diff --git a/awk/README.md b/awk/README.md
index 1a27ab81..243ea273 100644
--- a/awk/README.md
+++ b/awk/README.md
@@ -34,3 +34,4 @@
 - [space-age](./space-age/README.md)
 - [all-your-base](./all-your-base/README.md)
 - [high-scores](./high-scores/README.md)
+- [difference-of-squares](./difference-of-squares/README.md)
diff --git a/awk/difference-of-squares/.exercism/config.json b/awk/difference-of-squares/.exercism/config.json
new file mode 100644
index 00000000..6e5f0c4d
--- /dev/null
+++ b/awk/difference-of-squares/.exercism/config.json
@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "glennj"
+  ],
+  "files": {
+    "solution": [
+      "difference-of-squares.awk"
+    ],
+    "test": [
+      "test-difference-of-squares.bats"
+    ],
+    "example": [
+      ".meta/example.awk"
+    ]
+  },
+  "blurb": "Find the difference between the square of the sum and the sum of the squares of the first N natural numbers.",
+  "source": "Problem 6 at Project Euler",
+  "source_url": "https://projecteuler.net/problem=6"
+}
diff --git a/awk/difference-of-squares/.exercism/metadata.json b/awk/difference-of-squares/.exercism/metadata.json
new file mode 100644
index 00000000..413e43a6
--- /dev/null
+++ b/awk/difference-of-squares/.exercism/metadata.json
@@ -0,0 +1 @@
+{"track":"awk","exercise":"difference-of-squares","id":"488e8fafe396433c9d3455749663cac9","url":"https://exercism.org/tracks/awk/exercises/difference-of-squares","handle":"vpayno","is_requester":true,"auto_approve":false}
\ No newline at end of file
diff --git a/awk/difference-of-squares/.lint_default_vars b/awk/difference-of-squares/.lint_default_vars
new file mode 100644
index 00000000..e69de29b
diff --git a/awk/difference-of-squares/HELP.md b/awk/difference-of-squares/HELP.md
new file mode 100644
index 00000000..f7806eb2
--- /dev/null
+++ b/awk/difference-of-squares/HELP.md
@@ -0,0 +1,100 @@
+# Help
+
+## Running the tests
+
+Each exercise contains a test file.
+Run the tests using the `bats` program.
+```bash
+bats test-hello-world.bats
+```
+
+`bats` will need to be installed.
+See the [Testing on the Bash track][bash] page for instructions to install `bats` for your system.
+
+### bats is implemented in bash
+
+The bats file is a bash script, with some special functions recognized by the `bats` command.
+You'll see some tests that look like
+```sh
+gawk -f some-exercise.awk <<< "some,input,here"
+```
+That `<<<` syntax is a bash [Here String][here-string].
+It sends the string on the right-hand side into the standard input of the program on the left-hand side.
+It is ([approximately][so]) the same as
+```sh
+echo "some,input,here" | gawk -f some-exercise.awk
+```
+
+## Help for assert functions
+
+The tests use functions from the [bats-assert][bats-assert] library.
+Help for the various `assert*` functions can be found there.
+
+## Skipped tests
+
+Solving an exercise means making all its tests pass.
+By default, only one test (the first one) is executed when you run the tests.
+This is intentional, as it allows you to focus on just making that one test pass.
+Once it passes, you can enable the next test by commenting out or removing the
+
+    [[ $BATS_RUN_SKIPPED == true ]] || skip
+
+annotations prepending other tests.
+
+## Overriding skips
+
+To run all tests, including the ones with `skip` annotations, you can run:
+```bash
+BATS_RUN_SKIPPED=true bats test-some-exercise.bats
+```
+
+It can be convenient to use a wrapper function to save on typing: in `bash` you can do:
+```bash
+bats() {
+    BATS_RUN_SKIPPED=true command bats *.bats
+}
+```
+Then run tests with just:
+```bash
+bats
+```
+
+[bash]: https://exercism.org/docs/tracks/bash/tests
+[bats-assert]: https://github.com/bats-core/bats-assert
+[here-string]: https://www.gnu.org/software/bash/manual/bash.html#Here-Strings
+[so]: https://unix.stackexchange.com/a/80372/4667
+
+## Submitting your solution
+
+You can submit your solution using the `exercism submit difference-of-squares.awk` command.
+This command will upload your solution to the Exercism website and print the solution page's URL.
+
+It's possible to submit an incomplete solution which allows you to:
+
+- See how others have completed the exercise
+- Request help from a mentor
+
+## Need to get help?
+
+If you'd like help solving the exercise, check the following pages:
+
+- The [AWK track's documentation](https://exercism.org/docs/tracks/awk)
+- The [AWK track's programming category on the forum](https://forum.exercism.org/c/programming/awk)
+- [Exercism's programming category on the forum](https://forum.exercism.org/c/programming/5)
+- The [Frequently Asked Questions](https://exercism.org/docs/using/faqs)
+
+Should those resources not suffice, you could submit your (incomplete) solution to request mentoring.
+
+Places to look for help for AWK questions:
+
+* [Stack Overflow `awk` tag][so].
+* check the Resources section of the [Stack Overflow `awk` tag into page][so-info].
+* raise an issue at the [exercism/awk][github] Github repository.
+* IRC: `irc://irc.liberachat.net/#awk`, `irc://irc.liberachat.net/#exercism`
+    * see [Libera.chat][libera] if you're unfamiliar with IRC.
+
+
+[so]: https://stackoverflow.com/tags/awk
+[so-info]: https://stackoverflow.com/tags/awk/info
+[github]: https://github.com/exercism/awk
+[libera]: https://libera.chat
\ No newline at end of file
diff --git a/awk/difference-of-squares/README.md b/awk/difference-of-squares/README.md
new file mode 100644
index 00000000..62acb228
--- /dev/null
+++ b/awk/difference-of-squares/README.md
@@ -0,0 +1,36 @@
+# Difference of Squares
+
+Welcome to Difference of Squares on Exercism's AWK Track.
+If you need help running the tests or submitting your code, check out `HELP.md`.
+
+## Instructions
+
+Find the difference between the square of the sum and the sum of the squares of the first N natural numbers.
+
+The square of the sum of the first ten natural numbers is
+(1 + 2 + ... + 10)² = 55² = 3025.
+
+The sum of the squares of the first ten natural numbers is
+1² + 2² + ... + 10² = 385.
+
+Hence the difference between the square of the sum of the first ten natural numbers and the sum of the squares of the first ten natural numbers is 3025 - 385 = 2640.
+
+You are not expected to discover an efficient solution to this yourself from first principles; research is allowed, indeed, encouraged.
+Finding the best algorithm for the problem is a key skill in software engineering.
+
+## Source
+
+### Created by
+
+- @glennj
+
+### Based on
+
+Problem 6 at Project Euler - https://projecteuler.net/problem=6
+
+### My Solution
+
+- [my solution](./difference-of-squares.awk)
+- [awkunit tests](./difference-of-squares_test.awk)
+- [test cases](./test-cases.awk)
+- [run-tests](./run-tests-awk.txt)
diff --git a/awk/difference-of-squares/awkunit.awk b/awk/difference-of-squares/awkunit.awk
new file mode 120000
index 00000000..38a08adf
--- /dev/null
+++ b/awk/difference-of-squares/awkunit.awk
@@ -0,0 +1 @@
+../.lib/awkunit.awk
\ No newline at end of file
diff --git a/awk/difference-of-squares/bats-extra.bash b/awk/difference-of-squares/bats-extra.bash
new file mode 100644
index 00000000..54d48070
--- /dev/null
+++ b/awk/difference-of-squares/bats-extra.bash
@@ -0,0 +1,637 @@
+# This is the source code for bats-support and bats-assert, concatenated
+# * https://github.com/bats-core/bats-support
+# * https://github.com/bats-core/bats-assert
+#
+# Comments have been removed to save space. See the git repos for full source code.
+
+############################################################
+#
+# bats-support - Supporting library for Bats test helpers
+#
+# Written in 2016 by Zoltan Tombol <zoltan dot tombol at gmail dot com>
+#
+# To the extent possible under law, the author(s) have dedicated all
+# copyright and related and neighboring rights to this software to the
+# public domain worldwide. This software is distributed without any
+# warranty.
+#
+# You should have received a copy of the CC0 Public Domain Dedication
+# along with this software. If not, see
+# <http://creativecommons.org/publicdomain/zero/1.0/>.
+#
+
+fail() {
+  (( $# == 0 )) && batslib_err || batslib_err "$@"
+  return 1
+}
+
+batslib_is_caller() {
+  local -i is_mode_direct=1
+
+  # Handle options.
+  while (( $# > 0 )); do
+    case "$1" in
+      -i|--indirect) is_mode_direct=0; shift ;;
+      --) shift; break ;;
+      *) break ;;
+    esac
+  done
+
+  # Arguments.
+  local -r func="$1"
+
+  # Check call stack.
+  if (( is_mode_direct )); then
+    [[ $func == "${FUNCNAME[2]}" ]] && return 0
+  else
+    local -i depth
+    for (( depth=2; depth<${#FUNCNAME[@]}; ++depth )); do
+      [[ $func == "${FUNCNAME[$depth]}" ]] && return 0
+    done
+  fi
+
+  return 1
+}
+
+batslib_err() {
+  { if (( $# > 0 )); then
+      echo "$@"
+    else
+      cat -
+    fi
+  } >&2
+}
+
+batslib_count_lines() {
+  local -i n_lines=0
+  local line
+  while IFS='' read -r line || [[ -n $line ]]; do
+    (( ++n_lines ))
+  done < <(printf '%s' "$1")
+  echo "$n_lines"
+}
+
+batslib_is_single_line() {
+  for string in "$@"; do
+    (( $(batslib_count_lines "$string") > 1 )) && return 1
+  done
+  return 0
+}
+
+batslib_get_max_single_line_key_width() {
+  local -i max_len=-1
+  while (( $# != 0 )); do
+    local -i key_len="${#1}"
+    batslib_is_single_line "$2" && (( key_len > max_len )) && max_len="$key_len"
+    shift 2
+  done
+  echo "$max_len"
+}
+
+batslib_print_kv_single() {
+  local -ir col_width="$1"; shift
+  while (( $# != 0 )); do
+    printf '%-*s : %s\n' "$col_width" "$1" "$2"
+    shift 2
+  done
+}
+
+batslib_print_kv_multi() {
+  while (( $# != 0 )); do
+    printf '%s (%d lines):\n' "$1" "$( batslib_count_lines "$2" )"
+    printf '%s\n' "$2"
+    shift 2
+  done
+}
+
+batslib_print_kv_single_or_multi() {
+  local -ir width="$1"; shift
+  local -a pairs=( "$@" )
+
+  local -a values=()
+  local -i i
+  for (( i=1; i < ${#pairs[@]}; i+=2 )); do
+    values+=( "${pairs[$i]}" )
+  done
+
+  if batslib_is_single_line "${values[@]}"; then
+    batslib_print_kv_single "$width" "${pairs[@]}"
+  else
+    local -i i
+    for (( i=1; i < ${#pairs[@]}; i+=2 )); do
+      pairs[$i]="$( batslib_prefix < <(printf '%s' "${pairs[$i]}") )"
+    done
+    batslib_print_kv_multi "${pairs[@]}"
+  fi
+}
+
+batslib_prefix() {
+  local -r prefix="${1:-  }"
+  local line
+  while IFS='' read -r line || [[ -n $line ]]; do
+    printf '%s%s\n' "$prefix" "$line"
+  done
+}
+
+batslib_mark() {
+  local -r symbol="$1"; shift
+  # Sort line numbers.
+  set -- $( sort -nu <<< "$( printf '%d\n' "$@" )" )
+
+  local line
+  local -i idx=0
+  while IFS='' read -r line || [[ -n $line ]]; do
+    if (( ${1:--1} == idx )); then
+      printf '%s\n' "${symbol}${line:${#symbol}}"
+      shift
+    else
+      printf '%s\n' "$line"
+    fi
+    (( ++idx ))
+  done
+}
+
+batslib_decorate() {
+  echo
+  echo "-- $1 --"
+  cat -
+  echo '--'
+  echo
+}
+
+############################################################
+
+assert() {
+  if ! "$@"; then
+    batslib_print_kv_single 10 'expression' "$*" \
+    | batslib_decorate 'assertion failed' \
+    | fail
+  fi
+}
+
+assert_equal() {
+  if [[ $1 != "$2" ]]; then
+    batslib_print_kv_single_or_multi 8 \
+    'expected' "$2" \
+    'actual'   "$1" \
+    | batslib_decorate 'values do not equal' \
+    | fail
+  fi
+}
+
+assert_failure() {
+  : "${output?}"
+  : "${status?}"
+
+  (( $# > 0 )) && local -r expected="$1"
+  if (( status == 0 )); then
+    batslib_print_kv_single_or_multi 6 'output' "$output" \
+    | batslib_decorate 'command succeeded, but it was expected to fail' \
+    | fail
+  elif (( $# > 0 )) && (( status != expected )); then
+    { local -ir width=8
+      batslib_print_kv_single "$width" \
+      'expected' "$expected" \
+      'actual'   "$status"
+      batslib_print_kv_single_or_multi "$width" \
+      'output' "$output"
+    } \
+    | batslib_decorate 'command failed as expected, but status differs' \
+    | fail
+  fi
+}
+
+assert_line() {
+  local -i is_match_line=0
+  local -i is_mode_partial=0
+  local -i is_mode_regexp=0
+  : "${lines?}"
+
+  # Handle options.
+  while (( $# > 0 )); do
+    case "$1" in
+    -n|--index)
+      if (( $# < 2 )) || ! [[ $2 =~ ^([0-9]|[1-9][0-9]+)$ ]]; then
+        echo "\`--index' requires an integer argument: \`$2'" \
+        | batslib_decorate 'ERROR: assert_line' \
+        | fail
+        return $?
+      fi
+      is_match_line=1
+      local -ri idx="$2"
+      shift 2
+      ;;
+    -p|--partial) is_mode_partial=1; shift ;;
+    -e|--regexp) is_mode_regexp=1; shift ;;
+    --) shift; break ;;
+    *) break ;;
+    esac
+  done
+
+  if (( is_mode_partial )) && (( is_mode_regexp )); then
+    echo "\`--partial' and \`--regexp' are mutually exclusive" \
+    | batslib_decorate 'ERROR: assert_line' \
+    | fail
+    return $?
+  fi
+
+  # Arguments.
+  local -r expected="$1"
+
+  if (( is_mode_regexp == 1 )) && [[ '' =~ $expected ]] || (( $? == 2 )); then
+    echo "Invalid extended regular expression: \`$expected'" \
+    | batslib_decorate 'ERROR: assert_line' \
+    | fail
+    return $?
+  fi
+
+  # Matching.
+  if (( is_match_line )); then
+    # Specific line.
+    if (( is_mode_regexp )); then
+      if ! [[ ${lines[$idx]} =~ $expected ]]; then
+        batslib_print_kv_single 6 \
+        'index' "$idx" \
+        'regexp' "$expected" \
+        'line'  "${lines[$idx]}" \
+        | batslib_decorate 'regular expression does not match line' \
+        | fail
+      fi
+    elif (( is_mode_partial )); then
+      if [[ ${lines[$idx]} != *"$expected"* ]]; then
+        batslib_print_kv_single 9 \
+        'index'     "$idx" \
+        'substring' "$expected" \
+        'line'      "${lines[$idx]}" \
+        | batslib_decorate 'line does not contain substring' \
+        | fail
+      fi
+    else
+      if [[ ${lines[$idx]} != "$expected" ]]; then
+        batslib_print_kv_single 8 \
+        'index'    "$idx" \
+        'expected' "$expected" \
+        'actual'   "${lines[$idx]}" \
+        | batslib_decorate 'line differs' \
+        | fail
+      fi
+    fi
+  else
+    # Contained in output.
+    if (( is_mode_regexp )); then
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        [[ ${lines[$idx]} =~ $expected ]] && return 0
+      done
+      { local -ar single=( 'regexp' "$expected" )
+        local -ar may_be_multi=( 'output' "$output" )
+        local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+        batslib_print_kv_single "$width" "${single[@]}"
+        batslib_print_kv_single_or_multi "$width" "${may_be_multi[@]}"
+      } \
+      | batslib_decorate 'no output line matches regular expression' \
+      | fail
+    elif (( is_mode_partial )); then
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        [[ ${lines[$idx]} == *"$expected"* ]] && return 0
+      done
+      { local -ar single=( 'substring' "$expected" )
+        local -ar may_be_multi=( 'output' "$output" )
+        local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+        batslib_print_kv_single "$width" "${single[@]}"
+        batslib_print_kv_single_or_multi "$width" "${may_be_multi[@]}"
+      } \
+      | batslib_decorate 'no output line contains substring' \
+      | fail
+    else
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        [[ ${lines[$idx]} == "$expected" ]] && return 0
+      done
+      { local -ar single=( 'line' "$expected" )
+        local -ar may_be_multi=( 'output' "$output" )
+        local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+        batslib_print_kv_single "$width" "${single[@]}"
+        batslib_print_kv_single_or_multi "$width" "${may_be_multi[@]}"
+      } \
+      | batslib_decorate 'output does not contain line' \
+      | fail
+    fi
+  fi
+}
+
+assert_output() {
+  local -i is_mode_partial=0
+  local -i is_mode_regexp=0
+  local -i is_mode_nonempty=0
+  local -i use_stdin=0
+  : "${output?}"
+
+  # Handle options.
+  if (( $# == 0 )); then
+    is_mode_nonempty=1
+  fi
+
+  while (( $# > 0 )); do
+    case "$1" in
+    -p|--partial) is_mode_partial=1; shift ;;
+    -e|--regexp) is_mode_regexp=1; shift ;;
+    -|--stdin) use_stdin=1; shift ;;
+    --) shift; break ;;
+    *) break ;;
+    esac
+  done
+
+  if (( is_mode_partial )) && (( is_mode_regexp )); then
+    echo "\`--partial' and \`--regexp' are mutually exclusive" \
+    | batslib_decorate 'ERROR: assert_output' \
+    | fail
+    return $?
+  fi
+
+  # Arguments.
+  local expected
+  if (( use_stdin )); then
+    expected="$(cat -)"
+  else
+    expected="${1-}"
+  fi
+
+  # Matching.
+  if (( is_mode_nonempty )); then
+    if [ -z "$output" ]; then
+      echo 'expected non-empty output, but output was empty' \
+      | batslib_decorate 'no output' \
+      | fail
+    fi
+  elif (( is_mode_regexp )); then
+    if [[ '' =~ $expected ]] || (( $? == 2 )); then
+      echo "Invalid extended regular expression: \`$expected'" \
+      | batslib_decorate 'ERROR: assert_output' \
+      | fail
+    elif ! [[ $output =~ $expected ]]; then
+      batslib_print_kv_single_or_multi 6 \
+      'regexp'  "$expected" \
+      'output' "$output" \
+      | batslib_decorate 'regular expression does not match output' \
+      | fail
+    fi
+  elif (( is_mode_partial )); then
+    if [[ $output != *"$expected"* ]]; then
+      batslib_print_kv_single_or_multi 9 \
+      'substring' "$expected" \
+      'output'    "$output" \
+      | batslib_decorate 'output does not contain substring' \
+      | fail
+    fi
+  else
+    if [[ $output != "$expected" ]]; then
+      batslib_print_kv_single_or_multi 8 \
+      'expected' "$expected" \
+      'actual'   "$output" \
+      | batslib_decorate 'output differs' \
+      | fail
+    fi
+  fi
+}
+
+assert_success() {
+  : "${output?}"
+  : "${status?}"
+
+  if (( status != 0 )); then
+    { local -ir width=6
+      batslib_print_kv_single "$width" 'status' "$status"
+      batslib_print_kv_single_or_multi "$width" 'output' "$output"
+    } \
+    | batslib_decorate 'command failed' \
+    | fail
+  fi
+}
+
+refute() {
+  if "$@"; then
+    batslib_print_kv_single 10 'expression' "$*" \
+    | batslib_decorate 'assertion succeeded, but it was expected to fail' \
+    | fail
+  fi
+}
+
+refute_line() {
+  local -i is_match_line=0
+  local -i is_mode_partial=0
+  local -i is_mode_regexp=0
+  : "${lines?}"
+
+  # Handle options.
+  while (( $# > 0 )); do
+    case "$1" in
+    -n|--index)
+      if (( $# < 2 )) || ! [[ $2 =~ ^([0-9]|[1-9][0-9]+)$ ]]; then
+        echo "\`--index' requires an integer argument: \`$2'" \
+        | batslib_decorate 'ERROR: refute_line' \
+        | fail
+        return $?
+      fi
+      is_match_line=1
+      local -ri idx="$2"
+      shift 2
+      ;;
+    -p|--partial) is_mode_partial=1; shift ;;
+    -e|--regexp) is_mode_regexp=1; shift ;;
+    --) shift; break ;;
+    *) break ;;
+    esac
+  done
+
+  if (( is_mode_partial )) && (( is_mode_regexp )); then
+    echo "\`--partial' and \`--regexp' are mutually exclusive" \
+    | batslib_decorate 'ERROR: refute_line' \
+    | fail
+    return $?
+  fi
+
+  # Arguments.
+  local -r unexpected="$1"
+
+  if (( is_mode_regexp == 1 )) && [[ '' =~ $unexpected ]] || (( $? == 2 )); then
+    echo "Invalid extended regular expression: \`$unexpected'" \
+    | batslib_decorate 'ERROR: refute_line' \
+    | fail
+    return $?
+  fi
+
+  # Matching.
+  if (( is_match_line )); then
+    # Specific line.
+    if (( is_mode_regexp )); then
+      if [[ ${lines[$idx]} =~ $unexpected ]]; then
+        batslib_print_kv_single 6 \
+        'index' "$idx" \
+        'regexp' "$unexpected" \
+        'line'  "${lines[$idx]}" \
+        | batslib_decorate 'regular expression should not match line' \
+        | fail
+      fi
+    elif (( is_mode_partial )); then
+      if [[ ${lines[$idx]} == *"$unexpected"* ]]; then
+        batslib_print_kv_single 9 \
+        'index'     "$idx" \
+        'substring' "$unexpected" \
+        'line'      "${lines[$idx]}" \
+        | batslib_decorate 'line should not contain substring' \
+        | fail
+      fi
+    else
+      if [[ ${lines[$idx]} == "$unexpected" ]]; then
+        batslib_print_kv_single 5 \
+        'index' "$idx" \
+        'line'  "${lines[$idx]}" \
+        | batslib_decorate 'line should differ' \
+        | fail
+      fi
+    fi
+  else
+    # Line contained in output.
+    if (( is_mode_regexp )); then
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        if [[ ${lines[$idx]} =~ $unexpected ]]; then
+          { local -ar single=( 'regexp' "$unexpected" 'index' "$idx" )
+            local -a may_be_multi=( 'output' "$output" )
+            local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+            batslib_print_kv_single "$width" "${single[@]}"
+            if batslib_is_single_line "${may_be_multi[1]}"; then
+              batslib_print_kv_single "$width" "${may_be_multi[@]}"
+            else
+              may_be_multi[1]="$( printf '%s' "${may_be_multi[1]}" | batslib_prefix | batslib_mark '>' "$idx" )"
+              batslib_print_kv_multi "${may_be_multi[@]}"
+            fi
+          } \
+          | batslib_decorate 'no line should match the regular expression' \
+          | fail
+          return $?
+        fi
+      done
+    elif (( is_mode_partial )); then
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        if [[ ${lines[$idx]} == *"$unexpected"* ]]; then
+          { local -ar single=( 'substring' "$unexpected" 'index' "$idx" )
+            local -a may_be_multi=( 'output' "$output" )
+            local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+            batslib_print_kv_single "$width" "${single[@]}"
+            if batslib_is_single_line "${may_be_multi[1]}"; then
+              batslib_print_kv_single "$width" "${may_be_multi[@]}"
+            else
+              may_be_multi[1]="$( printf '%s' "${may_be_multi[1]}" | batslib_prefix | batslib_mark '>' "$idx" )"
+              batslib_print_kv_multi "${may_be_multi[@]}"
+            fi
+          } \
+          | batslib_decorate 'no line should contain substring' \
+          | fail
+          return $?
+        fi
+      done
+    else
+      local -i idx
+      for (( idx = 0; idx < ${#lines[@]}; ++idx )); do
+        if [[ ${lines[$idx]} == "$unexpected" ]]; then
+          { local -ar single=( 'line' "$unexpected" 'index' "$idx" )
+            local -a may_be_multi=( 'output' "$output" )
+            local -ir width="$( batslib_get_max_single_line_key_width "${single[@]}" "${may_be_multi[@]}" )"
+            batslib_print_kv_single "$width" "${single[@]}"
+            if batslib_is_single_line "${may_be_multi[1]}"; then
+              batslib_print_kv_single "$width" "${may_be_multi[@]}"
+            else
+              may_be_multi[1]="$( printf '%s' "${may_be_multi[1]}" | batslib_prefix | batslib_mark '>' "$idx" )"
+              batslib_print_kv_multi "${may_be_multi[@]}"
+            fi
+          } \
+          | batslib_decorate 'line should not be in output' \
+          | fail
+          return $?
+        fi
+      done
+    fi
+  fi
+}
+
+refute_output() {
+  local -i is_mode_partial=0
+  local -i is_mode_regexp=0
+  local -i is_mode_empty=0
+  local -i use_stdin=0
+  : "${output?}"
+
+  # Handle options.
+  if (( $# == 0 )); then
+    is_mode_empty=1
+  fi
+
+  while (( $# > 0 )); do
+    case "$1" in
+    -p|--partial) is_mode_partial=1; shift ;;
+    -e|--regexp) is_mode_regexp=1; shift ;;
+    -|--stdin) use_stdin=1; shift ;;
+    --) shift; break ;;
+    *) break ;;
+    esac
+  done
+
+  if (( is_mode_partial )) && (( is_mode_regexp )); then
+    echo "\`--partial' and \`--regexp' are mutually exclusive" \
+    | batslib_decorate 'ERROR: refute_output' \
+    | fail
+    return $?
+  fi
+
+  # Arguments.
+  local unexpected
+  if (( use_stdin )); then
+    unexpected="$(cat -)"
+  else
+    unexpected="${1-}"
+  fi
+
+  if (( is_mode_regexp == 1 )) && [[ '' =~ $unexpected ]] || (( $? == 2 )); then
+    echo "Invalid extended regular expression: \`$unexpected'" \
+    | batslib_decorate 'ERROR: refute_output' \
+    | fail
+    return $?
+  fi
+
+  # Matching.
+  if (( is_mode_empty )); then
+    if [ -n "$output" ]; then
+      batslib_print_kv_single_or_multi 6 \
+      'output' "$output" \
+      | batslib_decorate 'output non-empty, but expected no output' \
+      | fail
+    fi
+  elif (( is_mode_regexp )); then
+    if [[ $output =~ $unexpected ]]; then
+      batslib_print_kv_single_or_multi 6 \
+      'regexp'  "$unexpected" \
+      'output' "$output" \
+      | batslib_decorate 'regular expression should not match output' \
+      | fail
+    fi
+  elif (( is_mode_partial )); then
+    if [[ $output == *"$unexpected"* ]]; then
+      batslib_print_kv_single_or_multi 9 \
+      'substring' "$unexpected" \
+      'output'    "$output" \
+      | batslib_decorate 'output should not contain substring' \
+      | fail
+    fi
+  else
+    if [[ $output == "$unexpected" ]]; then
+      batslib_print_kv_single_or_multi 6 \
+      'output' "$output" \
+      | batslib_decorate 'output equals, but it was expected to differ' \
+      | fail
+    fi
+  fi
+}
diff --git a/awk/difference-of-squares/difference-of-squares.awk b/awk/difference-of-squares/difference-of-squares.awk
new file mode 100644
index 00000000..4f02d646
--- /dev/null
+++ b/awk/difference-of-squares/difference-of-squares.awk
@@ -0,0 +1,47 @@
+#!/usr/bin/gawk -M --lint --file
+
+function squareOfSum(number) {
+    return (number * (number + 1) / 2) ^ 2
+}
+
+function sumOfSquares(number) {
+    return (number * (number + 1) * (2 * number + 1)) / 6
+}
+
+function differenceOfSquares(number) {
+    return squareOfSum(number) - sumOfSquares(number)
+}
+
+BEGIN {
+}
+
+{
+    if (match($0, /^difference,/)) {
+        _ = split($0, part, ",")
+
+        print differenceOfSquares(part[2])
+
+        exit 0
+    }
+
+    if (match($0, /^square_of_sum,/)) {
+        _ = split($0, part, ",")
+
+        print squareOfSum(part[2])
+
+        exit 0
+    }
+
+    if (match($0, /^sum_of_squares,/)) {
+        _ = split($0, part, ",")
+
+        print sumOfSquares(part[2])
+
+        exit 0
+    }
+
+    print "error"
+}
+
+END {
+}
diff --git a/awk/difference-of-squares/difference-of-squares_test.awk b/awk/difference-of-squares/difference-of-squares_test.awk
new file mode 100644
index 00000000..1ad87fde
--- /dev/null
+++ b/awk/difference-of-squares/difference-of-squares_test.awk
@@ -0,0 +1,74 @@
+#!/usr/bin/gawk --bignum --lint --file
+
+@include "awkunit"
+@include "test-cases"
+@include "difference-of-squares"
+
+passed = 0
+testCount = 0
+
+function _debugTestPre() {
+    printf "Test %s: %s\n", (passed + 1), test_name
+    printf "     input -> [%s]\n", input
+}
+
+function _debugTestPost() {
+    passed = passed + 1
+    printf "    output -> [%s]\n", got
+    printf "    result -> passed\n\n"
+}
+
+function casesDifferenceOfSquares() {
+    printf "Running %d test cases\n\n", length(cases)
+    caseNum = 0
+
+    # orders array by index in for loop
+    PROCINFO["sorted_in"] = "@ind_str_asc"
+
+    # Associative arrays don't preserve insert order.
+    for (key in cases) {
+        input = key
+        want = cases[key]
+
+        test_name = "[" input "]"
+
+        _ = split(input, part, ",")
+
+        _debugTestPre()
+
+        got = 0
+
+        if (part[1] == "difference") {
+            got = differenceOfSquares(part[2])
+        } else if (part[1] == "square_of_sum") {
+            got = squareOfSum(part[2])
+        } else if (part[1] == "sum_of_squares") {
+            got = sumOfSquares(part[2])
+        }
+
+        assertEquals(got, want)
+
+        _debugTestPost()
+    }
+}
+
+BEGIN {
+    exit 0
+}
+
+END {
+    cmd = "grep --no-filename --count ^function\\ test *_test.awk"
+    cmd | getline testCount
+
+    printf "\nRunning %d tests...\n\n", testCount
+
+    testCount = testCount + length(cases)
+
+    # running tests with reduced duplication
+    casesDifferenceOfSquares()
+
+    print "\n" passed " out of " testCount " tests passed!"
+
+    # add exit here to keep it from looping
+    exit 0
+}
diff --git a/awk/difference-of-squares/run-tests-awk.txt b/awk/difference-of-squares/run-tests-awk.txt
new file mode 100644
index 00000000..7ae93d4b
--- /dev/null
+++ b/awk/difference-of-squares/run-tests-awk.txt
@@ -0,0 +1,158 @@
+Running automated test file(s):
+
+
+===============================================================================
+
+AWKLIBPATH=/usr/lib/x86_64-linux-gnu/gawk:../.lib
+
+/usr/lib/x86_64-linux-gnu/gawk
+filefuncs.so
+fnmatch.so
+fork.so
+inplace.so
+intdiv.so
+ordchr.so
+readdir.so
+readfile.so
+revoutput.so
+revtwoway.so
+rwarray.so
+time.so
+
+../.lib
+awkunit.awk
+awkunit.so
+
+gawk  --lint --file=./awkunit.awk < /dev/null > /dev/null
+gawk: ./awkunit.awk:3: warning: `load' is a gawk extension
+gawk: warning: function `assertEquals' defined but never called directly
+gawk: warning: function `assert' defined but never called directly
+gawk: ./awkunit.awk:26: warning: reference to uninitialized variable `_assert_exit'
+
+real	0m0.003s
+user	0m0.001s
+sys	0m0.002s
+
+gawk  --lint --file=./difference-of-squares.awk < /dev/null > /dev/null
+
+real	0m0.004s
+user	0m0.002s
+sys	0m0.002s
+
+gawk  --lint --file=./test-cases.awk < /dev/null > /dev/null
+
+real	0m0.003s
+user	0m0.001s
+sys	0m0.002s
+
+exit 0
+
+===============================================================================
+
+Running: bats ./test-difference-of-squares.bats
+1..9
+ok 1 square of sum 1
+ok 2 square of sum 5
+ok 3 square of sum 100
+ok 4 sum of squares 1
+ok 5 sum of squares 5
+ok 6 sum of squares 100
+ok 7 difference of squares 1
+ok 8 difference of squares 5
+ok 9 difference of squares 100
+
+real	0m0.351s
+user	0m0.210s
+sys	0m0.162s
+
+exit 0
+
+===============================================================================
+
+AWKLIBPATH=/usr/lib/x86_64-linux-gnu/gawk:../.lib
+
+/usr/lib/x86_64-linux-gnu/gawk
+filefuncs.so
+fnmatch.so
+fork.so
+inplace.so
+intdiv.so
+ordchr.so
+readdir.so
+readfile.so
+revoutput.so
+revtwoway.so
+rwarray.so
+time.so
+
+../.lib
+awkunit.awk
+awkunit.so
+
+Running: gawk  --file ./difference-of-squares_test.awk && printf \n%s\n Tests Passed! || printf \n%s\n Tests Failed!
+
+Running 0 tests...
+
+Running 8 test cases
+
+Test 1: [difference,1]
+     input -> [difference,1]
+    output -> [0]
+    result -> passed
+
+Test 2: [difference,100]
+     input -> [difference,100]
+    output -> [25164150]
+    result -> passed
+
+Test 3: [difference,5]
+     input -> [difference,5]
+    output -> [170]
+    result -> passed
+
+Test 4: [square_of_sum,1]
+     input -> [square_of_sum,1]
+    output -> [1]
+    result -> passed
+
+Test 5: [square_of_sum,100]
+     input -> [square_of_sum,100]
+    output -> [25502500]
+    result -> passed
+
+Test 6: [sum_of_squares,1]
+     input -> [sum_of_squares,1]
+    output -> [1]
+    result -> passed
+
+Test 7: [sum_of_squares,100]
+     input -> [sum_of_squares,100]
+    output -> [338350]
+    result -> passed
+
+Test 8: [sum_of_squares,5]
+     input -> [sum_of_squares,5]
+    output -> [55]
+    result -> passed
+
+
+8 out of 8 tests passed!
+
+real	0m0.009s
+user	0m0.005s
+sys	0m0.004s
+
+Tests Passed!
+
+exit 0
+
+===============================================================================
+
+Running: misspell .
+
+real	0m0.038s
+user	0m0.044s
+sys	0m0.014s
+
+===============================================================================
+
diff --git a/awk/difference-of-squares/test-cases.awk b/awk/difference-of-squares/test-cases.awk
new file mode 100644
index 00000000..0560a799
--- /dev/null
+++ b/awk/difference-of-squares/test-cases.awk
@@ -0,0 +1,20 @@
+#!/usr/bin/gawk --lint --file
+# test-cases.awk
+
+#   key: input
+# value: output
+
+BEGIN {
+    cases["square_of_sum,1"]="1"
+    cases["square_of_sum,100"]="225"
+    cases["square_of_sum,100"]="25502500"
+    cases["sum_of_squares,1"]="1"
+    cases["sum_of_squares,5"]="55"
+    cases["sum_of_squares,100"]="338350"
+    cases["difference,1"]="0"
+    cases["difference,5"]="170"
+    cases["difference,100"]="25164150"
+
+    # add exit here to keep it from waiting for input
+    exit 0
+}
diff --git a/awk/difference-of-squares/test-difference-of-squares.bats b/awk/difference-of-squares/test-difference-of-squares.bats
new file mode 100644
index 00000000..72d76f3f
--- /dev/null
+++ b/awk/difference-of-squares/test-difference-of-squares.bats
@@ -0,0 +1,66 @@
+#!/usr/bin/env bats
+load bats-extra
+
+# Square the sum of the numbers up to the given number
+@test "square of sum 1" {
+  #[[ $BATS_RUN_SKIPPED == "true" ]] || skip
+  run gawk -f difference-of-squares.awk <<< "square_of_sum,1"
+  assert_success
+  assert_output "1"
+}
+
+@test "square of sum 5" {
+  [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+  run gawk -f difference-of-squares.awk <<< "square_of_sum,5"
+  assert_success
+  assert_output "225"
+}
+
+@test "square of sum 100" {
+  [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+  run gawk -f difference-of-squares.awk <<< "square_of_sum,100"
+  assert_success
+  assert_output "25502500"
+}
+# Sum the squares of the numbers up to the given number
+@test "sum of squares 1" {
+  [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+  run gawk -f difference-of-squares.awk <<< "sum_of_squares,1"
+  assert_success
+  assert_output "1"
+}
+
+@test "sum of squares 5" {
+  [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+  run gawk -f difference-of-squares.awk <<< "sum_of_squares,5"
+  assert_success
+  assert_output "55"
+}
+
+@test "sum of squares 100" {
+  [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+  run gawk -f difference-of-squares.awk <<< "sum_of_squares,100"
+  assert_success
+  assert_output "338350"
+}
+# Subtract sum of squares from square of sums
+@test "difference of squares 1" {
+  [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+  run gawk -f difference-of-squares.awk <<< "difference,1"
+  assert_success
+  assert_output "0"
+}
+
+@test "difference of squares 5" {
+  [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+  run gawk -f difference-of-squares.awk <<< "difference,5"
+  assert_success
+  assert_output "170"
+}
+
+@test "difference of squares 100" {
+  [[ $BATS_RUN_SKIPPED == "true" ]] || skip
+  run gawk -f difference-of-squares.awk <<< "difference,100"
+  assert_success
+  assert_output "25164150"
+}