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+# Luhn
+
+Given a number determine whether or not it is valid per the Luhn formula.
+
+The [Luhn algorithm](https://en.wikipedia.org/wiki/Luhn_algorithm) is
+a simple checksum formula used to validate a variety of identification
+numbers, such as credit card numbers and Canadian Social Insurance
+Numbers.
+
+The task is to check if a given string is valid.
+
+Validating a Number
+------
+
+Strings of length 1 or less are not valid. Spaces are allowed in the input,
+but they should be stripped before checking. All other non-digit characters
+are disallowed.
+
+## Example 1: valid credit card number
+
+```text
+4539 1488 0343 6467
+```
+
+The first step of the Luhn algorithm is to double every second digit,
+starting from the right. We will be doubling
+
+```text
+4_3_ 1_8_ 0_4_ 6_6_
+```
+
+If doubling the number results in a number greater than 9 then subtract 9
+from the product. The results of our doubling:
+
+```text
+8569 2478 0383 3437
+```
+
+Then sum all of the digits:
+
+```text
+8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80
+```
+
+If the sum is evenly divisible by 10, then the number is valid. This number is valid!
+
+## Example 2: invalid credit card number
+
+```text
+8273 1232 7352 0569
+```
+
+Double the second digits, starting from the right
+
+```text
+7253 2262 5312 0539
+```
+
+Sum the digits
+
+```text
+7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57
+```
+
+57 is not evenly divisible by 10, so this number is not valid.
+
+
+Run the tests with:
+
+```bash
+bats luhn_test.sh
+```
+
+After the first test(s) pass, continue by commenting out or removing the
+`[[ $BATS_RUN_SKIPPED == true ]] || skip` 
+annotations prepending other tests.
+
+To run all tests, including the ones with `skip` annotations, run:
+
+```bash
+BATS_RUN_SKIPPED=true bats luhn_test.sh
+```
+
+## Source
+
+The Luhn Algorithm on Wikipedia [http://en.wikipedia.org/wiki/Luhn_algorithm](http://en.wikipedia.org/wiki/Luhn_algorithm)
+
+
+## External utilities
+`Bash` is a language to write "scripts" -- programs that can call
+external tools, such as
+[`sed`](https://www.gnu.org/software/sed/),
+[`awk`](https://www.gnu.org/software/gawk/),
+[`date`](https://www.gnu.org/software/coreutils/manual/html_node/date-invocation.html)
+and even programs written in other programming languages, 
+like [`Python`](https://www.python.org/).
+This track does not restrict the usage of these utilities, and as long
+as your solution is portable between systems and does not require
+installation of third party applications, feel free to use them to solve
+the exercise.
+
+For an extra challenge, if you would like to have a better understanding
+of the language, try to re-implement the solution in pure `Bash`,
+without using any external tools. Note that there are some types of
+problems that bash cannot solve, such as performing floating point
+arithmetic and manipulating dates: for those, you must call out to an
+external tool.
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others
+have completed the exercise.