Variables 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:

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? Click here for answer.

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