VARIABLE:
There are two types of variables namely:
i) Local Variable (shell variable)
ii) Environment Variable

 

SHELL VARIABLES:
Shell allows you to define and use variable.
If you want to define a variable a with numerical value 10.

after you log in are called Environment variables or System Variables.

ENVIRONMENT VARIABLES:
Environment variables are set using the syntax:
export  VAR=VALUE
or in two lines:
VAR=VALUE
export  VAR       

Example:
$HOME (for home folder)
$MAIL (directory for incoming mails)
$PATH (path for executables)
$LOGNAME (for username)
$TERM (for terminal-type)

Note:
If you export a variable, or set an alias on the command line, it only stays active for that one bash session. You must put it in your login script for it to stick.

Example:
vim  ~/.bashrc
export  HISTCONTROL=ignoreboth         
(add this line at the end of file, not to repeat command in history)

Note:

set	display all variables  (use  less command for pagewise)
env	display environment variables
echo	a single variable.