# Linux Shell Scripting

# Basic of Linux

If you are using Ubuntu then you must know about the following terms:-

1. Kernel
    
2. shell
    

### Kernel:-

is a central component of an operating system that manages the operations of computers and hardware.

It manages operations of memory and CPU time. It is a core component of an operating system.

Kernel acts as a bridge between applications and data processing performed at the hardware level using inter-process communication and system calls. 

### Shell:-

A shell is a special user program that provides an interface for the user to use operating system services. Shell accepts human-readable commands from users and converts them into something which the kernel can understand.

## Shell Scripting:-

As a shell can also take commands as input from the file we can write these commands in a file and can execute them in the shell to avoid this repetitive work. These files are called **Shell Scripts** or **Shell Programs**.

**A shell script comprises the following elements –**

* Shell Keywords – if, else, break etc.
    
* Shell commands – cd, ls, echo, pwd, touch etc.
    
* Control flow – if..then..else, case and shell loops etc.
    

**Why do we need shell scripts**

* To avoid repetitive work and automation
    
* System admins use shell scripting for routine backups
    
* System monitoring
    
* Adding new functionality to the shell etc.
    

## What is `#!/bin/bash?` can we write `#!/bin/sh` as well?

**Bash:-** It simplifies certain operations that are hard to accomplish efficiently with a GUI.

**#!/bin/bash**:- This is the first line of the shell path. It tells the shell to execute it via bash shell.

/**bin/bash vs /bin/sh**:- bash is a newer and improved version of the shell. The features offered and the advantages of Bash over Shell are very heavy. So bash is better.

## Write a Shell Script that prints `I will complete the #90DaysOofDevOps challenge`

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1681802764397/7cbe17e0-79f0-4e9c-bf10-3d18754ebdce.jpeg align="center")

According to the above image, nano is like a notepad devops\_first.sh is the filename. After entering that nano devops\_first.sh text editor will open and it shows like below diagram.

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1681802913226/e9a9ccb3-1df5-4492-bfe2-6bd8329f4894.png align="center")

According to this image **#!/bin/bash** This is the first line of the shell path.  
echo keyword is used to write the statement whatever we want.

## Write a Shell Script to take user input and print the variables.

***Shell Script: -*** following is the script as input

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1681803962929/d0839dbe-78e1-4c89-b5f6-fb36ea807ee3.png align="center")

***Output:-***

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1681804055295/ab68e836-fb3b-4b6d-8385-ffcf48c13b62.png align="center")

* ## Write an Example of If else in Shell Scripting by comparing 2 numbers
    
    **Shell Script:-**
    

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1681805761184/729dcc3d-e753-4e00-bfea-6b13d3fa7664.png align="center")

**Output:-**

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1681805781488/e010f9f6-d958-477a-96bd-15096485ba17.png align="center")