Solution to atbash_cipher in Bash

bash/atbash-cipher/README.md

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+# Atbash Cipher
+
+Welcome to Atbash Cipher on Exercism's Bash Track.
+If you need help running the tests or submitting your code, check out `HELP.md`.
+
+## Instructions
+
+Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.
+
+The Atbash cipher is a simple substitution cipher that relies on transposing all the letters in the alphabet such that the resulting alphabet is backwards.
+The first letter is replaced with the last letter, the second with the second-last, and so on.
+
+An Atbash cipher for the Latin alphabet would be as follows:
+
+```text
+Plain:  abcdefghijklmnopqrstuvwxyz
+Cipher: zyxwvutsrqponmlkjihgfedcba
+```
+
+It is a very weak cipher because it only has one possible key, and it is a simple mono-alphabetic substitution cipher.
+However, this may not have been an issue in the cipher's time.
+
+Ciphertext is written out in groups of fixed length, the traditional group size being 5 letters, leaving numbers unchanged, and punctuation is excluded.
+This is to make it harder to guess things based on word boundaries.
+All text will be encoded as lowercase letters.
+
+## Examples
+
+- Encoding `test` gives `gvhg`
+- Encoding `x123 yes` gives `c123b vh`
+- Decoding `gvhg` gives `test`
+- Decoding `gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt` gives `thequickbrownfoxjumpsoverthelazydog`
+
+## Source
+
+### Created by
+
+- @sjwarner-bp
+
+### Contributed to by
+
+- @bkhl
+- @budmc29
+- @glennj
+- @guygastineau
+- @IsaacG
+- @kotp
+- @Smarticles101
+- @ZapAnton
+
+### Based on
+
+Wikipedia - https://en.wikipedia.org/wiki/Atbash