A.P. Lawrence

How and where to start writing first Shell Script in UNIX Part 2

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Tue Aug 31 16:14:20 2004 How and where to start writing first Shell Script in UNIX&
Posted by Rakesh Awasthi
Referencing: /Blog/B1073.html

This article is a follow up of my last article “How and where to start writing first Shell Script in UNIX”. It covers the shell variables, both User defined as well as system shell variable.

What is a variable?

A variable is a name associated with a value. It’s a string that can be used to represent and manipulate your data.

In UNIX shell script programming you need not declare a variable. You just assign a value to it. There are two types of variables:

1. User shell variables
2. System shell variables

User shell variables

To use a variable just initialize it, there is no need to declare it.

Myname=rakesh

Age=20

In the above two lines ‘Myname’ and ‘Age’ are two variables having ‘rakesh’ and ‘20’ as the value associated to them respectively.

Remember ‘20’ is a string not a numeric value and can’t be used for mathematical operations. I shall explain how to do mathematical operations in my follow-ups.

Declaring a Null String

A null string can be declared in any of the following three ways:

variable=

variable=””

variable=’’

Making a value Read-Only

I don’t want to change the value of my variable inside the UNIX shell script accidentally.

readonly name=rakesh

Now the value of ‘name’ can’t be changed in your UNIX shell script. The read-only value can’t be even unset. I shall explain unset in the meantime. To get a list of all read-only variables type this on the prompt string

readonly

What is a valid variable name?

I wonder what a valid variable name is! What variable name should be given for better understanding of there usage and the associated value?

Only digits, alphabets and underscore ( _ ) are allowed in defining a variable name. Variable names are case sensitive, so Age and age are two different variables in UNIX shell scripts.

What is unset?

Unset erases all the reference of the variable from shell’s memory. To erase the value of variable ‘age’ type on the prompt string

unset age

This way you can use the same variable again in the program by associating a different value to it. Though you need not flush the value of a variable before reusing it but it’s a good practice to do show before associating a value of different data type to it.

System shell variables

Those who are familiar with the DOS shell might know what happens when you type set on the command prompt. It gives you the list of all environment shell variables. The environment shell variables are also known as system or public shell variables. Try doing the same with your UNIX shell. Type

set

on your prompt string and eventually you will get list of all system shell variables.

There are few commonly used system shell variables are

HOME

MAIL

PATH

EXINIT

PS1

PS2

USER

SHELL

TERM

You can use these shell variables in your program.

How to print or use the value of the variable

Now please tell me how can I use these variables in my UNIX shell script.

In fact it’s very simple. All you need to do is to put a dollar ( $ ) before your variable name.

$varname

will give you the value ‘varname’ is holding. And to print the value type

echo $varname