Understanding Linux Shell

    Shell: A Command-Line Interpretor that connects a user to Operating System and allows to execute the commands or by creating text script.

    Process: Any task that a user run in the system is called a process. A process is little more complex than just a task.

    File: It resides on hard disk (hdd) and contains data owned by a user.

    X-windows aka windows: A mode of Linux where screen (monitor) can be split in small “parts” called windows
	, that allow a user to do several things at the same time and/or switch from one task to another easily and view graphics in a nice way.

    Text terminal: A monitor that has only the capability of displaying text stuff, no graphics or a very basic graphics display.

    Session: Time between logging on and logging out of the system.


Types of Shell 



    Bourne shell (sh)
    Korn shell (ksh)
    Bourne Again shell (bash)
    POSIX shell (sh)


Bourne shell :  The Bourne shell was one of the major shells used in early versions and became a de facto standard. 
		It was written by Stephen Bourne at Bell Labs. 
		Every Unix-like system has at least one shell compatible with the Bourne shell. 
		The Bourne shell program name is “sh” and it is typically located in the file system hierarchy at /bin/sh.

C shell	     :  The C shell was developed by Bill Joy for the Berkeley Software Distribution. 
	        Its syntax is modelled after the C programming language. 
                It is used primarily for interactive terminal use, but less frequently for scripting and operating system control. 
                C shell has many interactive commands.


Shell scripting advantages and disadvantages

	Advantages:

	    To automate the frequently performed operations
	    To run sequence of commands as a single command
	    Easy to use
	    Portable (It can be executed in any Unix-like operating systems without any modifications)

	Disadvantages:
	    Slow execution speed compared to any programming languages
	    A new process launched for almost every shell command executed


-->-- FAQ | Extension of your Shell Script | We are using Bourne Shell | So .sh is the extension of the Script


    To get a Linux shell, you need to start a terminal.
    To see what shell you have, run: echo $SHELL.
    In Linux, the dollar sign ($) stands for a shell variable.
    The ‘echo‘ command just returns whatever you type in.
    The pipeline instruction (|) comes to rescue, when chaining several commands.

    Linux commands have their own syntax, Linux won’t forgive you whatsoever is the mistakes. 
    If you get a command wrong, you won’t flunk or damage anything, but it won’t work.
    
    #!/bin/sh – It is called shebang. It is written at the top of a shell script and it passes the instruction to the program /bin/sh.

#!/bin/sh -->-- First Line of your Script 

Process of writing and executing a script

    Open terminal.
    Navigate to the place where you want to create script using ‘cd‘ command.
    Cd (enter) [This will bring the prompt at Your home Directory].
    touch hello.sh (Here we named the script as hello, remember the ‘.sh‘ extension is compulsory).
    vi hello.sh (nano hello.sh) [You can use your favourite editor, to edit the script].
    chmod 744 hello.sh (making the script executable).
    sh hello.sh or ./hello.sh (running the script)

Writing your First Script
==========================

#!/bin/bash
# My first script

echo "Welcome to Unix World!"


Sample Output
-------------
Hello World!



#!/bin/bash (is the shebang.)
# My first script (is comment, anything following '#' is a comment)
echo “Hello World!” (is the main part of this script)


Writing your Second Script
==========================
#! /bin/bash
echo "Hello $USER"
echo "Hey i am" $USER " and will be telling you about the current processes"
echo "Running processes List"
ps


Create a file with above codes, save it to anything you want, but with extension “.sh“, make it executable and run it, from you terminal.


Sample Output
-------------

Hello tecmint
Hey i am tecmint and will be telling you about the current processes
Running processes List
  PID TTY          TIME CMD
 1111 pts/0    00:00:00 bash
 1287 pts/0    00:00:00 sh
 1288 pts/0    00:00:00 ps


Writing your Third Script
==========================
Moving to, write our third and last script for this article. This script acts as an interactive script. 
Why don’t you, yourself execute this simple yet interactive script and tell us how you felt.


#! /bin/bash
echo "Hey what's Your First Name?";
read a;
echo "welcome Mr./Mrs. $a, would you like to tell us, Your Last Name";
read b;
echo "Thanks Mr./Mrs. $a $b for telling us your name";
echo "*******************"
echo "Mr./Mrs. $b, it's time to say you good bye"

Sample Output
-------------

Hey what's Your First Name?
Avishek
welcome Mr./Mrs. Avishek, would you like to tell us, Your Last Name
Kumar
Thanks Mr./Mrs. Avishek Kumar for telling us your name
******************************************************
Mr./Mrs. Kumar, it's time to say you good bye

There are three types of shell variables available. They are,

	a)Local variables
	b)Environment variables
	c)Shell variables or Special variables

	Local variables:
		A local variable is a variable, its value will be visible to the current shell.
	
	Environment variables:
		An environment variable is a variable, its value will be visible to all the child shell's or child processes started from the shell.
	
	Shell variables or Special variables:
		A shell variable or special variable is a variable set by shell for its necessity.
		Some of these variables are environment variables whereas others are local variables 


Command Line Arguments

	Command line arguments are important part of writing scripts. Command line arguments define
	the expected input into a shell script. For example, we may want to pass a file name or folder
	name or some other type of argument to a shell script.

Several special variables exist to help manage command-line arguments to a script:

	  $# - represents the total number of arguments (much like argv) – except command
	• $0 - represents the name of the script, as invoked
	• $1, $2, $3, .., $8, $9 - The first 9 command line arguments
	• $* - all command line arguments OR
	• $@ - all command line arguments

Evaluating Expr

	Shell scripts are not intended to do complex mathematical expressions. But expr program can be
	used to manipulate variables, normally interpreted as strings, asintegers. 
	Consider the following "adder" script:

	sum=`expr $1 + $2`
	printf "%s + %s = %s\n" $1 $2 $sum

Arithmetic Operators

	+ (Addition) 		Adds values on either side of the operator 				`expr $a + $b` will give 30
	- (Subtraction) 	Subtracts right hand operand from left hand operand 			`expr $a - $b` will give -10
	* (Multiplication) 	Multiplies values on either side of the operator 			`expr $a \* $b` will give 200
	/ (Division) 		Divides left hand operand by right hand operand				`expr $b / $a` will give 2
	% (Modulus) 		Divides left hand operand by right hand operand and returns remainder 	`expr $b % $a` will give 0
	= (Assignment) 		Assigns right operand in left operand 					a = $b would assign value of b into a
	== (Equality) 		Compares two numbers, if both are same then returns true. 		[ $a == $b ] would return false.
	!= (Not Equality) 	Compares two numbers, if both are different then returns true. 		[ $a != $b ] would return true.

Relational Operators

	-eq 	Checks if the value of two operands are equal or not; 

		if yes, then the condition becomes true. 						[ $a -eq $b ] is not true.

	-ne 	Checks if the value of two operands are equal or not; 
		if values are not equal, then the condition becomes true. 				[ $a -ne $b ] is true.

	-gt 	Checks if the value of left operand is greater than the value of right operand; 
		if yes, then the condition becomes true. 						[ $a -gt $b ] is not true.

	-lt 	Checks if the value of left operand is less than the value of right operand; 
		if yes, then the condition becomes true. 						[ $a -lt $b ] is true.

	-ge 	Checks if the value of left operand is greater than or equal to the value of right operand; 
		if yes, then the condition becomes true. 						[ $a -ge $b ] is not true.

	-le 	Checks if the value of left operand is less than or equal to the value of right operand; 
		if yes, then the condition becomes true. 						[ $a -le $b ] is true.


Boolean Operators

	! 	This is logical negation. 
		This inverts a true condition into false and vice versa. 	[ ! false ] is true.

	-o 	This is logical OR. If one of the operands is true, 
		then the condition becomes true. 				[ $a -lt 20 -o $b -gt 100 ] is true.

	-a 	This is logical AND. If both the operands are true, 
		then the condition becomes true otherwise false. 		[ $a -lt 20 -a $b -gt 100 ] is false. 

String Operators

	= 	Checks if the value of two operands are equal or not; 
		if yes, then the condition becomes true. 			[ $a = $b ] is not true.

	!= 	Checks if the value of two operands are equal or not; 
		if values are not equal then the condition becomes true. 	[ $a != $b ] is true.

	-z 	Checks if the given string operand size is zero; 
		if it is zero length, then it returns true.		 	[ -z $a ] is not true.

	-n 	Checks if the given string operand size is non-zero;
		if it is nonzero length, then it returns true. 			[ -n $a ] is not false.

	str 	Checks if str is not the empty string; 
		if it is empty, then it returns false.


Condition and Looping


IF THEN

		if command
		then
			command
			command
			...
			command
		fi

IF THEN ELSE

		if command
		then
			command
			command
			...
			command
		else
			command
			command
			...
			command
		fi


Example 

		if [ "$LOGNAME"="guna" ]
		then
			printf "%s is logged in" $LOGNAME
		else
			printf "Intruder! Intruder!"
		fi


ELSE IF


		if command
		   command
		   command
		   ...
		   command
			then
			command
			command
			...
			command
		elif command
		then
			command
			command
			...
			command
		elif command
		then
			command
			command
			...
		command
		fi
	
SWITCH CASE Statement

		case var
		in
		pat) command
			command	
			...
			command
			;; # Two ;;'s serve as the break
		pat) command
			command
			...
			command
			;; # Two ;;'s serve as the break
		pat) command
			command
			...
			command
			;; # Two ;;'s serve as the break
		esac

Example

	#!/bin/sh
	case "$2"
	in
	"+") ans=`expr $1 + $3`
		printf "%d %s %d = %d\n" $1 $2 $3 $ans
		;;
	"-") ans=`expr $1 - $3`
		printf "%d %s %d = %d\n" $1 $2 $3 $ans
		;;
	"\*") ans=`expr "$1 * $3"`
		printf "%d %s %d = %d\n" $1 $2 $3 $ans
		;;
	"/") ans=`expr $1 / $3`
	printf "%d %s %d = %d\n" $1 $2 $3 $ans
		;;

	
	Execution         sh myscript.sh 2 + 3		| 	./myscript.sh 2 + 3 


The for Loop

	The for loop provides a tool for processing a list of input. The input to the for loop is a list of values. Each
	iteration through the loop it extracts one value into a variable and then enters the body of the loop. the
	loop stops when the extract fails because there are no more values in the list.


	for var in "$@"
	do
		printf "%s\n" $var
	done



	for var in "$@"
	do
		if [ "$var" = "quit" ]
			then
			break
		fi	
		printf "%s\n" $var
	done


	for var in "$@"
	do
		if [ "$var" = "me" ]
		then
			continue
		elif [ "$var" = "mine" ]
		then
			continue
		elif [ "$var" = "myself" ]
		then
			continue
		fi

	if [ "$var" = "quit" ]
	then
		break
	fi
		printf "%s\n" $var
	done


While and Until Loop

	ls | sort |
	while read file
	do
		echo $file
	done



Write a shell script sum.sh that takes an unspecified number of command line arguments (up to 9) of ints 
and finds their sum. Modify the code to add a number to the sum only if the number is greater than 10


	#! /bin/sh
	sum=0
	for var in "$@"
	do
		if [ $var –gt 10 ]
		then
			sum=`expr $sum + $var`
		fi
	done
	printf "%s\n" $sum


	-- run the script as
	% sh 2.1.sh 2 4 5   (or)  .\sh2.1.sh 2 4 5