Variable substitution

Shell Programming and Scripting
1

Hi,
I have to write a shell script in which I have to substitute a variable within a variable. For example,

var1=aaa
var2=file.$var1.txt

The output should be,
echo $var2
file.aaa.txt

Can someone please help me in getting this. I tried using eval, but it didnt work. I might be using it in a wrong way. Appreciate your response.

2 
var1=aaa
var2=file.${var1}.txt
echo $var2
3

Thanks