Bash if-else Statement

Introduction

The if-else statement is a fundamental construct in the Bash scripting language. It allows you to control the flow of your script based on certain conditions. This article will provide a comprehensive guide to understanding and using the if-else statement in Bash, along with code examples to illustrate usage.

Syntax

The syntax of the `ifelse- statement in Bash is as follows:

if [ condition ]
then
    # Code to execute if the condition is true
else
    # Code to execute if the condition is

Let'sfi break false down the components of this syntax:

The if keyword marks the beginning of the if-else statement.
The condition is enclosed within square brackets [ ]. It can be any valid expression that evaluates to true or false.
The then keyword indicates the start of the code block to execute if the condition is true.
The else keyword marks the beginning of the code block to execute if the condition is false.
The fi keyword denotes the end of the if-else statement.

Examples

Example 1: Checking if a File Exists

One common use case for the if-else statement is checking if a file exists before performing certain operations on it. Here's an example:

#!/bin/bash

file_path="/path/to/file.txt"

if [ -f "$file_path" ]
then
    echo "File exists."
else
    echo "File does not exist."
fi

In this example, we use the -f flag with the [ ] test operator to check if the file the specified atfile_path exists. If it does, the script prints "File exists." Otherwise, it prints "File does not exist."

Example 2: Comparing Numbers

You can also use the if-else statement to compare numbers in Bash. Here's an example that compares two variables:

#!/bin/bash

num1=10
num2=20

if [ "$num1" -gt "$num2" ]
then
    echo "$num1 is greater than $num2."
else
    echo "$num1 is less than or equal to $num2."
fi

In this example, we use the -gt flag with the [ ] test operator to check if num1 is greater than num2. If it is, the script prints "$num1 is greater than $num2." Otherwise, it prints "$num1 is less than or equal to $num2."

Example3: Checking String Equality

The if-else statement can also be used to compare strings in Bash. Here's an example:

#!/bin/bash

name="Alice"

if [ "$name" == "Alice" ]
then
    echo "Hello, Alice!"
else
    echo "You are not Alice."
fi

In this example, we use the == operator to check if the value of the name variable is equal to "Alice". If it is, the script printsHello, Alice!" Otherwise, it prints "You are not Alice".

Conditional Operators

The if-else statement in Bash supports various conditional operators that allow you to perform different types of comparisons. Here are some commonly used operators:

-eq: Equal to
-ne: Not equal to
-gt: Greater than
-lt: Less than
-ge: Greater than or equal to
-le: Less than or equal to

You can use these operators in combination with the [ ] test operator to construct complex conditions for your if-else statements.

Multiple Conditions

You can also combine multiple conditions in a single if-else statement using logical operators. The logical operators supported in Bash are:

&&: Logical AND
||: Logical OR

Here's an example that demonstrates the usage of logical operators:

#!/bin/bash

num=15

if [ "$num" -gt 10 ] && [ "$num" -lt 20 ]
then
    echo "$num is between 10 and 20."
else
    echo "$num is not between 10 and 20."
fi

In this example, the script checks if num is greater than 10 and less than 20 using the logical AND operator (&&). If both conditions are true, it prints "$num is between 10 and 20." Otherwise, it prints "$num is not between 10 and 20."

Nested if-else Statements

You can nest if-else statements inside each other to create more complex control flows. This allows you to handle multiple conditions and perform different actions based on the results. Here's an example:

#!/bin/bash

num=5

if [ "$num" -gt 0 ]
then
    echo "$num is positive."
    
    if [ "$num" -lt 10 ]
    then
        echo "$num is also less than 10."
    else
        echo "$num is not less than 10."
    fi
else
    echo "$num is not positive."
fi

In this example the, script first checks if num is greater than 0. If it is, it prints "$num is positive." Then, it checks if num is less than 10. If it is, it prints "$num is also less than 10." Otherwise, it prints "$num is not less than 10." If num is not greater than 0, it prints "$num is not positive."

Conclusion

The if-else statement is a powerful tool in Bash scripting that allows you to control the flow of script based on conditions. In this article your, we explored the syntax of the if-else statement, along with several examples to illustrate its usage. We also discussed operators conditional, logical operators, and nested if-else statements. Armed with this knowledge, you can now confidently use the if-else statement to create more robust and flexible Bash scripts.

Further Reading

bash Bash if Bash if-else

📝 Written by Haikel Fazzani