How to replace a letter of string at some position

Question

Suppose I want to replace the nth letter of some string, how can I do that?

I tried something like this but it's not correct:

#!/bin/bash
index= # let say 2
s='Hello'
echo ${s/$index/'a'} # This should print Healo

Answer 1

There is a Advanced Bash-Scripting Guide that shows you how to do substring and concatenation.

Let's say:

#!/bin/bash
index=2
s=Hello
echo ${s:0:index-1}a${s:index}

Answer 2

Another solution with sed:

$ echo "hello" | sed 's/\(.\{2\}\)./\1a/'