01. Bash Shell Script

What is a Bash Script?

A Bash script is a text file containing a series of commands that you would normally type in the terminal.
Instead of executing commands manually, a script automates them — saving time and reducing errors.

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Step 1: Locate Your Bash Interpreter

Before writing a script, find out where Bash is installed on your system.

$ which bash
/bin/bash

This tells you that the Bash interpreter is located in /bin/bash.
You’ll need this path for the shebang line (explained next).

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What is a Shebang?

Every Bash script starts with a special line called a shebang.

#!/bin/bash

Explanation:

• #! — Marks the file as a script and tells the system which interpreter to use.

• /bin/bash — The path to the Bash interpreter.

This line ensures the script runs in the correct shell, regardless of the user’s default shell.

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Step 2: Create a Simple Script

Let’s create your first Bash script using nano editor.

$ nano hello_world.sh

Paste the following content:

#!/bin/bash
# 01. Bash Shell Script
STRING="Hello World"
# print variable on a screen
echo $STRING

Explanation:

• STRING="Hello World" — Creates a variable named STRING.

• echo $STRING — Prints the variable’s value to the terminal.

Save and exit the file (Ctrl + O, Enter, then Ctrl + X).

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Step 3: Make the Script Executable

Before running a script, you must give it execute permission.

$ chmod +x hello_world.sh

Explanation:

• chmod — Changes file permissions.

• +x — Adds execute permission to the file.

• Now the script can be run directly from the command line.

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Step 4: Run the Script

Execute your script using:

$ ./hello_world.sh

Output:

Hello World

Congratulations! You’ve written and executed your first Bash script.

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Example 2: Simple Backup Script

You can automate repetitive tasks, such as creating backups.

Script:

#!/bin/bash
tar -czf myhome_directory.tar.gz /home/linuxconfig

Explanation:

• tar — Archive utility used to combine and compress files.

• -c — Create a new archive.

• -z — Compress the archive using gzip.

• -f — Specify the filename (myhome_directory.tar.gz).

• /home/linuxconfig — The directory being backed up.

Result:

Running this script creates a compressed backup of /home/linuxconfig in the current directory.

$ ./backup.sh

Output:

myhome_directory.tar.gz created successfully

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Why Use Bash Scripts?

Benefit	Description
Automation	Run repetitive tasks automatically.
Consistency	Same commands executed every time — fewer errors.
Expandability	Start simple, add logic later (loops, conditions, etc.).
Efficiency	Save time and effort by not typing long commands repeatedly.

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Summary

Step	Command	Purpose
1️	which bash	Find Bash interpreter path
2️	nano hello_world.sh	Create script file
3️	chmod +x hello_world.sh	Make script executable
4️	./hello_world.sh	Run the script

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