How to Write a Shell Script

Create a Shell Script file with extension .sh using Touch command.
Open file using command vi/vim.
Press esc i to insert/write something on file.
Start writing your Shell Script using specific indentation/syntax.

#! This is called Shebang followed by bin followed by executable (in our case it is bash).

What is The Purpose of (#!) Shebang in Shell Scripting

Shebang (#!) is the first line in a shell script that tells the computer which program should run the script. It helps the system know how to interpret the script correctly. Common Shebangs and Their Differences

Shebang	Interpreter	What It Does
`#!/bin/bash`	Bash (Bourne Again Shell)	Most commonly used, supports many features.
`#!/bin/sh`	Bourne Shell (or its link)	May point to different shells depending on the system.
`#!/bin/dash`	Dash (Debian Almquist Shell)	A lightweight and fast shell but lacks some Bash features.
`#!/bin/ksh`	Korn Shell	An older shell, less common now.
`#!/usr/bin/env bash`	Finds `bash` in system paths	Useful for portability across different systems.

Why Use a Shebang?

Specifies the shell: Ensures the script runs in the correct shell, avoiding unexpected errors.
Allows direct execution: Without it, you'd need to run the script as bash script.sh instead of ./script.sh.Differences Between /bin/sh and /bin/bash

Linux distributions use different links:
- - On Ubuntu, /bin/sh points to /bin/dash (which lacks some Bash features).
    - On older systems, /bin/sh used to be the original Bourne Shell.
    - On other systems, /bin/sh may still link to Bash.
- If you write a Bash script but use #!/bin/sh on Ubuntu, it might not work because Dash doesn't support all Bash features.

💡 Tip: If you're writing a script that requires Bash features, always use #!/bin/bash.

After writing first line you can start writing your script.

How to Edit a Script in Vim (Linux Text Editor)

Enter insert mode: Press i
Save and exit: Press Esc, then type :wq!
Quit without saving: Press Esc, then type :q!

Conclusion

By understanding shebang and different shell interpreters, you can write scripts that work smoothly on various systems.

Introduction to Automation and Shell Scripting in Linux (Part 02)