#!/bin/sh
#
# Execute a command as another user, with access to the X display
#
# Copyright (C) 2013  Antonio Ospite <ospite@studenti.unina.it>
#
# 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/>.


# This is the equivalent of sux[1] but simpler and using sudo.
# [1] http://fgouget.free.fr/sux/

set -e

usage() {
  echo "usage: $(basename $0) [OPTION]... <sudo_options>"
  echo "Execute a command as another user, with access to the X display"
  echo
  echo "List of OPTIONs:"
  echo
  echo "  -u user     Execute the program as the specified user (default is root)"
  echo "  -d display  Set the X display"
  echo "  -h|--help   Show this help text"
  echo
  sudo -h | grep -v "^[ ]\+-u user"
}

USERNAME=root

while true;
do
  case "$1" in
    -u)
      [ "x$2" != "x" ] || { usage 1>&2; exit 1; }
      USERNAME="$2"
      shift 2
      ;;

    -d)
      [ "x$2" != "x" ] || { usage 1>&2; exit 1; }
      DISPLAY="$2"
      shift 2
      ;;

    -h | --help)
      usage
      exit 0
      ;;

    *)
      break
      ;;
  esac
done

id $USERNAME > /dev/null || { echo "Invalid user." 1>&2; exit 1; }

[ -n "$DISPLAY" ] || { echo "Cannot get the DISPLAY env variable." 1>&2; exit 1; }

[ "x$@" != "x" ] || { usage 1>&2; exit 1; }


# Get the authorization cookie from the current user.
COOKIE="$(xauth nextract - $DISPLAY)"

# XAUTHORITY needs to be unset now, so that the default $HOME/.Xauthority will
# be used for the target user and not some locked auth file from the current
# user.
unset XAUTHORITY

# Authorize the user.
# Use "sudo -H" to cover the case when env_keep+="HOME" is set in /etc/sudoers
echo "$COOKIE" | sudo -H -u "$USERNAME" xauth nmerge -

# Execute the command.
# NOTE: -i or -s can be passed in order to open a shell
sudo DISPLAY="$DISPLAY" -u "$USERNAME" "$@"