num1 is indeed greater than 5, the script prints: "num1 is greater than 5 OR num2 is less than 5".

Excellent work! In this lesson, you explored the fundamental concepts of control structures and logical operators in shell scripting. You learned about if-else statements and how to combine conditions using && and || operators.

These constructs are powerful tools that enable you to write more intelligent and responsive scripts. Understanding and applying these concepts will greatly enhance your ability to automate tasks and solve real-world problems.

Now, it's time to put your newfound knowledge to the test! Move on to the practice sections where you will apply what you've learned through hands-on exercises. Happy scripting!