Bash-Beginners-Guide

Chapter 10. More on variables
In this chapter, we will discuss the advanced use of variables and arguments. Upon
completion, you will be able to:
♦ Declare and use an array of variables
♦ Specify the sort of variable you want to use
♦ Make variables read-only
♦ Use set to assign a value to a variable
10.1. Types of variables
10.1.1. General assignment of values
As we already saw, Bash understands many different kinds of variables or parameters. Thus far, we haven't
bothered much with what kind of variables we assigned, so our variables could hold any value that we
assigned to them. A simple command line example demonstrates this:
[bob in ~] VARIABLE=12
[bob in ~] echo $VARIABLE
12
[bob in ~] VARIABLE=string
[bob in ~] echo $VARIABLE
string
There are cases when you want to avoid this kind of behavior, for instance when handling telephone and other
numbers. Apart from integers and variables, you may also want to specify a variable that is a constant. This is
often done at the beginning of a script, when the value of the constant is declared. After that, there are only
references to the constant variable name, so that when the constant needs to be changed, it only has to be done
once. A variable may also be a series of variables of any type, a so-called array of variables (VAR0VAR1,
VAR2, ... VARN).
10.1.2. Using the declare built-in
Using a declare statement, we can limit the value assignment to variables.
The syntax for declare is the following:
declare OPTION(s) VARIABLE=value
The following options are used to determine the type of data the variable can hold and to assign it attributes:
Table 10-1. Options to the declare built-in
Option Meaning
Chapter 10. More on variables 121