Mastering the Bash If Statement

The if statement is one of the most fundamental constructs in Bash scripting, enabling scripts to make decisions based on conditions. Whether you’re a beginner or an experienced developer, understanding how to use the if statement effectively is essential for writing powerful and flexible shell scripts.

Syntax of the Bash if Statement

The basic syntax of the if statement in Bash is:

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

The fi keyword marks the end of the if statement. The [ condition ] part evaluates to either true or false.

Example: Basic if Statement

Here’s a simple script to check if a file exists:

#!/bin/bash

if [ -f "myfile.txt" ]; then
    echo "File exists."
else
    echo "File does not exist."
fi

Explanation:

• -f: Tests if “myfile.txt” exists and is a regular file.

Common Conditions in Bash

Here are some commonly used conditions with if statements:

1. File Conditions:
   • -e file: Check if the file exists.
   • -d file: Check if it’s a directory.
   • -r file: Check if it’s readable.
2. String Comparisons:
   • =: Check if two strings are equal.
   • !=: Check if two strings are not equal.
   • -z string: Check if the string is empty.
3. Numeric Comparisons:
   • -eq: Equal to.
   • -lt: Less than.
   • -gt: Greater than.

Advanced: Using elif for Multiple Conditions

The elif keyword allows you to check multiple conditions:

#!/bin/bash

read -p "Enter your age: " age

if [ "$age" -lt 18 ]; then
    echo "You are a minor."
elif [ "$age" -ge 18 ] && [ "$age" -lt 65 ]; then
    echo "You are an adult."
else
    echo "You are a senior."
fi

Nesting and Logical Operators

You can combine conditions using logical operators:

• AND (&&)
• OR (||)

if [ -f "file1.txt" ] && [ -f "file2.txt" ]; then
    echo "Both files exist."
fi

Conclusion

The Bash if statement is a versatile tool that forms the backbone of conditional logic in shell scripting. By mastering its syntax and capabilities, you can write scripts that adapt to various scenarios and automate complex tasks efficiently. Start experimenting with the examples above to elevate your Bash scripting skills!