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

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

Do you mean something like ROT13:
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}

Answer 2

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 3

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

Answer 4

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 5

Or
cat data.txt | tr 'a-zA-Z' 'n-za-mN-ZA-M'
It will also work

Answer 6

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

Answer 7

Problem statement and how this command can help you:
For example The password is stored in the file data.txt, where 13 positions have rotated all lowercase (a-z) and uppercase (A-Z) letters.
The data.txt file contains 1 line encrypted with the ROT13 ( rotation by 13) algorithm. In order to decrypt it, I have to replace every letter with the letter 13 positions ahead.
file contains the data as shown below
cat data.txt
Gur cnffjbeq vf WIAOOSFzMjXXBC0KoSKBbJ8puQm5lIEi
after rotation to 13 character, the password will look like this.
The password is JVNBBFSmZwKKOP0XbFXOoW8chDz5yVRv
The command to Do that is given below.
cat data.txt | tr '[A-Za-z]' '[N-ZA-Mn-za-m]'
Explanation of the Command
cat data.txt read all the character in data.txt file and then pass to tr command, tr commands takes two arguments, the first argument [A-Za-z] read only the characters made of A-Z or a-z. and in the second argument is rotation regular expression.
[13th character from A - ZA-12th character from A and same expression as for small letters]
[N-ZA-Mn-za-m]
N : 13th character from A.
Z : to the end.
A : first character.
N : just a previous character from the 13th character. to complete the circle.
repeat the same expression for small letters.
We rotated by 13, you can replace the 13th and Previous character by any x position to rotate the string by x characters