Unix - Using Shell Arrays

In this chapter, we will discuss how to use shell arrays in Unix. A shell variable is capable enough to hold a single value. These variables are called scalar variables.
Shell supports a different type of variable called an array variable. This can hold multiple values at the same time. Arrays provide a method of grouping a set of variables. Instead of creating a new name for each variable that is required, you can use a single array variable that stores all the other variables.
All the naming rules discussed for Shell Variables would be applicable while naming arrays.

Defining Array Values

The difference between an array variable and a scalar variable can be explained as follows.
Suppose you are trying to represent the names of various students as a set of variables. Each of the individual variables is a scalar variable as follows −

NAME01="Zara"
NAME02="Qadir"
NAME03="Mahnaz"
NAME04="Ayan"
NAME05="Daisy"

We can use a single array to store all the above mentioned names. Following is the simplest method of creating an array variable. This helps assign a value to one of its indices.

array_name[index]=value

Here array_name is the name of the array, index is the index of the item in the array that you want to set, and value is the value you want to set for that item.
As an example, the following commands −

NAME[0]="Zara"
NAME[1]="Qadir"
NAME[2]="Mahnaz"
NAME[3]="Ayan"
NAME[4]="Daisy"

If you are using the ksh shell, here is the syntax of array initialization −

set -A array_name value1 value2 ... valuen

If you are using the bash shell, here is the syntax of array initialization −

array_name = (value1 ... valuen)

Accessing Array Values

After you have set any array variable, you access it as follows −

${array_name[index]}

Here array_name is the name of the array, and index is the index of the value to be accessed. Following is an example to understand the concept −

#!/bin/sh

NAME[0]="Zara"
NAME[1]="Qadir"
NAME[2]="Mahnaz"
NAME[3]="Ayan"
NAME[4]="Daisy"
echo "First Index: ${NAME[0]}"
echo "Second Index: ${NAME[1]}"

The above example will generate the following result −

$./test.sh
First Index: Zara
Second Index: Qadir

You can access all the items in an array in one of the following ways −

${array_name[*]}
${array_name[@]}

Here array_name is the name of the array you are interested in. Following example will help you understand the concept −

#!/bin/sh

NAME[0]="Zara"
NAME[1]="Qadir"
NAME[2]="Mahnaz"
NAME[3]="Ayan"
NAME[4]="Daisy"
echo "First Method: ${NAME[*]}"
echo "Second Method: ${NAME[@]}"

The above example will generate the following result −

$./test.sh
First Method: Zara Qadir Mahnaz Ayan Daisy
Second Method: Zara Qadir Mahnaz Ayan Daisy