Added initial port of original OSX project.

- This is a copy of the [OSX](https://github.com/bkuhlmann/osx) project
  originally released on 2012-03-31. The OSX project has been
  deprecated. All future development and support will take place with
  this project instead.
- This project uses the *macOS* name in order to better match the
  updated branding and terminology used by Apple.

lib/verifiers.sh

@@ -0,0 +1,108 @@
+#! /bin/bash
+
+# DESCRIPTION
+# Defines verification/validation functions.
+
+# Verifies Homebrew software exists.
+# Parameters:
+# $1 = The file name.
+verify_homebrew() {
+  local application="$1"
+  local applications="$2"
+
+  if [[ "${applications[*]}" != *"$application"* ]]; then
+    printf " - Missing: $application\n"
+  fi
+}
+export -f verify_homebrew
+
+# Checks for missing Homebrew software.
+verify_homebrews() {
+  printf "Checking Homebrew software...\n"
+
+  local applications="$(brew list)"
+
+  while read line; do
+    # Skip blank or comment lines.
+    if [[ "$line" == "brew install"* ]]; then
+      local application=$(printf "$line" | awk '{print $3}')
+
+      # Exception: "gpg" is the binary but is listed as "gnugp".
+      if [[ "$application" == "gpg" ]]; then
+        application="gnupg"
+      fi
+
+      # Exception: "hg" is the binary but is listed as "mercurial".
+      if [[ "$application" == "hg" ]]; then
+        application="mercurial"
+      fi
+
+      verify_homebrew "$application" "${applications[*]}"
+    fi
+  done < "$PWD/scripts/homebrew.sh"
+
+  printf "Homebrew check complete.\n"
+}
+export -f verify_homebrews
+
+# Verifies application exists.
+# Parameters:
+# $1 = The file name.
+verify_application() {
+  local file_name="$1"
+
+  # Display the missing install if not found.
+  local install_path=$(get_install_path "$file_name")
+
+  if [[ ! -e "$install_path" ]]; then
+    printf " - Missing: $file_name\n"
+  fi
+}
+export -f verify_application
+
+# Checks for missing applications suffixed by "APP_NAME" as defined in settings.sh.
+verify_applications() {
+  printf "\nChecking application software...\n"
+
+  # Only use environment keys that end with "APP_NAME".
+  local file_names=$(set | awk -F "=" '{print $1}' | grep ".*APP_NAME")
+
+  # For each application name, check to see if the application is installed. Otherwise, skip.
+  for name in $file_names; do
+    # Pass the key value to verfication.
+    verify_application "${!name}"
+  done
+
+  printf "Application software check complete.\n"
+}
+export -f verify_applications
+
+# Verifies path exists.
+# Parameters:
+# $1 = The path.
+verify_path() {
+  local path="$1"
+
+  # Display the missing path if not found.
+  if [[ ! -e "$path" ]]; then
+    printf " - Missing: $path\n"
+  fi
+}
+export -f verify_path
+
+# Checks for missing extensions suffixed by "EXTENSION_PATH" as defined in settings.sh.
+verify_extensions() {
+  printf "\nChecking application extensions...\n"
+
+  # Only use environment keys that end with "EXTENSION_PATH".
+  local extensions=$(set | awk -F "=" '{print $1}' | grep ".*EXTENSION_PATH")
+
+  # For each extension, check to see if the extension is installed. Otherwise, skip.
+  for extension in $extensions; do
+    # Evaluate/extract the key (extension) value and pass it on for verfication.
+    verify_path "${!extension}"
+  done
+
+  printf "Application extension check complete.\n"
+}
+export -f verify_extensions