How to shift each letter of the string by a given number of letters?

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 want to rotate also the capitals you could use something like this:

cat data.txt | tr '[a-z]' '[n-za-m]' | tr '[A-Z]' '[N-ZA-M]'

where data.txt has whatever you want to rotate.

Answer 2

$ alpha=abcdefghijklmnopqrstuvwxyz
$ rot=3
$ sed "y/${alpha}/${alpha:$rot}${alpha::$rot}/" <<< 'foobar'
irredu

Answer 3

Shift by 12 characters(A becomes M, and vice versa)

Encryption
----------
$> echo ABCDE | tr '[A-Z]' '[M-ZA-L]' // prints MNOPQ

Decryption
----------
$> echo MNOPQ | tr '[M-ZA-L]' '[A-Z]' // prints ABCDE

In the encryption example, we are piping ABCDE to the command tr which is given two arguments. The first one is a matching string. It will match certain strings in your input(in our case ABCDE). The second argument works upon the result of the first argument and modifies it accordingly. So, we're basically matching any uppercase letter present in the input ABCDE and passing it to the second argument. The second argument replaces the characters with their 12th next counterpart. Now, this part is important to understand and might confuse some people, we're basically going from [M-L] in the second argument. Since the tr command doesn't accept this directly, we're breaking it up into two separate chunks. First chunk is [M-Z] and the second one is [A-L]. It's basically like a search-and-replace mechanism. You search with the first argument, modify with the second argument, as simple as that.

For the second example, I've just swapped the first argument with the second one in the tr command. Which acts perfectly as a decryptor. You could write it the same way as the first example, but I find it less time consuming when I have the encryption algorithm and I can just swap the arguments to have a decryption algorithm as well.

Answer 4

Or

cat data.txt | tr 'a-zA-Z' 'n-za-mN-ZA-M' 

It will also work

Answer 5

Without using tr, shift 1 to 25 characters and can be decrypted using 26 - original key

#!/bin/bash

#set -x

i=0
for letters in {A..Z}
do
abc_cap[$i]="$letters"
((i++))
done

i=0
for letters in {a..z}
do
abc_small[$i]="$letters"
((i++))
done

read -r -p "Enter message to be encrypted/decrypted: " -a message
read -r -p "Enter shift amount (26 - orig key for decrypt): " shift_amount

echo -n "Encrypted message: "

if [ "$shift_amount" -gt 25 ] || [ "$shift_amount" -lt 1 ]
then  
echo "Shift amount out of range"
exit  
fi  

for word in "${message[@]}"
do
 
while read -r -n 1 letter
do
if [[ "$letter" = [a-z] ]]
then
        for a in "${!abc_small[@]}"
        do
        if [ "${abc_small[$a]}" = "$letter" ]
        then
        a=$(echo "($a + $shift_amount) % 26" | bc)
        echo -n "${abc_small[$a]}"
        fi
        done

elif [[ "$letter" = [A-Z] ]]
then
        for a in "${!abc_cap[@]}"
        do
       if [ "${abc_cap[$a]}" = "$letter" ]
        then
        a=$(echo "($a + $shift_amount) % 26" | bc)
        echo -n "${abc_cap[$a]}"
        fi
        done

elif [[ "$letter" = "" ]]
then echo -n " "
 
else echo -n "$letter"
fi
 
done < <(echo "$word")
done
echo
exit