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PART I

What is a Shell ?

- its an interpreter to pass commands given by user to the kernel
- commands can be executed line by line
- there are different types of shells

Types of Shells

find it out   -> cat /etc/shells
current shell -> echo $SHELL

Handout

Bashworkshop.md
    
    line by explaining of all commands
    simple commands
    usage of tabs
    simple regular expression while explaining grep

single line command

five-longest-ping-data.md

    making complex commands from simple ones

PART II

where / when / what scripting ?

- many single line commands can be grouped together and put into a single file called script
- when the script is an input to a shell its called shell script and here specifically its called bash scripting as the shell is bash
-	mostly in interpreted language
-	small scale programs/automation
-	super-set of this language is the programming language(ex: Python)
-	ex: Bash, Python

Why to Write Shell Script ?

Shell script can take input from user, file and output them on screen.
Useful to create our own commands.
Save lots of time.
To automate some task of day today life.
System Administration part can be also automated.

Variables:

 System Variables and Environment 
    Very commonly used variables maintained by shell, represented with capital letters 
    env command to get a list of all system variables.
    Example:
            $HOME
            $RANDOM

Loosely typed
    - need not specify data type during variable declaration
    Example :
        i=5         // i will hold an integer value now 
        i=hello     // i will now be overwritten to string hello 
    - could change the type during the run-time
      assignment
    - leave no space before and after the equal to sign
        Example:
        a="Hello World"   //Correct  
        a ="Hello world"  //Wrong !!
        a= "hello world"  //Wrong !!
    - you will have to use a $ symbol in front of the variable to make bash understand that its a variable
    
String
    - represented by anything inside double quotes
    - Example:
    a="Hi there..."     //works
    a="0123456789"      //works
    a=hello             //works
    a=hello world       //WONT work, you will have to escape the space using backslash
    a=hello\ world      //will work

NULL Variables 
    Example:
    a=
    a=""

Quotes

	"Double Quotes" - Anything enclose in double quotes removed meaning of that Double Quotes characters (except \ and $).
	'Single quotes' - Enclosed in single quotes remains unchanged.
	`Back quote` - echo "todays date is `date` "To execute command 			//usually the key above tab

I/P and O/P

echo command
    //printf in bash 
    Example:
        echo "sudev"
        echo $HOME
        echo $a			

read command
    //scanf in bash 
    Example:    
        read a // you dont have to declare variable "a" to do this

Try it out !!!

QUESTION:
    
    Write a script to take the first name, last name and the age of a person as input. 
    Output should give the full name and the age of the person in a proper sentence.

SOLUTION:

    #!/bin/bash

    echo "First name: "
    read fname

    echo "Last name: "
    read lname

    echo "Age: "
    read age

    fullName=$fname" "$lname

    echo "Your name is $fullName and your age is $age"

    exit

IMPORTANT

Note: Using the #!/bin/bash shebang line.

	Reason - It's a convention so the *nix shell knows what kind of interpreter to run.

	Even today (where most systems run "bash", the "Bourne Again Shell"), scripts can be in bash, python, perl, ruby, PHP, etc, etc. For example, 	  you might see "!/bin/perl" or "/bin/perl5".

	PPS: Remember - under *nix, associating a suffix with a file type is merely a convention, not a "rule". An "executable" can be a binary 			  program, any one of a million script types and other things as well. Hence the need for "#!/bin/bash".

PART III

Shell Arithmetic

	expr
			
			expr is used to evaluate an expression in bash.
			Example 
				
				expr 8 / 2 										//works 
				epxr 8/ 2											//wont work, space problems 
				expr 8 / $(expr 4 \* 2) 			//you has to be carefull with this like 
																			// escaping some wildchars and again $() to evaluate individual expr's
	
Let

    Let is used in a similar fashion as expr, small change in the syntax
    Example                                        
            
            let "m = 3 + 34 - $t" // spaces within the double quotes are not an issue
 
 	Test 
 			
 			[[ ]] 
 			
 			use this to do all the testing conditions 
 			
 			like to check if a variable "ab" is greater than 10
 			
 			[[ $ab -gt 10 ]] 
 			
 			operations -gt -lt -eq -ne -ge -le

Command line Arguments

input is given as <script name> arg1 arg2 arg3....

inside the script arg1, arg2, arg3 all can be accessed using the variables
$1, $2, $3, etc respectively

by default $0 is represented by the script name

all the arguements can be collectively accessed using the variable $@

PART IV

Ageneda:

    * to learn about different constructs in bash
    * these include: if condition, for and while loops, switch cases
    * learn to read a bash script
    * built large programs from prototypes(small programs which do not
      consider all test cases)
    * learn how different commands work

Programs to be discussed:

    * timer.sh
    * uniq
    * zipo.sh
    * licenser.sh
    * dictionary.sh
    * pron.sh
    * debug