Centering a string in another

I am using Ubuntu 18.04

I have a string of length N which I want to center in another string.

Thus setting some string of length N

old_string="***"

I want a new string (length 55) with the old string centered in it.

new_string="     ***     "

How can I do this in bash ?

Although in actual fact the trailing spaces are redundant, so I can-simply have

new_string="     ***"

Assuming you use the bash shell, try this:

short='---' 
long='abcdefgh'
half=$(( ${#long} / 2 ))
result=${long:0:${half}}${short}${long:${half}}
printf "%s\n" $short $long $half $result

Result:

---
abcdefgh
4
abcd---efgh

Reference:

man bash
/ Parameter Expansion