How to use variable in bash script

Today we will learn more about variable in shell.
To process our data/information, data must be kept in computers RAM memory. RAM memory is divided into small locations, and each location had unique number called memory location/address, which is used to hold our data. Programmer can give a unique name to this memory location/address called memory variable or variable (Its a named storage location that may take different values, but only one at a time).
In Linux (Shell), there are two types of variable:

1. System variables - Created and maintained by Linux itself. This type of variable defined in CAPITAL LETTERS
2. User defined variables (UDV) - Created and maintained by user. This type of variable defined in lower letters. 

You can see system variables by giving command like $ set, some of the important System variables are:

System Variable	Meaning
BASH=/bin/bash	Our shell name
BASH_VERSION=1.14.7(1)	Our shell version name
COLUMNS=80	No. of columns for our screen
HOME=/home/vivek	Our home directory
LINES=25	No. of columns for our screen
LOGNAME=students	students Our logging name
OSTYPE=Linux	Our Os type
PATH=/usr/bin:/sbin:/bin:/usr/sbin	Our path settings
PS1=[\u@\h \W]\$	Our prompt settings
PWD=/home/students/Common	Our current working directory
SHELL=/bin/bash	Our shell name
USERNAME=vivek	User name who is currently login to this PC

How to define User defined variables (UDV)

To define UDV use following syntax
Syntax: 
variable name=value
'value' is assigned to given 'variable name' and Value must be on right side = sign.

Example:$ no=10# this is ok
$ 10=no# Error, NOT Ok, Value must be on right side of = sign.
To define variable called 'vech' having value Bus
$ vech=Bus
To define variable called n having value 10
$ n=10



Rules for Naming variable name (Both UDV and System Variable)

(1) Variable name must begin with Alphanumeric character or underscore character (_), followed by one or more Alphanumeric character. For e.g. Valid shell variable are as follows
HOME
SYSTEM_VERSION
vech
no
(2) Don't put spaces on either side of the equal sign when assigning value to variable. For e.g. In following variable declaration there will be no error
$ no=10
But there will be problem for any of the following variable declaration:
$ no =10
$ no= 10
$ no = 10
(3) Variables are case-sensitive, just like filename in Linux. For e.g.
$ no=10
$ No=11
$ NO=20
$ nO=2
Above all are different variable name, so to print value 20 we have to use $ echo $NO and not any of the following
$ echo $no # will print 10 but not 20
$ echo $No# will print 11 but not 20
$ echo $nO# will print 2 but not 20
(4) You can define NULL variable as follows (NULL variable is variable which has no value at the time of definition) For e.g.
$ vech=
$ vech=""
Try to print it's value by issuing following command
$ echo $vech
Nothing will be shown because variable has no value i.e. NULL variable.
(5) Do not use ?,* etc, to name your variable names.

How to print or access value of UDV (User defined variables)

To print or access UDV use following syntax
Syntax: 
$variablename
Define variable vech and n as follows:
$ vech=Bus
$ n=10
To print contains of variable 'vech' type
$ echo $vech
It will print 'Bus',To print contains of variable 'n' type command as follows
$ echo $n
Caution: Do not try $ echo vech, as it will print vech instead its value 'Bus' and $ echo n, as it will print n instead its value '10', You must use $ followed by variable name.

source: http://www.freeos.com