Checks whether the given hostname matches the own external IP address. The external IP address is determined by querying https://myip.babioch.de.

check_myip

#! /bin/sh

# Copyright (c) 2017 Karol Babioch <karol@babioch.de>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# Verbosity flag
VERBOSE="False"

# Usage help
help()
{
  cat << EOF

  Copyright (c) 2017 Karol Babioch <karol@babioch.de>

  Checks whether the given hostname matches the own external IP address. The
  external IP address is determined by querying https://myip.babioch.de.

  Usage: $0 [OPTIONS]... HOSTNAME

  OPTIONS:
    -v, --verbose       Enable verbose output
    -h, --help          Show this usage help

EOF
}

# Output a message and exit with given code
# $1: Exit code
# $2: Message (optional)
check_exit()
{
  if [[ ! -z "$2" ]]; then

    echo "$2"

  fi

  exit $1
}

# Output information in verbose mode
# $1: Message
verbose()
{
  echo "$1" >&3
}

# Validate IP address with regular expression
# $1: IP
valid_ip()
{
  if [[ "$1" =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then

    return 0

  fi

  return 1
}

# Parse options provided as command line arguments
for i in "$@"
do

  case "$i" in

    # Verbose mode
    -v|--verbose)
      VERBOSE="True"
      shift
      ;;

    # Usage help
    -h|--help)
      help
      exit
      ;;

  esac

done

# Check verbosity flag and attach new stream to stdout or /dev/null
if [[ "$VERBOSE" == "True" ]]; then

    exec 3>&1

else

    exec 3> /dev/null

fi

# Check number of arguments
if [[ "$#" != 1 ]]; then

  check_exit 3 "Wrong number of arguments"

fi

HOSTNAME="$1"
IPA=$(curl -s https://myip.babioch.de)
IPB=$(dig +short "$HOSTNAME")

# Output debug information
verbose "Hostname: $HOSTNAME"
verbose "My IP: $IPA"
verbose "Hostname's IP: $IPB"

if ! valid_ip "$IPA"; then
  check_exit 3 "Couldn't determine my IP address"
fi

if ! valid_ip "$IPB"; then
  check_exit 3 "Couldn't determine IP of $HOSTNAME"
fi

if [[ "$IPA" == "$IPB" ]]; then
  echo "My IP matches the one of $HOSTNAME: $IPB"
  exit 0
fi

check_exit 2 "My IP ($IPA) differs from the one of $HOSTNAME ($IPB)"

Potential ToDos: Check if necessary tools are available, e.g. dig(1)