Variables in Linux 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

NOTE that Some of the above settings can be different in your PC/Linux environment. You can print any of the above variables contains as follows:
$ echo $USERNAME
$ echo $HOME

Exercise:
1) If you want to print your home directory location then you give command:
a)$ echo $HOME

OR

(b)$ echo HOME

Which of the above command is correct & why? 

Ans.: (a) command is correct, since we have to print the contains of variable (HOME) and not the HOME. You must use $ followed by variable name to print variables cotaines.

Caution: Do not modify System variable this can some time create problems.

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