1. Edit your ~/.profile file to add a custom folder to your PATH
	export PATH="$PATH:/path/to/script_directory
2. Reload the ~/.profile file
	source ~/.profile

* Variable
* Position Parameter
* Special Parameter

* User defined variable
* Shell Variable 

List of Metacharachters

	1. SpaceTab
        2. NewLine
	3. | 
	4. &
	5. ;
	6. (
	7. )
	8. <
	9. >

1. Simple command : Set of word terminated by oprators
	+ The first word is command line 
	+ Subsequent words are taken as indivisual arguments to that command.
	
2. Oprators: The Token that contain at least 1 unquoted metacharachters.

	List of Control oprators 		List of Redirection oprators
		NewLine						<
		|						>
		||						<<
		&						>>
		&&						<&
		;						>&
		;;						>|
		;&						<<-
		|&						<>
		(
		)
		
Example 
echo $name > out    Here: Space and > unquoted metacharachters	

Example 1 
echo a b c d echo 1 2 3  - Tokenisation 

Example 2
echo a b c d; echo 1 2 3 

This is interpreted as two simple commands because there is control oprator(;) that end the first command.

Example 3
echo $name > out   
Interpreted as one simple command, including redirection operator

Compound command : Start with reserved word and are terminated by corresponding reserved word. 

Example

if [[2 -gt 1]];then
 echo "Welcome"
fi

- Earliar stages are given higer precedence than later one.
- Same stages given equal precedence and processed from left to right.

Four Stage of Processing Expansions

1. Brace Expansions
2. Parameter Expansions ==> Arithmatic Expansions ==> command Substitution ==> Tild Expansions ==> Word Splitting ==> Globbing 

- echo "Hello" 

Result : echo Hello 

The double quote removed because they are not quoted do not result from expansion 

- echo '"Hello"'

Result : echo "Hello" 

The double quote are retained, however because they are quoted by single quote.

- echo \"Hello\" 

Result : "Hello"

The backslashes are removed, because they are unquoted and don't result from an expansion 
The double quote are retained, because they are quoted by ther preceding backslashes. 

- path="C:\Users\Amit\Documents"

Result : echo C:\Users\Amit\Documents

1. Not All data stream 
2. A data stream can only connect to one location at a time 
3. Redirection processed from left to right

Example 1 
command < file  : Redirect the containt of file to the standard input command.

Example 2
command > file : Truncat the file and then the standard output of command to it

Example 3 
command >> file  : Appends standard output of command to file.

Example 4
command 2 > file : Truncat file and then redirects standard error of command to it. 

Example 5 
command 2 >> file  : Appends standard error to a file 

Example 6 
command 2 & > file  : Truncat file, and then redirects both standard output and standard error of command to it.

Example 7 
command &>> file  : Appends both standard output and standard error of command to file.

	#!/bin/bash
	echo "My name is $1"
	echo "My home directory is $2"
	echo "My Favourite colour is $3" 

-p  : Promt to user about what information they must enter
-t : timeout if the user doesn't enter value within time second
-s : Prevent the input that the user enters from being shown into the terminal  
-N : Limit the user response to exactly charachters 

	#!/bin/bash
	read -t 5 -p "Input your first name within 5 seconds: " name
	read -n 2 -p "Input your age (max 2 digits): " age
	read -s -N 5 -p "Enter your zip code (exactly 5 digits): " zipcode
	echo "$name, $age, $zipcode" >> data.csv

	PS3="Please select an option below: "
	select variable in options; do
		commands...
		break
	done

	#!/bin/bash
	PS3="What is the day of the week?: "
	select day in mon tue wed thu fri sat sun; do
		echo "The day of the week is $day"
		break
	done

	if test1; then 
	
			command  #Only run if test1 passes 
	
	elif test2; then 
	
			command  #Only run if test1 fail and test2 passes 
		
	elif testN; then 
	
			command  #Only run if All previous fail and testN passes 	
			
			
	else 
			command  #only run if all test fail 
	
	fi

	#!/bin/bash
	
	read -p "Please enter a number" number
	
	if [ $number -gt 0 ]; then
	
	echo "Your number is greater than 0"
	
	elif [ $number -lt 0 ]; then
	
	echo "Your number is less than 0"
	
	else
	
	echo "Your number is 0!"
	
	fi

	case "$variable" in # don't forget the $ and the double quotes!
	pattern1)
	Commands ...
	;;
	pattern2)
	Commands ...
	;;
	patternN)
	Commands ...
	;;
	*)
	Commands ... # run these if no other pattern matches
	;;
	esac

	#!/bin/bash
	
	read -p "Please enter a number: " number
	
	case "$number" in
	
	"") echo "You didn't enter anything!"
	
	[0-9]) echo "you have entered a single digit number";;
	
	[0-9][0-9]) echo "you have entered a two digit number";;
	
	[0-9][0-9][0-9]) echo "you have entered a three digit number";;
	
	*) echo "you have entered a number that is more than three digits";;
	
	esac