Shell Programming and Scripting

Code:

#!/bin/bash

a1="04:29:39 - System health check failed"

i=$1
echo "a $((a$i))"

The script above gives the following error
$113> sh tryt.sh 1
tryt.sh: line 6: 04:29:39 - System health check failed: syntax error in expression (error token is ":29:39 - System health check failed")

It also fails when I try to compare in if condition. How to compare a$i to a value in if condition. My purpose is to display the statement.
Can someone explain?

You can't use that construct for indirection; it has a limited use only if a1 holds a numerical value. Recent bashs provide variable indirection:echo ${!a1}.