What is Conditional Execution in Linux?

Conditional execution is a fundamental concept in Linux and other programming languages that allows you to control the flow of a program based on certain conditions. To answer the question of what is conditional execution in Linux in simple words, it enables you to make decisions in your script or command based on whether specific conditions are true or false. This is achieved through conditional statements and operators, which are essential for writing powerful and flexible scripts in Linux.

Conditional execution is essential in automating tasks, creating scripts, and handling various situations in the Linux environment. By using conditional statements, you can perform different actions based on the state of variables, user input, or system conditions. Understanding what is conditional execution in Linux, how it works, and how to use it effectively can significantly enhance your Linux scripting skills and make your programs more robust and versatile.

In this article, we will look at what is conditional execution in Linux, explore the syntax of conditional execution, different types of test operators, logical operators (&& and ||), IF/THEN statements, ELSE and ELIF clauses, and the case statement in Linux. Let's dive in and discover the power of what is conditional execution in the Linux operating system.

Syntax

Before we dive into the various aspects of conditional execution, let's start with the basic syntax. In Linux, conditional execution is often implemented using the if statement. The general syntax for an if statement is as follows:

The if statement checks the specified condition, and if it evaluates to true (non-zero), the commands inside the then block are executed. If the condition is false (zero), the commands inside the then block are skipped, and the program proceeds to the next part of the script.

For example, let's say we want to check if a variable age is greater than 18. If it is, we print a message indicating that the person is an adult:

In this example, the condition [ $age -gt 18 ] checks if the value of the variable age is greater than 18. Since age is assigned the value 20, the condition is true, and the message "You are an adult" will be printed.

File Test Operators

File test operators in Linux are used to check various attributes of files, such as existence, type, permissions, and modification time. These operators are often used in conditional statements to decide the flow of the program based on file-related conditions.

Here are some common file test operators:

1. -e FILE

   This operator checks if the specified file exists. If the file exists, the condition evaluates to true. For example:

2. -f FILE

   The -f operator checks if the given file is a regular file (not a directory or a device file). For instance:

3. -d FILE

   The -d operator checks if the provided file is a directory. For example:

4. -r FILE

   This operator verifies if the specified file is readable. If the file is readable, the condition becomes true. For instance:

5. -w FILE

   The -w operator checks if the given file is writable. If it is writable, the condition evaluates to true. For example:

6. -x FILE

   This operator determines if the specified file is executable. If the file is executable, the condition becomes true. For instance:

7. -s FILE

   The -s operator checks if the file exists and has a size greater than zero (i.e., it is not empty). For example:

String Test Operators

String test operators in Linux are used to compare strings and perform conditional execution based on the results. These operators enable you to check for string equality, non-equality, and emptiness.

Here are some common string test operators:

1. = and !=

   The = operator checks if two strings are equal, and the != operator checks if two strings are not equal. For example:

2. -z STRING

   The -z operator checks if the provided string is empty (has zero length). For instance:

3. -n STRING

   This operator verifies if the given string is not empty (has a length greater than zero). For example:

Arithmetic Tests

Arithmetic tests in Linux involve performing numerical comparisons using arithmetic operators. These tests are especially useful for evaluating mathematical conditions within conditional statements.

Here are some common arithmetic test operators:

1. -eq and -ne

   The -eq operator checks if two numbers are equal, and the -ne operator checks if two numbers are not equal. For example:

2. -lt and -le

   The -lt operator checks if the first number is less than the second number, and the -le operator checks if the first number is less than or equal to the second number. For instance:

3. -gt and -ge

   The -gt operator checks if the first number is greater than the second number, and the -ge operator checks if the first number is greater than or equal to the second number. For example:

&& and ||

In conditional execution, && (AND) and || (OR) are logical operators used to combine multiple conditions. They provide a way to execute commands based on the evaluation of more than one condition.

1. && (AND) Operator

   The && operator allows you to execute a command only if the preceding command or condition evaluates to true. The subsequent command will not be executed if the preceding condition is false. Here's an example:

   In this example, the message will be displayed only if both conditions, $age -gt 18 and $balance -gt 0, are true.

2. || (OR) Operator

   On the other hand, the || operator allows you to execute a command if the preceding command or condition evaluates to false. If the preceding condition is true, the subsequent command will not be executed. Here's an example:

   In this example, the message will be displayed if either condition, $age -lt 18 or $balance -lt 0, is true.

IF/THEN

The IF/THEN statement is the most basic form of conditional execution in Linux. It allows you to execute a block of commands if a specified condition is true. If the condition is false, the block of commands is skipped, and the program continues with the next part of the script.

Here's a more detailed representation of the IF/THEN statement:

For example, let's say we want to check if a user's age is greater than or equal to 21, and if it is, we display a message indicating they are eligible to purchase alcohol:

In this script, the user is prompted to enter their age. If the entered age is greater than or equal to 21, the message "You are eligible to purchase alcohol." will be displayed.

The IF/THEN statement can also be extended with the ELSE and ELIF clauses, as we'll see in the following sections.

ELSE

The ELSE clause and the IF/THEN statement are used to provide an alternative set of commands to execute when the condition specified in the IF statement is false.

The syntax for using ELSE is as follows:

For example, let's modify the previous age check script to include an ELSE clause. If the user's age is less than 21, we'll display a message indicating they are not eligible to purchase alcohol:

In this updated script, if the entered age is less than 21, the message "You are not eligible to purchase alcohol." will be displayed.

The ELSE clause allows you to handle both true and false conditions and make your scripts more comprehensive and user-friendly.

ELIF

The ELIF clause (short for "else if") is used in conjunction with the IF/THEN statement to specify additional conditions to check if the initial IF condition is false.

The syntax for using ELIF is as follows:

You can have multiple ELIF clauses to test multiple conditions in sequence until a true condition is found, or none of the conditions are true, and the ELSE block is executed.

Let's see an example of using ELIF to check a user's exam score:

In this example, the script will determine the appropriate grade based on the exam score entered by the user.

The ELIF clause provides a flexible way to handle multiple conditions and make complex decisions in your Linux scripts.

case Statement

The case statement in Linux provides a powerful way to perform conditional execution based on the value of a variable. It is particularly useful when you need to compare the same variable against multiple values.

The syntax for using the case statement is as follows:

The case statement compares the value of $variable against each pattern until a match is found. When a match is found, the corresponding block of commands is executed. If none of the patterns match, the commands under *) are executed.

Here's an example of using the case statement to handle different fruits entered by the user:

In this script, the user's input is compared against three fruit names: "apple", "banana", and "orange". The appropriate message is displayed based on the user's selection. If the input does not match any of the specified fruits, the "Unknown fruit." message will be displayed.

The case statement is an elegant way to simplify conditional execution when dealing with multiple possible values of a variable.