Improved version of the Private Internet Access port forwarding script with better cross-platform support

port_forwarding.sh

#!/usr/bin/env bash
#
# Enable port forwarding when using Private Internet Access
#
# Usage:
#  ./port_forwarding.sh

error( )
{
  echo "$@" 1>&2
  exit 1
}

error_and_usage( )
{
  echo "$@" 1>&2
  usage_and_exit 1
}

usage( )
{
  echo "Usage: `dirname $0`/$PROGRAM"
}

usage_and_exit( )
{
  usage
  exit $1
}

version( )
{
  echo "$PROGRAM version $VERSION"
}


port_forward_assignment( )
{
  client_id_file="/usr/local/etc/openvpn/pia_client_id"
  if [ ! -f "$client_id_file" ]; then
    if hash shasum 2>/dev/null; then
      head -n 100 /dev/urandom | shasum -a 256 | tr -d " -" > "$client_id_file"
    elif hash sha256sum 2>/dev/null; then
      head -n 100 /dev/urandom | sha256sum | tr -d " -" > "$client_id_file"
    else
      echo "Please install shasum or sha256sum, and make sure it is visible in your \$PATH"
      exit 1
    fi
  fi
  client_id=`cat "$client_id_file"`
  json=`curl "http://209.222.18.222:2000/?client_id=$client_id" 2>/dev/null`
  if [ "$json" == "" ]; then
    json='Port forwarding is already activated on this connection, has expired, or you are not connected to a PIA region that supports port forwarding'
  fi

  echo $json
}

EXITCODE=0
PROGRAM=`basename $0`
VERSION=2.1

while test $# -gt 0
do
  case $1 in
  --usage | --help | -h )
    usage_and_exit 0
    ;;
  --version | -v )
    version
    exit 0
    ;;
  *)
    error_and_usage "Unrecognized option: $1"
    ;;
  esac
  shift
done

port_forward_assignment

exit 0

This script improves on the original by checking for the presence of shasum or sha256sum, rather than testing against platform names, in order to be more widely compatible (with FreeBSD, for example). It also displays a relevant error message if neither utility is found.
Also adapted a user-submitted change to store and re-use the ClientID, which will return an already opened port, if one exists.