Solved luhn in Bash

bash/luhn/luhn.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+
+#!/usr/bin/env bash
+
+# Strip all spaces from input
+input=$(echo $1 | tr -d ' ')
+
+# Check input length
+if [ ${#input} -le 1 ]
+then
+  echo "false"
+  exit 0
+fi
+
+# Check for non-digit characters
+if [[ $input =~ [^[:digit:]] ]]
+then
+  echo "false"
+  exit 0
+fi
+
+# Double every second digit from the right
+doubled_input=""
+for (( i=${#input}-1; i>=0; i-- ))
+do
+  # Checks if the current digit is a second digit
+  if [ $(((${#input}-i) % 2)) -eq 0 ]
+  then
+    doubled_digit=$(( ${input:$i:1} * 2 ))
+    if [ $doubled_digit -gt 9 ]
+    then
+      doubled_digit=$(( doubled_digit - 9 ))
+    fi
+    # Adds the new digit to the left of "doubled_input"
+    doubled_input="${doubled_digit}${doubled_input}"
+  # If is not a second digit, copy it to the new string.
+  else
+    doubled_input="${input:$i:1}${doubled_input}"
+  fi
+done
+
+# Calculate the sum of all digits
+sum=0
+for (( i=0; i<${#doubled_input}; i++ ))
+do
+  sum=$(( sum + ${doubled_input:$i:1} ))
+done
+
+# Check if sum is divisible by 10
+if [ $(( sum % 10 )) -eq 0 ]
+then
+  echo "true"
+else
+  echo "false"
+fi