Question

How can i shift each letter of a string by a given number of letters down or up in bash, without using a hardcoded dictionary?

Answer 1

If you mean something like ROT13, you can use tr:

pax$ echo 'hello there' | tr 'a-z' 'n-za-m'
uryyb gurer

pax$ echo 'hello there' | tr 'a-z' 'n-za-m' | tr 'a-z' 'n-za-m'
hello there

For a more general solution where you want to provide an arbitrary rotation (0 through 26), you can use:

#!/usr/bin/bash
    
dual=abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz
phrase='hello there'
rotat=13
newphrase=$(echo $phrase | tr "${dual:0:26}" "${dual:${rotat}:26}")
echo ${newphrase}