How to Use Bash If Statements (With 4 Examples)

Bash (short for “Bourne Again SHell”) is a popular command-line shell and scripting language used in Unix-like operating systems. It provides powerful features for automating tasks, making decisions based on conditions, and controlling the flow of scripts. One of the most fundamental and versatile constructs in Bash scripting is the if statement. In this article, we will explore the usage of if statements in Bash, along with four practical examples to illustrate its capabilities.

What is the if statement?

The if statement in Bash allows you to execute a block of code conditionally based on the evaluation of an expression. The syntax for the if statement is as follows:

bash

if [ condition ]; then
# Code block to execute if the condition is true
else
# Code block to execute if the condition is false
fi


The condition is a logical expression that can be evaluated to either true or false. If the condition is true, the code inside the first block is executed; otherwise, the code inside the else block (if provided) is executed. The fi keyword marks the end of the if statement.

Using Comparison Operators

In Bash, you can use various comparison operators to create conditions for the if statement. Here are some commonly used comparison operators:

• Equal: == or =
• Not equal: !=
• Less than: <
• Greater than: >
• Less than or equal: <=
• Greater than or equal: >=

Now, let’s dive into four examples to understand the practical application of the if statement.

Example 1: Checking File Existence

Suppose you want to check whether a file exists in a directory before performing certain actions. The following Bash script demonstrates how to do this:

bash

#!/bin/bash

file_path=“/path/to/your/file.txt”

if [ -f “$file_path“ ]; then
echo “File exists at: $file_path“
else
echo “File not found at: $file_path“
fi

Explanation:

• The -f flag is used with the file path to check if it exists and is a regular file.
• The script first assigns the file path to the file_path variable. Replace /path/to/your/file.txt with the actual path of the file you want to check.
• The if statement checks if the file exists using [ -f "$file_path" ]. If the condition evaluates to true, it prints the message confirming the file’s existence; otherwise, it prints the message stating that the file was not found.

Example 2: User Input Validation

User input validation is a common requirement in many Bash scripts. You may want to ensure that users enter specific types of data or adhere to certain formats. Let’s see how you can use an if statement to validate user input for a positive number:

bash

#!/bin/bash

read -p “Enter a positive number: “ num

if [ “$num“ -gt 0 ]; then
echo “You entered a positive number: $num“
else
echo “Invalid input. Please enter a positive number.”
fi

Explanation:

• The script uses the read command to prompt the user to enter a positive number.
• The user input is stored in the num variable.
• The if statement checks whether the value of num is greater than zero (-gt 0). If the condition is true, it prints the message confirming the input as a positive number; otherwise, it notifies the user of invalid input.

Example 3: Nested if Statements

In Bash scripting, you can also use nested if statements to create more complex conditions. Let’s consider an example where we check both the file existence and its read permission:

bash

#!/bin/bash

file_path=“/path/to/your/file.txt”

if [ -f “$file_path“ ]; then
if [ -r “$file_path“ ]; then
echo “File exists and is readable.”
else
echo “File is not readable.”
fi
else
echo “File not found at: $file_path“
fi

Explanation:

• The script first checks if the file exists using the outer if statement with [ -f "$file_path" ].
• If the file exists, it enters the inner if statement to check if the file is readable using [ -r "$file_path" ].
• Depending on the evaluation of both conditions, the script prints the appropriate message.

Example 4: Using Logical Operators

Logical operators allow you to combine multiple conditions to create more sophisticated if statements. Here, we’ll demonstrate an example where we check if a number is both positive and even:

bash

#!/bin/bash

read -p “Enter a number: “ num

if [ “$num“ -gt 0 ] && [ “$((num % 2))” -eq 0 ]; then
echo “You entered a positive and even number: $num“
else
echo “Invalid input. Please enter a positive and even number.”
fi

Explanation:

• The script prompts the user to enter a number using the read command, and the input is stored in the num variable.
• The if statement contains two conditions separated by && (AND operator). The first condition checks if the number is greater than zero (-gt 0), and the second condition checks if the number is even (using $((num % 2)) -eq 0).
• Only if both conditions evaluate to true, the script prints the message confirming the input as a positive and even number.

Conclusion

In this article, we explored the usage of if statements in Bash scripting. We learned how to create conditions using comparison operators, validate user input, utilize nested if statements for more complex scenarios, and combine conditions using logical operators. The if statement is a fundamental tool in Bash scripting that enables you to make decisions and control the flow of your scripts based on various conditions.

As you continue your journey with Bash scripting, you’ll discover even more possibilities for using if statements in combination with loops, functions, and other constructs to build powerful and automated solutions for various tasks. Experiment with different scenarios, and practice writing Bash scripts to deepen your understanding of this versatile language.