Unix/Linux Shell Variables

Variables in Unix/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 Unix/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.

    You can see system variables by giving command like $ set, There is a long list of system variable

    so if you interested to see all the system variable then you should redirect the all system variable

    into a text file by using command as "set > systemVariable.txt". Now check systemVariable.txt in

    your current directory.

Some of the important System variables are as following:
   

System Variable	Description
BASH=/bin/bash	System Shell Name
BASH_VERSION=4.2.25(1)-release	System Shell Version Name
COLUMNS=80	Number of Column for your screen
LINES=34	Number of Lines for your screen
HOME=/home/mukesh	Your Shell's Home Directory
LOGNAME=mukesh	Logging Name of Current System
OSTYPE=Linux	Our Operating System Type
PATH=/usr/bin:/sbin:/bin:/usr/sbin	Our System's Path Setting
PS1=[\u@\h \W]\$	Our Prompt Settings
SHELL=/bin/bash	Our Shell Name
PWD=/home/mukesh/Documents	Our Current Working Directory
USERNAME=mukesh	Username who is currently log in into system

   
   
    Note :Some of the above settings can be different in your Unix/Linux environment as per your system.
    You can print any of the above variables contains as follows:

    $ echo $USERNAME
    $ echo $BASH_VERSION



2) User defined variables : 

•   Created and maintained by user. This type of variable defined in lower letters.
•   User-defined variable can be assigned a value.
•   The value of a named variable can be retrieved by preceding the variable name with a ‘$’ sign
•   Used extensively in shell-scripts.
•   Used for reading data, storing and displaying it.

Accepting Data and Displaying It on Screen:


•  To accept input data from user you can use read command.
•  In shell scripts , read is used for accepting data from the user and storing it for other processing.
•  Example : $ read yourname

Type command "read yourname" and hit enter. Type your name and hit enter again. So now your name has been stored into user defined varaible name yourname.

Displaying Value:
To display the value you can use the echo command.
So to display your name which you have stored into a variable yourname type the following command.

  echo $yourname