Bash Examples: If, Else If, Else

Bash

Bash Examples: If, Else If, Else

March 1, 2023

This guide is part of the "Snippets" series. This series is focused on providing simple and accessible tutorials on various topics relating to development!

Bash is a commonly used command-line interface for Unix-based systems. It provides a variety of tools and utilities for managing and automating tasks on your computer. The IF, ELIF, and ELSE statements are used in Bash to perform conditional operations based on the evaluation of a certain condition.

Here's an example of how to use the IF statement:

if [ condition ]; then
    # do something
fi

The condition in this statement is the test that will be evaluated to either true or false. If the condition is true, then the commands inside the if block will be executed. If the condition is false, the commands inside the if block will be skipped.

For example, let's say you want to print a message if a variable called num is greater than 10. Here's how you could write that in Bash:

if [ $num -gt 10 ]; then
    echo "The number is greater than 10."
fi

In this example, the -gt flag is used to compare the variable num to the value 10. If num is greater than 10, then the message "The number is greater than 10." will be printed to the console.

Now, let's say you want to perform a different action if the num variable is less than or equal to 10. In that case, you can use the ELIF statement:

if [ $num -gt 10 ]; then
    echo "The number is greater than 10."
elif [ $num -le 10 ]; then
    echo "The number is less than or equal to 10."
fi

In this example, the elif statement is used to perform a different action if the if condition is false. If num is less than or equal to 10, then the message "The number is less than or equal to 10." will be printed to the console.

Finally, you can use the ELSE statement to perform a default action if none of the conditions in the if or elif statements are true:

if [ $num -gt 10 ]; then
    echo "The number is greater than 10."
elif [ $num -le 10 ]; then
    echo "The number is less than or equal to 10."
else
    echo "The number is not a valid input."
fi

In this example, the else statement is used to perform a default action if the if and elif conditions are both false. If num is not a valid input, then the message "The number is not a valid input." will be printed to the console.

Checking if a file exists

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

Checking if a number is positive, negative, or zero

read -p "Enter a number: " num

if [ $num -gt 0 ]; then
    echo "The number is positive."
elif [ $num -lt 0 ]; then
    echo "The number is negative."
else
    echo "The number is zero."
fi

Checking if a string is empty or not

read -p "Enter a string: " str

if [ -z "$str" ]; then
    echo "The string is empty."
else
    echo "The string is not empty."
fi

Checking if a directory exists and creating it if necessary

if [ -d mydirectory ]; then
    echo "The directory exists."
else
    mkdir mydirectory
    echo "The directory has been created."
fi

How to run a Bash file

To save and run a Bash file, you need to create a file with the .sh extension and then make it executable. Here's how you can do it:

Open a text editor (such as Vim, Nano, or Sublime Text) and create a new file.
Add the Bash commands that you want to run in the file. For example, you could add the following commands:

#!/bin/bash
echo "Hello, World!"

This script will print the message "Hello, World!" to the console.

Save the file with a .sh extension. For example, you could save the file as myscript.sh.
Make the file executable by running the following command in the terminal:

chmod +x myscript.sh

This command sets the executable bit on the file, allowing you to run it as a script.

Run the script by typing the following command in the terminal:

./myscript.sh

This command runs the Bash script and prints the output to the console.

The console output will depend on the commands in your Bash script. In the example above, the output would be:

Hello, World!

If your script contains multiple commands, each command's output will be printed to the console as it is executed.

That's the basic process for saving and running a Bash file.