Control Structures and Logical Operators in Shell Scripting | CodeSignal Learn

Control Structures and Logical Operators in Shell Scripting | CodeSignal Learn

Introduction to Control Structures and Logical Operators

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!

Syntax of if/elif/else Statements

The if/elif/else control structure allows you to perform different actions based on various conditions. Here’s the syntax: Shell if [ condition1 ] then # Commands to execute if condition1 is true elif [ condition2 ] then # Commands to execute if condition1 is false and condition2 is true else # Commands to execute if both condition1 and condition2 are false fi 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 .

Using If-Else Statements

Let's take a look at an example to determine if a number is positive, negative, or neither. Shell #!/bin/bash # Script to demonstrate if-else control structure number=5 if [ $number -gt 0 ] then echo "The number is positive" # Prints: "The number is positive" elif [ $number -lt 0 ] then echo "The number is negative" # This won't execute else echo "The number is zero" # This won't execute fi 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".

Introducing Logical Operators

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Using the && Operator

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: Shell #!/bin/bash # Variables num1=10 num2=20 str1="Hello" str2="World" # Using && (AND) Operator if [ $num1 -lt $num2 ] && [ "$str1" != "$str2" ] then 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" else echo "Condition is false" # This won't execute fi 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" .

Using the || Operator

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: Shell #!/bin/bash # Variables num1=10 num2=20 # Using || (OR) Operator if [ $num1 -gt 5 ] || [ $num2 -lt 5 ] then echo "num1 is greater than 5 OR num2 is less than 5" # Prints: "num1 is greater than 5 OR num2 is less than 5" else echo "Neither condition is true" # This won't execute fi 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" .