Control Structures and Logical Operators in Shell Scripting

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:

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.

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

#!/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".

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: