Bash Script Rotation Cipher

Hello I need to write a bash script rotation cipher. Unfortunately I can't get any further now. Next I need to use If-Else condition to convert the letters to numbers and shift plus 5 by the letter. Can someone please help me how to do it?

Thank you

#!/bin/bash
#ROT=$1
ROT=5
TEXT=$2
CRYPT_TEXT=""

echo $ROT
echo $TEXT

echo "crypted: $CRYPT_TEXT"

for c in $(echo $TEXT | sed -e 's/\(.\)/\1\n/g')
do
  echo $c
done

Conversion to upper case can be done in Bash using:

TEXT="foobar" 
echo ${TEXT^^}

A rotation cipher could be implemented using tr, e.g rot13:

echo $TEXT | tr 'A-Za-z' 'N-ZA-Mn-za-m'
# sbbone

rot5 would look like this:

echo $TEXT | tr 'A-Za-z' 'F-ZA-Ef-za-e'
# kttgfw

A partial version without tr command:

#!/bin/bash

TEXT="AZ"

for (( i=0; i<${#TEXT}; i++ )); do
  printf "%s -> %d\\n" "${TEXT:$i:1}" \'${TEXT:$i:1}
  printf -v val "%d" \'${TEXT:$i:1}
  shifted=$(($val + 5))
  echo "shifted: $shifted"
  printf "\\$(printf '%03o' $shifted)\n"

  # A-Z is in range:
  # 65-90
  if [[ $shifted -gt 90 ]];then
    # if value is greated than Z letter you need to subtrack 26
    # so that 91 would become letter A
    echo "$shifted val too large"
    corrected=$(( $shifted - 26))
    echo "corrected ord value $corrected"
    printf "\\$(printf '%03o' $corrected)\n"
  fi
done

the output should look like:

A -> 65
shifted: 70
F
Z -> 90
shifted: 95
_
95 val too large
corrected ord value 69
E

The script converts letters to its corresponding ASCII codes, performs shift and converts codes back to letters. You need to make sure that it works for uppper and lower case letters (or support only one of them). I'll leave the rest as an exercise for the kind reader.