# Bash Scripting : A Beginner's Guide

Bash scripting is a powerful way to automate tasks in Unix-based operating systems. This guide will cover the essentials of Bash scripting, including how to use comments, the `echo` and `read` commands, built-in variables, and how to handle script arguments. Additionally, we'll provide an example script to list all files and directory sizes in the current path.

### **Comments**

Comments are essential for adding explanations or annotations to your script, making it easier to understand and maintain.

**Single Line Comments**

Single-line comments begin with the `#` symbol.

```bash
#!/bin/bash
# This is a single-line comment
```

**Multi Line Comments**

We can have Multiline comment by adding ": << nameOfComment" at start of first line and to End it, we need to add "nameOfComment" to end of last line of comment

```bash
#!/bin/bash
: << EOF 
This is a multi-line comment.
It can span multiple lines.
All the text between the EOF markers is ignored.
EOF
```

### Displaying Messages: `echo`

The `echo` command is used to display messages on the terminal.

```bash
#!/bin/bash
str='Hello World'
echo "$str"
```

### Reading User Input: `read`

The `read` command is used to take input from the user at runtime.

```bash
#!/bin/bash
echo "Enter 1st Num"
read firstNum

echo "Enter 2nd Num"
read secondNum

sum=$((firstNum + secondNum))
echo "Sum is $sum"
```

### Using Built-in Variables

Bash has several built-in variables that can be used in scripts, such as `PWD` for the current directory and `USER` for the current user.

```bash
#!/bin/bash
echo "Current Directory: $PWD"
echo "Current User: $USER"
```

### Taking Arguments in Scripts

You can pass arguments to a script while running it and access them using `$1`, `$2`, etc.

```bash
#!/bin/bash
# index.sh /home/ubuntu txt
dir=$1 # dir value will become /home/ubuntu
extn=$2 # extn value will become txt

ls "$dir"/*."$extn"
```

### Example Script: List Files and Directory Sizes

This script lists all files and directories in the current path along with their sizes.

```bash
#!/bin/bash

list_file_and_directory() {
    echo "Files and Directories in current Path"
    for item in *; do
        if [ -f "$item" ]; then
            size=$(ls -l "$item" | awk '{print $5}')
            echo "- $item ($size KB)"
        elif [ -d "$item" ]; then
            size=$(du -sh "$item" | awk '{print $1}')
            echo "- $item ($size)"
        fi
    done
    echo "..."
}

list_file_and_directory
```

Bash scripting is a versatile tool that can help you automate a wide range of tasks. By understanding the basics, such as comments, `echo`, `read`, built-in variables, and handling arguments, you can create powerful scripts to simplify your workflow. Happy scripting!