Solution to atbash_cipher in Bash

bash/atbash-cipher/atbash_cipher.sh

@@ -0,0 +1,57 @@
+#!/usr/bin/env bash
+
+# Get the mode (encode or decode)
+mode=$1
+
+# Get the input string
+input=$2
+
+PLAIN="abcdefghijklmnopqrstuvwxyz"
+CIPHER="zyxwvutsrqponmlkjihgfedcba"
+
+encode () {
+  local encoded=""
+  for (( i=0; i<${#1}; i++ )); do
+    char="${1:$i:1}"
+    # Lowercase
+    char="${char,,}"
+    # Is number
+    [[ -n "${char##*[!0-9]*}" ]] && {
+      encoded+="$char"
+      continue
+    }
+    # Cut anything after matched
+    before="${PLAIN/$char*/}"
+    encoded+="${CIPHER:${#before}:1}"
+  done
+  # Insert a space for every 5 chars
+  encoded=$(echo "$encoded" | sed "s/.\{5\}/& /g;s/ $//")
+  echo "$encoded"
+}
+
+decode () {
+  local decoded=""
+  for (( i=0; i<${#1}; i++ )); do
+    char="${1:$i:1}"
+    # Lowercase
+    char="${char,,}"
+    # Is number
+    [[ -n "${char##*[!0-9]*}" ]] && {
+      decoded+="$char"
+      continue
+    }
+    # Cut anything after matched
+    before="${CIPHER/$char*/}"
+    decoded+="${PLAIN:${#before}:1}"
+  done
+  echo "$decoded"
+}
+
+
+# # Encode or decode the input string based on the mode
+case $mode in
+    encode)
+        encode "$input";;
+    decode)
+        decode "$input";;
+esac