#!/usr/bin/env bash
###############################################################################
#     __               __
#    / /_  ____  _____/ /______
#   / __ \/ __ \/ ___/ __/ ___/
#  / / / / /_/ (__  ) /_(__  )
# /_/ /_/\____/____/\__/____/
#
# A program for managing host file entries.
#
# https://github.com/xwmx/hosts
#
# Based on Bash Boilerplate: https://github.com/xwmx/bash-boilerplate
#
# The MIT License (MIT)
#
# Copyright (c) 2015 William Melody • hi@williammelody.com
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.
###############################################################################

###############################################################################
# Strict Mode
###############################################################################

set -o nounset
set -o errexit
set -o pipefail
set -o noglob
IFS=$'\n\t'

###############################################################################
# Environment & Globals
###############################################################################

# $_VERSION
#
# The most recent program version.
_VERSION="3.6.4"

# $_ME
#
# This program's basename.
_ME="$(basename "${0}")"

# $HOSTS_DEFAULT_SUBCOMMAND
#
# The command to be run by default, when no command name is specified.
[[ -n "${HOSTS_DEFAULT_COMMAND:-}" ]] &&
  HOSTS_DEFAULT_SUBCOMMAND="${HOSTS_DEFAULT_COMMAND}"
HOSTS_DEFAULT_SUBCOMMAND="${HOSTS_DEFAULT_SUBCOMMAND:-list}"

# $HOSTS_PATH
#
# The path to the hosts file. This will almost always be /etc/hosts
HOSTS_PATH="${HOSTS_PATH:-/etc/hosts}"

# $_REPO
#
# The <user>/<repo> identifier for the git repository.
_REPO="xwmx/hosts"

# $_REPO_RAW_URL
#
# The base URL for raw files.
_REPO_RAW_URL="https://raw.githubusercontent.com/${_REPO}/master"

###############################################################################
# Debug
###############################################################################

# _debug()
#
# Usage:
#   _debug printf "Debug info. Variable: %s\\n" "$0"
#
# A simple function for executing a specified command if the `$_USE_DEBUG`
# variable has been set. The command is expected to print a message and
# should typically be either `echo`, `printf`, or `cat`.
__DEBUG_COUNTER=0
_debug() {
  if [[ "${_USE_DEBUG:-"0"}" -eq 1 ]]
  then
    __DEBUG_COUNTER=$((__DEBUG_COUNTER+1))
    {
      # Prefix debug message with "bug (U+1F41B)"
      printf "🐛  %s " "${__DEBUG_COUNTER}"
      "${@}"
      printf "―――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――\\n"
    } 1>&2
  fi
}

###############################################################################
# Error Messaging
###############################################################################

# _exit_1()
#
# Usage:
#   _exit_1 <command>
#
# Description:
#   Exit with status 1 after executing the specified with output redirected
#   to standard error. The command is expected to print a message and should
#   typically be either `echo`, `printf`, or `cat`.
_exit_1() {
  {
    printf "%s " "$(tput setaf 1)!$(tput sgr0)"
    "${@}"
  } 1>&2
  exit 1
}

# _return_1()
#
# Usage:
#   _return_1 <command>
#
# Description:
#   Return with status 1 after executing the specified with output redirected
#   to standard error. The command is expected to print a message and should
#   typically be either `echo`, `printf`, or `cat`.
_return_1() {
  {
    printf "%s " "$(tput setaf 1)!$(tput sgr0)"
    "${@}"
  } 1>&2
  return 1
}

# _warn()
#
# Usage:
#   _warn <command>
#
# Description:
#   Print the specified command with output redirected to standard error.
#   The command is expected to print a message and should typically be either
#   `echo`, `printf`, or `cat`.
_warn() {
  {
    printf "%s " "$(tput setaf 1)!$(tput sgr0)"
    "${@}"
  } 1>&2
}

###############################################################################
# Helpers
###############################################################################

# Space and tab for regular expressions
#
# sed regular expressions have slightly different behaviors dependending on
# the environment, and POSIX [[:space:]] matches whitespace characters other
# than just space and tab. These variables provide an easier, portable way to
# test for just these two characters.
export _TAB_=$'\t'
export _SPACE_=$' '
export _TAB_SPACE_="${_TAB_}${_SPACE_}"
export _TAB_SPACE_CC_="[${_TAB_SPACE_}]"

# _contains()
#
# Usage:
#   _contains "${item}" "${list[@]}"
#
# Returns:
#   0  If the item is included in the list.
#   1  If not.
_contains() {
  local _query="${1:-}"
  shift

  if [[ -z "${_query}"  ]] ||
     [[ -z "${*:-}"     ]]
  then
    return 1
  fi

  for __element in "${@}"
  do
    [[ "${__element}" == "${_query}" ]] && return 0
  done

  return 1
}

# _download_from()
#
# Usage:
#   _download_from <url> [<outfile>]
#
# Description:
#   Download the file at <url> and print to standard output or <outfile>.
#   Uses `curl` if available, falling back to `wget`.
_download_from() {
  local _downloaded=0
  local _target_path="${2:-}"
  local _url="${1:-}"

  if [[ -z "${_url}" ]] ||
     [[ ! "${_url}" =~ ^ftp|^http|^file|^mailto|^news|^telnet|^gopher ]]
  then
    return 1
  fi

  if [[ -n "${_target_path}" ]]
  then
    if hash "curl" 2>/dev/null
    then
      curl -s -L "${_url}" -o "${_target_path}" &&
        _downloaded=1
    elif hash "wget" 2>/dev/null
    then
      wget --quiet -O "${_target_path}" "${_url}" 2>/dev/null &&
        _downloaded=1
    fi
  else
    if hash "curl" 2>/dev/null
    then
      curl -s -L "${_url}" &&
        _downloaded=1
    elif hash "wget" 2>/dev/null
    then
      wget --quiet -O - "${_url}" 2>/dev/null &&
        _downloaded=1
    fi
  fi

  if ! ((_downloaded))
  then
    return 1
  fi
}

# _join()
#
# Usage:
#   _join "," a b c
#   _join "${an_array[@]}"
#
# Returns:
#   The list or array of items joined into a string with elements divided by
#   the optional separator if one is provided.
#
# More information:
#   https://stackoverflow.com/a/17841619
_join() {
  local _delimiter="${1}"
  shift
  printf "%s" "${1}"
  shift
  printf "%s" "${@/#/${_delimiter}}" | tr -d '[:space:]'
}

# _print_entries()
#
# Usage:
#   _print_entries <entries>
_print_entries() {
  local _input="${1:-}"
  [[ -n "${_input}" ]] || return 0

  local _newline=$'\n'

  if [[ -n "${2:-}" ]]
  then
    _input+="${_newline}${2:-}"
  fi

  local _max_length=0

  while IFS=$'\t ' read -r -a _parts
  do
    if [[ "${_parts[0]}" =~ disabled ]]
    then
      _parts=("${_parts[@]:1}")
    fi

    if [[ "${_max_length}" -lt "${#_parts[0]}" ]]
    then
      _max_length="${#_parts[0]}"
    fi
  done <<< "${_input}"

  local _max_tab_equivalent="$((_max_length / 8))"

  while IFS=$'\t ' read -r -a _parts
  do
    if [[ "${_parts[0]}" =~ disabled ]]
    then
      _parts=("${_parts[@]:1}")
      printf "disabled:\\n"
    fi

    local _current_tab_equivalent=$((${#_parts[0]} / 8))
    local _tab_count=$((_max_tab_equivalent - _current_tab_equivalent + 1))
    local _tabs=
    _tabs="$(printf "%*s" ${_tab_count} | tr " " '\t')"

    if [[ -n "${_parts[2]:-}" ]]
    then
      printf "%s%s  %s\t%s\\n"  \
        "${_parts[0]}"          \
        "${_tabs}"              \
        "${_parts[1]}"          \
        "$(printf "%s" "${_parts[*]:2}" | tr '\r\n' ' ')"
    else
      printf "%s%s  %s\\n"  \
        "${_parts[0]}"      \
        "${_tabs}"          \
        "${_parts[1]}"
    fi

  done <<< "${_input}"
}

# _sed_i()
#
# `sed -i` takes an extension on macOS, but that extension can cause errors in
# GNU `sed`.
#
# https://stackoverflow.com/q/43171648
# https://stackoverflow.com/a/16746032
_sed_i() {
  if sed --help >/dev/null 2>&1
  then # GNU
    sed -i "${@}"
  else # BSD
    sed -i '' "${@}"
  fi
}

# _verify_write_permissions
#
# Usage:
#   _verify_write_permissions
#
# Print a helpful error message when the specified operation can't be
# performed due to the lack of write permissions. If `$_AUTO_SUDO` is enabled,
# then run the command with sudo and exit with the command's exit value.
_verify_write_permissions() {
  if [[ ! -w "${HOSTS_PATH}" ]]
  then
    if ((_AUTO_SUDO))
    then
      local _my_path
      _my_path="$(cd "$(dirname "$0")"; pwd)/${_ME}"

      sudo "${_my_path}" "${_SUBCOMMAND}" "${_COMMAND_PARAMETERS[@]:-}"
      exit $?
    else
      _exit_1 printf \
"You don't have permission to perform this operation. Try again with:
sudo !!\\n"
    fi
  fi
}

###############################################################################
# desc
###############################################################################

# desc()
#
# Usage:
#   desc <name> <description>
#   desc --get <name>
#
# Options:
#   --get  Print the description for <name> if one has been set.
#
# Examples:
# ```
#   desc "list" <<HEREDOC
# Usage:
#   ${_ME} list
#
# Description:
#   List items.
# HEREDOC
#
# desc --get "list"
# ```
#
# Set or print a description for a specified command or function <name>. The
# <description> text can be passed as the second argument or as standard input.
#
# To make the <description> text available to other functions, `desc()` assigns
# the text to a variable with the format `$___desc_<name>`.
#
# When the `--get` option is used, the description for <name> is printed, if
# one has been set.
desc() {
  _debug printf "desc() \${*}: %s\\n" "$@"
  [[ -z "${1:-}" ]] && _exit_1 printf "desc(): No command name specified.\\n"

  if [[ "${1}" == "--get" ]]
  then # get ------------------------------------------------------------------
    [[ -z "${2:-}" ]] && _exit_1 printf "desc(): No command name specified.\\n"

    local _name="${2:-}"
    local _desc_var="___desc_${_name}"

    if [[ -n "${!_desc_var:-}" ]]
    then
      printf "%s\\n" "${!_desc_var}"
    else
      printf "No additional information for \`%s\`\\n" "${_name}"
    fi
  else # set ------------------------------------------------------------------
    if [[ -n "${2:-}" ]]
    then # argument is present
      read -r -d '' "___desc_${1}" <<HEREDOC || true
${2}
HEREDOC

      _debug printf "desc() set with argument: \${___desc_%s}\\n" "${1}"
    else # no argument is present, so assume piped input
      # `read` exits with non-zero status when a delimeter is not found, so
      # avoid errors by ending statement with `|| true`.
      read -r -d '' "___desc_${1}" || true

      _debug printf "desc() set with pipe: \${___desc_%s}\\n" "${1}"
    fi
  fi
}

###############################################################################
# Help
###############################################################################

# help ################################################################### help

desc "help" <<HEREDOC
Usage:
  ${_ME} help [<command>]

Description:
  Display help information for ${_ME} or a specified command.
HEREDOC
help() {
  if [[ -n "${1:-}" ]]
  then
    desc --get "${1}"
  else
    cat <<HEREDOC
    __               __
   / /_  ____  _____/ /______
  / __ \\/ __ \\/ ___/ __/ ___/
 / / / / /_/ (__  ) /_(__  )
/_/ /_/\\____/____/\\__/____/

A program for managing host file entries.

Usage:
  ${_ME} [<search string>]
  ${_ME} add <ip> <hostname> [<comment>]
  ${_ME} backups [create | (compare | delete | restore | show) <filename>]
  ${_ME} block <hostname>...
  ${_ME} completions (check | install [-d | --download] | uninstall)
  ${_ME} disable (<ip> | <hostname> | <search string>)
  ${_ME} disabled
  ${_ME} edit
  ${_ME} enable (<ip> | <hostname> | <search string>)
  ${_ME} enabled
  ${_ME} file
  ${_ME} list [enabled | disabled | <search string>]
  ${_ME} search <search string>
  ${_ME} show (<ip> | <hostname> | <search string>)
  ${_ME} subcommands [--raw]
  ${_ME} remove (<ip> | <hostname> | <search string>) [--force]
  ${_ME} unblock <hostname>...
  ${_ME} --auto-sudo
  ${_ME} -h | --help
  ${_ME} --version

Options:
  --auto-sudo  Run write commands with \`sudo\` automatically.
  -h --help    Display this help information.
  --version    Display version information.

Help:
  ${_ME} help [<command>]

More Information:
  https://github.com/xwmx/hosts
HEREDOC
  fi
}

###############################################################################
# Commands
# ========.....................................................................
#
# Example command group structure:
#
# desc example ""   - Optional. A short description for the command.
# example() { : }   - The command called by the user.
#
#
# desc example <<HEREDOC
#   Usage:
#     ${_ME} example
#
#   Description:
#     Print "Hello, World!"
#
#     For usage formatting conventions see:
#     - http://docopt.org/
#     - http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html
# HEREDOC
# example() {
#   printf "Hello, World!\\n"
# }
#
###############################################################################

# add ##################################################################### add

desc "add" <<HEREDOC
Usage:
  ${_ME} add <ip> <hostname> [<comment>]

Description:
  Add a given IP address and hostname pair, along with an optional comment.

Exit status:
  0   Entry successfully added.
  1   Invalid parameters or entry exists.
HEREDOC
add() {
  _debug printf "add() \${1}: %s\\n" "${1:-}"
  _debug printf "add() \${2}: %s\\n" "${2:-}"
  _debug printf "add() \${3}: %s\\n" "${3:-}"

  _verify_write_permissions

  local _ip="${1:-}"
  local _hostname="${2:-}"
  local _comment="${*:3}"
  local _tabs=$'\t'

  if [[ -z "${_ip:-}" ]]
  then
    help "add"
    exit 1
  elif [[ -z "${_hostname:-}" ]]
  then
    printf "Please include a hostname\\n"
    help "add"
    exit 1
  elif grep \
    -e "^${_ip}[[:space:]]\+${_hostname}$" \
    -e "^${_ip}[[:space:]]\+${_hostname}[[:space:]]\+.*$" "${HOSTS_PATH}"
  then
    _exit_1 printf \
      "Duplicate address/host combination, %s unchanged.\\n" \
      "${HOSTS_PATH}"
  else
    if [[ "${#_ip}" -lt 8 ]]
    then
      _tabs=$'\t\t'
    fi

    if [[ -n "${_comment:-}" ]]
    then
      local _formatted_comment
      _formatted_comment=$(_join " " "${_comment[@]}")

      printf "%s%s%s\\t# %s\\n" \
        "${_ip}"                \
        "${_tabs}"              \
        "${_hostname}"          \
        "${_formatted_comment}" >> "${HOSTS_PATH}"
      printf "Added:\\n%s%s%s\\t# %s\\n"  \
        "${_ip}"                          \
        "${_tabs}"                        \
        "${_hostname}"                    \
        "${_formatted_comment}"
    else
      printf "%s%s%s\\n"  \
        "${_ip}"          \
        "${_tabs}"        \
        "${_hostname}" >> "${HOSTS_PATH}"
      printf "Added:\\n%s%s%s\\n" \
        "${_ip}"                  \
        "${_tabs}"                \
        "${_hostname}"
    fi
  fi
}

# backups ############################################################# backups

desc "backups" <<HEREDOC
Usage:
  ${_ME} backups
  ${_ME} backups create
  ${_ME} backups compare <filename>
  ${_ME} backups delete  <filename>
  ${_ME} backups restore <filename> [--skip-backup]
  ${_ME} backups show    <filename>

Subcommands:
  backups           List available backups.
  backups create    Create a new backup of the hosts file.
  backups compare   Compare a backup file with the current hosts file.
  backups delete    Delete the specified backup.
  backups restore   Replace the contents of the hosts file with a
                    specified backup. The hosts file is automatically
                    backed up before being overwritten unless the
                    '--skip-backup' flag is specified.
  backups show      Show the contents of the specified backup file.

Description:
  Manage backups.

Exit status:
  0   Success.
  1   Invalid parameters or backup not found.
HEREDOC
backups() {
  local _filename=
  local _hosts_dirname=
  _hosts_dirname="$(dirname "${HOSTS_PATH}")"
  local _skip_backup=0
  local _subcommand=

  for __arg in "${@:-}"
  do
    case "${__arg}" in
      --skip-backup)
        _skip_backup=1
        ;;
      compare|create|delete|restore|show)
        _subcommand="${__arg}"
        ;;
      *)
        if [[ -z "${_filename:-}" ]]
        then
          _filename="${__arg}"
        fi
        ;;
    esac
  done

  case "${_subcommand}" in
    compare)
      if [[ -z "${_filename:-}" ]]
      then
        help "backups"
        exit 1
      elif [[ ! -e "${_hosts_dirname}/${_filename}" ]]
      then
        _exit_1 printf "Backup not found: %s\\n" "${_filename:-}"
      fi

      diff -u "${HOSTS_PATH}" "${_hosts_dirname}/${_filename}"
      ;;
    create)
      _verify_write_permissions

      local _timestamp
      _timestamp="$(date +"%Y%m%d%H%M%S")"

      cp "${HOSTS_PATH}" "${HOSTS_PATH}--backup-${_timestamp}" && \
        printf "Backed up to %s--backup-%s\\n" "${HOSTS_PATH}" "${_timestamp}"
      ;;
    delete)
      if [[ -z "${_filename:-}" ]]
      then
        help "backups"
        exit 1
      fi

      _verify_write_permissions

      if [[ "${HOSTS_PATH}" != "${_hosts_dirname}/${_filename:-}" ]] &&
         [[ -e "${_hosts_dirname}/${_filename:-}"                 ]]
      then
        rm "${_hosts_dirname}/${_filename:-}" &&
          printf "Backup deleted: %s\\n" "${_hosts_dirname}/${_filename:-}"
      else
        _exit_1 printf "Backup not found: %s\\n" "${_filename:-}"
      fi
      ;;
    restore)
      if [[ -z "${_filename:-}" ]]
      then
        help "backups"
        exit 1
      elif [[ ! -e "${_hosts_dirname}/${_filename}" ]]
      then
        _exit_1 printf "Backup not found: %s\\n" "${_filename:-}"
      fi

      _verify_write_permissions

      if ! ((_skip_backup))
      then
        backups create
      fi

      cat "${_hosts_dirname}/${_filename}" > "${HOSTS_PATH}" &&
        printf "Restored from backup: %s\\n" "${_filename}"
      ;;
    show)
      if [[ -z "${_filename:-}" ]]
      then
        help "backups"
        exit 1
      elif [[ ! -e "${_hosts_dirname}/${_filename}" ]]
      then
        _exit_1 printf "Backup not found: %s\\n" "${_filename:-}"
      fi

      cat "${_hosts_dirname}/${_filename:-}"
      ;;
    *)
      local _filenames=()

      set +f
      for __filename in $(cd "${_hosts_dirname}" && ls -1 hosts* 2> /dev/null)
      do
        if [[ "${__filename:-}" != "hosts"              ]] &&
           [[ ! "${__filename:-}" =~ ^hosts_test..{6}$  ]]
        then
          _filenames+=("${__filename:-}")
        fi
      done
      set -f

      if ((${#_filenames[@]}))
      then
        for __filename in "${_filenames[@]:-}"
        do
          printf "%s\\n" "${__filename}"
        done
      else
        printf \
          "No backups found. Create a new backup:\\n %s backups create\\n" \
          "${_ME}"
      fi
      ;;
  esac
}

# block ################################################################# block

desc "block" <<HEREDOC
Usage:
  ${_ME} block <hostname>...

Description:
  Block one or more hostnames by adding new entries assigned to \`127.0.0.1\`
  for IPv4 and both \`fe80::1%lo0\` and \`::1\` for IPv6.

Exit status:
  0   <hostname> successfully blocked.
  1   Invalid parameters or entry exists.
HEREDOC
block() {
  _verify_write_permissions

  if [[ -z "${1:-}" ]]
  then
    help "block"
    exit 1
  fi

  for __hostname in "${@}"
  do
    add 127.0.0.1 "${__hostname}"
    # block IPv6
    add "fe80::1%lo0" "${__hostname}"
    add "::1" "${__hostname}"
  done
}

# completions ##################################################### completions

desc "completions" <<HEREDOC
Usage:
  ${_ME} completions (check | install [-d | --download] | uninstall)

Options:
  -d, --download  Download the completion scripts and install.

Description:
  Manage completion scripts. For more information, visit:
  https://github.com/${_REPO}/blob/master/etc/README.md

Exit status:
  0   Completions successfully installed.
  1   Invalid parameters or other error.
HEREDOC
completions() {
  local _BASH_COMP_NAME="hosts"
  local _ZSH_COMP_NAME="_hosts"

  # Usage: _completions_check
  _completions_check() {
    local _bash_completion_path=
    _bash_completion_path="$(_get_bash_completion_path)"

    local _exists=0

    if [[ -n "${_bash_completion_path:-}" ]] &&
       [[ -d "${_bash_completion_path}"   ]]
    then
      if [[ -w "${_bash_completion_path}"   ]]
      then
        if [[ -e "${_bash_completion_path}/${_BASH_COMP_NAME}" ]]
        then
          _exists=1
          printf "Exists: %s\\n" "${_bash_completion_path}/${_BASH_COMP_NAME}"
        fi
      else
        _warn printf "Permission denied: %s\\n" "${_bash_completion_path}"
      fi
    fi

    local _zsh_completion_path="/usr/local/share/zsh/site-functions"

    if [[ -d "${_zsh_completion_path}" ]]
    then
      if [[ -w "${_zsh_completion_path}" ]]
      then
        if [[ -e "${_zsh_completion_path}/${_ZSH_COMP_NAME}" ]]
        then
          _exists=1
          printf "Exists: %s\\n" "${_zsh_completion_path}/${_ZSH_COMP_NAME}"
        fi
      else
        _warn printf "Permission denied: %s\\n" "${_zsh_completion_path}"
      fi
    fi

    if ! ((_exists))
    then
      _exit_1 printf "Completion scripts not found.\\n"
    fi
  }

  # Usage: _get_bash_completion_path
  _get_bash_completion_path() {
    local _bash_completion_path=

    if [[ -n "${BASH_COMPLETION_COMPAT_DIR:-}" ]]
    then
      _bash_completion_path="${BASH_COMPLETION_COMPAT_DIR}"
    fi

    if [[ -z "${_bash_completion_path:-}" ]]
    then
      local _maybe_path
      _maybe_path="$(
        pkg-config \
          --variable=completionsdir bash-completion 2>/dev/null || true
      )"

      if [[ -n "${_maybe_path:-}" ]]
      then
        _bash_completion_path="${_maybe_path}"
      fi
    fi

    if [[ -z "${_bash_completion_path:-}"       ]] &&
       [[ -d "/usr/local/etc/bash_completion.d" ]]
    then
      _bash_completion_path="/usr/local/etc/bash_completion.d"
    fi

    if [[ -z "${_bash_completion_path:-}" ]] &&
       [[ -d "/etc/bash_completion.d"     ]]
    then
      _bash_completion_path="/etc/bash_completion.d"
    fi

    printf "%s\\n" "${_bash_completion_path:-}"
  }

  # Usage: _completions_install [--download]
  _completions_install() {
    local _download=0
    if [[ "${1:-}" == "--download" ]]
    then
      _download=1
    fi

    local _my_path="${0}"

    if [[ -L "${_my_path}" ]]
    then
      if hash "realpath" 2>/dev/null
      then
        _my_path="$(realpath "${_my_path}")"
      else
        _my_path="$(readlink "${_my_path}")"
      fi
    fi

    local _my_dir=
    _my_dir="$(cd "$(dirname "${_my_path}")"; pwd)"
    if [[ -z "${_my_dir}" ]] || [[ ! -d "${_my_dir}" ]]
    then
      exit 1
    fi

    if [[ -z "${_REPO:-}" ]] || [[ -z "${_REPO_RAW_URL:-}" ]]
    then
      _exit_1 printf "Source Git repository not configured.\\n"
    fi

    for __shell in bash zsh
    do
      local _completion_source="${_my_dir}/etc/${_ME}-completion.${__shell}"

      if ((_download))
      then
        if [[ ! -f "${_completion_source}" ]]
        then
          _completion_source="$(mktemp)"
          local _completion_url="${_REPO_RAW_URL}/etc/${_ME}-completion.${__shell}"

          if ! _download_from "${_completion_url}" "${_completion_source}"
          then
            _exit_1 printf "Unable to download Completion script from %s\\n" \
              "${_completion_source}"
          fi
        fi
      fi

      if [[ ! -f "${_completion_source}" ]]
      then
        cat <<HEREDOC
Unable to find source ${__shell} completion script. You can try downloading
and installing the latest version with the following command (\`sudo\` might
be necessary):
  ${_ME} completions install --download

More information: ${__shell}
  https://github.com/${_REPO}/blob/master/etc/README.md
HEREDOC
      else
        local _completion_path=
        local _completion_target=
        if [[ "${__shell}" == "bash" ]]
        then
          _completion_path="$(_get_bash_completion_path)"
          _completion_target="${_completion_path}/${_BASH_COMP_NAME}"
        elif [[ "${__shell}" == "zsh" ]]
        then
          _completion_path="/usr/local/share/zsh/site-functions"
          _completion_target="${_completion_path}/${_ZSH_COMP_NAME}"
        fi

        if [[ -n "${_completion_path:-}" ]] &&
           [[ -d "${_completion_path}"   ]]
        then
          if [[ -w "${_completion_path}" ]]
          then
            if [[ ! -e "${_completion_target}" ]]
            then
              cp \
                "${_completion_source}" \
                "${_completion_target}"
              chmod +r "${_completion_target}"
              printf "Completion script installed: %s\\n" \
                "${_completion_target}"
            else
              _warn printf "Exists: %s\\n" "${_completion_target}"
            fi
          else
            _warn printf "Permission denied: %s\\n" "${_completion_path}"
          fi
        fi
      fi
    done
  }

  # Usage: _completions_uninstall
  _completions_uninstall() {
    local _completion_path=
    local _completion_target=

    for __shell in bash zsh
    do
      if [[ "${__shell}" == "bash" ]]
      then
        _completion_path="$(_get_bash_completion_path)"
        _completion_target="${_completion_path}/${_BASH_COMP_NAME}"
      elif [[ "${__shell}" == "zsh" ]]
      then
        _completion_path="/usr/local/share/zsh/site-functions"
        _completion_target="${_completion_path}/${_ZSH_COMP_NAME}"
      fi

      if [[ -n "${_completion_path:-}" ]] &&
         [[ -d "${_completion_path}"   ]]
      then
        if [[ -w "${_completion_path}"   ]] &&
           [[ -w "${_completion_target}" ]]
        then
          if [[ -f "${_completion_target}" ]]
          then
            rm "${_completion_target}"
            printf "Completion script removed: %s\\n" \
              "${_completion_target}"
          fi
        else
          _warn printf "Permission denied: %s\\n" "${_completion_path}"
        fi
      fi
    done
  }

  local _subcommand="${1:-}"

  case "${_subcommand}" in
    check)
      _completions_check
      ;;
    install)
      if [[ "${2:-}" =~ ^-d|--download$ ]]
      then
        _completions_install --download
      else
        _completions_install
      fi
      ;;
    uninstall)
      _completions_uninstall
      ;;
    *)
      help "completions"
      return 0
      ;;
  esac
}

# disable ############################################################# disable

desc "disable" <<HEREDOC
Usage:
  ${_ME} disable (<ip> | <hostname> | <search string>)

Description:
  Disable one or more entries based on a given ip address, hostname, or
  search string.

Exit status:
  0   Entry successfully disabled.
  1   Invalid parameters or entry not found.
HEREDOC
disable() {
  _verify_write_permissions

  local _search_string="${1:-}"

  if [[ -z "${_search_string:-}" ]]
  then
    help "disable"
    exit 1
  else
    _debug printf "disable() \${_search_string}: %s\\n" "${_search_string}"

    # Regular Expression Notes
    #
    # - Note double periods in regular expression in order to emulate /.+/,
    #   which apparently doesn't work properly with all versions of sed.

    local _regex_ip
    _regex_ip="^\\(${_search_string}[${_TAB_SPACE_}]..*\\)$"

    local _regex_commented_hostname
    _regex_commented_hostname="^\\([^#]..*[${_TAB_SPACE_}]${_search_string}[${_TAB_SPACE_}]..*\\)$"

    local _regex_hostname
    _regex_hostname="^\\([^#]..*[${_TAB_SPACE_}]${_search_string}\\)$"

    local _targets
    _targets=$(
      sed -n                                      \
        -e "s/${_regex_ip}/\\1/p"                 \
        -e "s/${_regex_commented_hostname}/\\1/p" \
        -e "s/${_regex_hostname}/\\1/p"           \
        "${HOSTS_PATH}"
    )

    _debug printf "disable() \${_targets}: %s\\n" "${_targets}"

    if [[ -z "${_targets:-}" ]]
    then
      _exit_1 printf "Not found: %s\\n" "${_search_string}"
    fi

    printf "Disabling:\\n%s\\n" "${_targets}"

    _sed_i                                                    \
      -e "s/${_regex_ip}/\\#disabled: \\1/g"                  \
      -e "s/${_regex_commented_hostname}/\\#disabled: \\1/g"  \
      -e "s/${_regex_hostname}/\\#disabled: \\1/g"            \
      "${HOSTS_PATH}"
  fi
}

# disabled ########################################################### disabled

desc "disabled" <<HEREDOC
Usage:
  ${_ME} disabled

Description:
  List all disabled entries. This is an alias for \`hosts list disabled\`.

Exit status:
  0   One or more disabled entries found.
  1   Invalid parameters or no disabled entries found.
HEREDOC
disabled() {
  list disabled
}

# edit ################################################################### edit

desc "edit" <<HEREDOC
Usage:
  ${_ME} edit

Description:
  Open the ${HOSTS_PATH} file in your \$EDITOR.
HEREDOC
edit() {
  _verify_write_permissions

  if [[ -z "${EDITOR}" ]]
  then
    _exit_1 printf "\$EDITOR not set.\\n"
  else
    "${EDITOR}" "${HOSTS_PATH}"
  fi
}

# enable ############################################################### enable

desc "enable" <<HEREDOC
Usage:
  ${_ME} enable (<ip> | <hostname> | <search string>)

Description:
  Enable one or more disabled entries based on a given ip address, hostname,
  or search string.

Exit status:
  0   Entry successfully enabled.
  1   Invalid parameters or entry not found.
HEREDOC
enable() {
  _verify_write_permissions

  local _search_string="${1:-}"

  if [[ -z "${_search_string:-}" ]]
  then
    help "enable"
    exit 1
  else
    _debug printf "enable() \${_search_string}: %s\\n" "${_search_string}"

    # Regular Expression Notes
    #
    # - Note double periods in regular expression in order to emulate /.+/,
    #   which apparently doesn't work properly with all versions of sed.

    local _regex_ip
    _regex_ip="^\\#disabled: \\(${_search_string}[${_TAB_SPACE_}]..*\\)$"

    local _regex_commented_hostname
    _regex_commented_hostname="^\\#disabled: \\(..*[${_TAB_SPACE_}]${_search_string}[${_TAB_SPACE_}]..*\\)$"

    local _regex_hostname
    _regex_hostname="^\\#disabled: \\(..*[${_TAB_SPACE_}]${_search_string}\\)$"

    local _targets
    _targets=$(
      sed -n                                      \
        -e "s/${_regex_ip}/\\1/p"                 \
        -e "s/${_regex_commented_hostname}/\\1/p" \
        -e "s/${_regex_hostname}/\\1/p"           \
        "${HOSTS_PATH}"
    )

    _debug printf "enable() \${targets}: %s\\n" "${_targets}"

    if [[ -z "${_targets:-}" ]]
    then
      _exit_1 printf "Not found: %s\\n" "${_search_string}"
    fi

    printf "Enabling:\\n%s\\n" "${_targets}"

    _sed_i                                      \
      -e "s/${_regex_ip}/\\1/g"                 \
      -e "s/${_regex_commented_hostname}/\\1/g" \
      -e "s/${_regex_hostname}/\\1/g"           \
      "${HOSTS_PATH}"
  fi
}

# enabled ############################################################# enabled

desc "enabled" <<HEREDOC
Usage:
  ${_ME} enabled

Description:
  List all enabled entries. This is an alias for \`hosts list enabled\`.

Exit status:
  0   One or more enabled entries found.
  1   Invalid parameters or no enabled entries found.
HEREDOC
enabled() {
  list enabled
}

# file ################################################################### file

desc "file" <<HEREDOC
Usage:
  ${_ME} file

Description:
  Print the entire contents of the ${HOSTS_PATH} file.
HEREDOC
file() {
  cat "${HOSTS_PATH}"
}

# list ################################################################### list

desc "list" <<HEREDOC
Usage:
  ${_ME} list [enabled | disabled | <search string>]

Description:
  List the existing IP / hostname pairs, optionally limited to a specified
  state. When provided with a seach string, all matching enabled entries will
  be printed.

Exit status:
  0   One or more matching entries found.
  1   Invalid parameters or entry not found.
HEREDOC
list() {
  local _disabled_entries
  _disabled_entries=$(
    sed -n "s/^\\#disabled: \\(.*\\)$/\\1/p" "${HOSTS_PATH}"
  )

  # NOTE: use separate expressions since using `|` for the 'or' results in
  # inconsistent behavior.
  local _enabled_entries
  _enabled_entries="$(
    grep -v -e '^$' -e '^\s*\#' "${HOSTS_PATH}"
  )"

  if [[ -n "${1:-}" ]]
  then
    if [[ "${1}" == "disabled" ]]
    then
      [[ -z "${_disabled_entries}" ]] && return 1
      _print_entries "${_disabled_entries}"
    elif [[ "${1}" == "enabled" ]]
    then
      [[ -z "${_enabled_entries}" ]] && return 1
      _print_entries "${_enabled_entries}"
    else
      show "${1}"
    fi
  else
    [[ -z "${_enabled_entries}" ]] &&
    [[ -z "${_enabled_entries}" ]] &&
      return 1

    _print_entries "${_enabled_entries:-}"

    if [[ -n "${_disabled_entries:-}" ]]
    then
      [[ -n "${_enabled_entries:-}" ]] && printf "\\n"
      printf "Disabled:\\n"
      printf "%s\\n" "---------"
      _print_entries "${_disabled_entries}"
    fi
  fi
}

# remove ############################################################### remove

desc "remove" <<HEREDOC
Usage:
  ${_ME} remove (<ip> | <hostname> | <search string>) [--force]
  ${_ME} remove <ip> <hostname>

Options:
  --force  Skip the confirmation prompt.

Description:
  Remove one or more entries based on a given IP address, hostname, or search
  string. If an IP and hostname are both provided, only entries matching the
  IP and hostname pair will be removed.

Exit status:
  0   Entry successfully removed.
  1   Invalid parameters or entry not found.
HEREDOC
remove() {
  _verify_write_permissions

  local _is_search_pair=0
  local _force_skip_prompt=0
  local _arguments=()
  local _search_ip=
  local _search_hostname=
  local _search_string=

  _debug printf "remove() \${1}: %s\\n" "${1:-}"
  _debug printf "remove() \${2}: %s\\n" "${2:-}"

  for __arg in "${@:-}"
  do
    case "${__arg}" in
      --force)
        _force_skip_prompt=1
        ;;
      *)
        _arguments+=("${__arg}")
        ;;
    esac
  done

  _debug printf "remove() \${arguments[1]}: %s\\n" "${_arguments[0]:-}"
  _debug printf "remove() \${arguments[2]}: %s\\n" "${_arguments[1]:-}"

  if [[ -z "${_arguments[0]:-}" ]]
  then
    help "remove"
    exit 1
  elif [[ -n "${_arguments[1]:-}" ]]
  then
    _search_ip="${_arguments[0]}"
    _search_hostname="${_arguments[1]}"
    _is_search_pair=1

    _debug printf "remove() \${_is_search_pair}: %s\\n" "${_is_search_pair}"
  else
    _search_string="${_arguments[0]:-}"

    _debug printf "remove() \${_search_string}: %s\\n" "${_search_string}"
  fi

  # Regular Expression Notes
  #
  # Note double periods in regular expression in order to emulate /.+/,
  # which apparently doesn't work properly with all versions of sed.

  # IP / Hostname pair regular expressions:
  local _regex_ip_hostname_commented
  _regex_ip_hostname_commented="^\\(${_search_ip}[${_TAB_SPACE_}]*${_search_hostname}[${_TAB_SPACE_}]..*\\)$"

  local _regex_ip_hostname
  _regex_ip_hostname="^\\(${_search_ip}[${_TAB_SPACE_}]*${_search_hostname}\\)$"

  # Search string regular expressions:
  local _regex_ip
  _regex_ip="^\\(${_search_string}[${_TAB_SPACE_}]..*\\)$"

  local _regex_commented_hostname
  _regex_commented_hostname="^\\(..*[${_TAB_SPACE_}]${_search_string}[${_TAB_SPACE_}]..*\\)$"

  local _regex_hostname
  _regex_hostname="^\\(..*[${_TAB_SPACE_}]${_search_string}\\)$"

  local _target_entries

  if ((_is_search_pair))
  then
    _target_entries=$(
      sed -n                                          \
        -e "s/${_regex_ip_hostname_commented}/\\1/p"  \
        -e "s/${_regex_ip_hostname}/\\1/p"            \
        "${HOSTS_PATH}"
    )
  else
    _target_entries=$(
      sed -n                                      \
        -e "s/${_regex_ip}/\\1/p"                 \
        -e "s/${_regex_commented_hostname}/\\1/p" \
        -e "s/${_regex_hostname}/\\1/p"           \
        "${HOSTS_PATH}"
    )
  fi

  if [[ -z "${_target_entries:-}" ]]
  then
    _exit_1 printf "No matching entries found.\\n"
  fi

  if ! ((_force_skip_prompt))
  then
    printf "Removing the following entries:\\n%s\\n" "${_target_entries}"

    while true
    do
      read -r -p "Are you sure you want to proceed? [y/N] " _yn

      case "${_yn}" in
        [Yy]* )
          break
          ;;
        * )
          printf "Exiting...\\n"
          exit 0
          ;;
      esac
    done
  fi

  if ((_is_search_pair))
  then
    _sed_i                                    \
      -e "/${_regex_ip_hostname_commented}/d" \
      -e "/${_regex_ip_hostname}/d"           \
      "${HOSTS_PATH}"
  else
    _sed_i                                    \
      -e "/${_regex_ip}/d"                    \
      -e "/${_regex_commented_hostname}/d"    \
      -e "/${_regex_hostname}/d"              \
      "${HOSTS_PATH}"
  fi

  printf "Removed:\\n%s\\n" "${_target_entries}"
}
desc "delete" "$(desc --get 'remove')"
delete() { remove "${@}"; }

# search ############################################################### search

desc "search" <<HEREDOC
Usage:
  ${_ME} search <search string>

Description:
  Search entries for <search string>.

Exit status:
  0   One or more matching entries found.
  1   Invalid parameters or entry not found.
HEREDOC
search() {
  if [[ -z "${1:-}" ]]
  then
    help "search"
    exit 1
  fi

  list "$@"
}

# show ################################################################### show

desc "show" <<HEREDOC
Usage:
  ${_ME} show (<ip> | <hostname> | <search string>)

Description:
  Print entries matching a given IP address, hostname, or search string.

Exit status:
  0   One or more matching entries found.
  1   Invalid parameters or entry not found.
HEREDOC
show() {
  if [[ -n "${1:-}" ]]
  then
    # Run `sed` before `grep` to avoid conflict that supresses `sed` output.
    local _disabled_entries
    _disabled_entries="$(
      sed -n "s/^\\#disabled: \\(.*${1}.*\\)$/\\1/p" "${HOSTS_PATH}"
    )"

    local _enabled_entries
    _enabled_entries="$(
      grep --invert-match "^#" "${HOSTS_PATH}" | grep "${1}" || true
    )"

    if [[ -z "${_disabled_entries}" ]] &&
       [[ -z "${_enabled_entries}"  ]]
    then
      _return_1 printf "No matching entries.\\n"
    fi

    _print_entries "${_enabled_entries}"

    if [[ -n "${_disabled_entries}" ]]
    then
      [[ -n "${_enabled_entries}" ]] && printf "\\n"
      printf "Disabled:\\n"
      printf "%s\\n" "---------"
      _print_entries "${_disabled_entries}"
    fi
  else
    help show
    exit 1
  fi
}

# subcommands ##################################################### subcommands

desc "subcommands" <<HEREDOC
Usage:
  ${_ME} subcommands [--raw]

Options:
  --raw  Display the subcommand list without formatting.

Description:
  Display the list of available subcommands.
HEREDOC
subcommands() {
  if [[ "${1:-}" == "--raw" ]]
  then
    printf "%s\\n" "${_DEFINED_SUBCOMMANDS[@]}"
  else
    printf "Available subcommands:\\n"
    printf "  %s\\n" "${_DEFINED_SUBCOMMANDS[@]}"
  fi
}
desc "commands" "$(desc --get 'subcommands')"
commands() { subcommands "${@}"; }

# unblock ############################################################# unblock

desc "unblock" <<HEREDOC
Usage:
  ${_ME} unblock <hostname>...

Description:
  Unblock one or more hostnames by removing the entries from the hosts file.

Exit status:
  0   <hostname> successfully unblocked.
  1   Invalid parameters or entry not found.
HEREDOC
unblock() {
  _verify_write_permissions

  if [[ -z "${1:-}" ]]
  then
    help unblock
    exit 1
  fi

  for __hostname in "${@}"
  do
    remove 127.0.0.1      "${__hostname}" --force
    # unblock IPv6
    remove "fe80::1%lo0"  "${__hostname}" --force
    remove "::1"          "${__hostname}" --force
  done
}

# version ############################################################# version

desc "version" <<HEREDOC
Usage:
  ${_ME} (version | --version)

Description:
  Display the current program version.

  To save you the trouble, the current version is ${_VERSION}
HEREDOC
version() {
  printf "%s\\n" "${_VERSION}"
}

###############################################################################
# Load Program & Run
###############################################################################

# Parse Options ###############################################################

_SUBCOMMAND=
_COMMAND_PARAMETERS=()
_USE_DEBUG=0
_AUTO_SUDO=0

_SUBCOMMANDS=(
  add
  backups
  block
  commands
  completions
  delete
  disable
  disabled
  edit
  enable
  enabled
  file
  help
  list
  remove
  search
  show
  subcommands
  unblock
  version
)
_SUBCOMMANDS_PATTERN="$(_join '|' "${_SUBCOMMANDS[@]}")"

while [[ ${#} -gt 0 ]]
do
  __opt="${1}"

  shift

  case "${__opt}" in
    -h|--help)
      _SUBCOMMAND="help"
      ;;
    --version)
      _SUBCOMMAND="version"
      ;;
    --debug)
      _USE_DEBUG=1
      ;;
    --auto-sudo|--sudo)
      _AUTO_SUDO=1
      ;;
    *)
      if [[ -z "${_SUBCOMMAND:-}"                   ]] &&
         [[ "${__opt:-}" =~ ${_SUBCOMMANDS_PATTERN} ]]
      then
        _SUBCOMMAND="${__opt}"
      else
        _COMMAND_PARAMETERS+=("${__opt}")
      fi
      ;;
  esac
done

_debug printf              \
  "\${_SUBCOMMAND}: %s\\n" \
  "${_SUBCOMMAND}"
_debug printf                           \
  "\${_COMMAND_PARAMETERS[*]:-}: %s\\n" \
  "${_COMMAND_PARAMETERS[*]:-}"

# Load Commands ###############################################################

# Initialize $_DEFINED_SUBCOMMANDS array.
_DEFINED_SUBCOMMANDS=()

# _load_subcommands()
#
# Usage:
#   _load_subcommands
#
# Loads all of the commands sourced in the environment.
_load_subcommands() {
  _debug printf "_load_subcommands(): entering...\\n"

  local _function_list
  _function_list=($(declare -F))

  for __name in "${_function_list[@]}"
  do
    # Each element has the format `declare -f function_name`, so set the name
    # to only the 'function_name' part of the string.
    local _function_name
    _function_name="$(printf "%s" "${__name}" | awk '{ print $3 }')"

    # Add the function name to the $_DEFINED_SUBCOMMANDS array unless it starts
    # with an underscore or is desc() since these are treated as having
    # 'private' visibility.
    if [[ ! "${_function_name}" =~ ^_(.*) ]] &&
       [[ "${_function_name}" != "desc"   ]]
    then
      _DEFINED_SUBCOMMANDS+=("${_function_name}")
    fi
  done

  _debug printf \
    "_load_subcommands() \${_DEFINED_SUBCOMMANDS[*]:-}:\\n%s\\n" \
    "${_DEFINED_SUBCOMMANDS[*]:-}"
}

# Main ########################################################################

# _main()
#
# Usage:
#   _main
#
# The primary function for starting the program.
#
# NOTE: must be called at end of program after all commands have been defined.
_main() {
  _debug printf "main(): entering...\\n"
  _debug printf "main() \${_SUBCOMMAND} (upon entering): %s\\n" "${_SUBCOMMAND}"

  if [[ -z "${_SUBCOMMAND}" ]]
  then
    _SUBCOMMAND="${HOSTS_DEFAULT_SUBCOMMAND}"
  fi

  _load_subcommands

  if _contains "${_SUBCOMMAND}" "${_DEFINED_SUBCOMMANDS[@]:-}"
  then
    "${_SUBCOMMAND}" "${_COMMAND_PARAMETERS[@]:-}"
  else
    _exit_1 printf "Unknown subcommand: %s\\n" "${_SUBCOMMAND}"
  fi
}

_main