/usr/bin/rstart is in x11-session-utils 7.7+2build1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 | #! /bin/sh
# $Xorg: client.cpp,v 1.4 2000/12/20 16:41:43 pookie Exp $
#
# Copyright (c) 1993 Quarterdeck Office Systems
#
# Permission to use, copy, modify, distribute, and sell this software
# and software and its documentation for any purpose is hereby granted
# without fee, provided that the above copyright notice appear in all
# copies and that both that copyright notice and this permission
# notice appear in supporting documentation, and that the name
# Quarterdeck Office Systems, Inc. not be used in advertising or
# publicity pertaining to distribution of this software without
# specific, written prior permission.
#
# THIS SOFTWARE IS PROVIDED "AS-IS".  QUARTERDECK OFFICE SYSTEMS,
# INC., DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
# INCLUDING WITHOUT LIMITATION ALL IMPLIED WARRANTIES OF
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR
# NONINFRINGEMENT.  IN NO EVENT SHALL QUARTERDECK OFFICE SYSTEMS,
# INC., BE LIABLE FOR ANY DAMAGES WHATSOEVER, INCLUDING SPECIAL,
# INCIDENTAL OR CONSEQUENTIAL DAMAGES, INCLUDING LOSS OF USE, DATA, OR
# PROFITS, EVEN IF ADVISED OF THE POSSIBILITY THEREOF, AND REGARDLESS
# OF WHETHER IN AN ACTION IN CONTRACT, TORT OR NEGLIGENCE, ARISING OUT
# OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
usage() {
    if [ -n "$1" ]
    then
	echo "`basename $0`: $1"
    fi
    echo ""
    echo "Usage:  `basename $0` [options] hostname command args ..."
    echo ""
    echo "where options include:"
    echo "    -c context    run command in the specified context"
    echo "    -g            interpret command as a generic command"
    echo "    -l username   run command as the specified user"
    echo "    -v            enable verbose output"
    exit 1
}
if [ $# -eq 0 ]
then
    usage
fi
context=X
verbose=DETACH
name=
kind=CMD
while :
do
	case $1 in
	-c)
		if [ $# -lt 2 ]
		then
			usage "-c option requires an argument"
		fi
		context=$2
		shift; shift
		;;
	-g)
		kind=GENERIC-CMD
		shift
		;;
	-l)
		if [ $# -lt 2 ]
		then
			usage "-l option requires an argument"
		fi
		name="-l $2"
		shift; shift
		;;
	-v)
		verbose=NODETACH
		shift
		;;
	*)
		if [ $# -eq 0 ]
		then
			usage "missing host name"
		fi
		host=$1
		shift
		break
		;;
	esac
done
if [ $# -eq 0 ]
then
	usage "missing command"
fi
case $verbose in
DETACH)
	exec > /dev/null
	;;
esac
case "$DISPLAY" in
:*)	disp="`uname -n`$DISPLAY"
	echo expanded $DISPLAY to $disp
	;;
*)	disp="$DISPLAY"
	;;
esac
(
cat << /
CONTEXT $context
MISC X DISPLAY=$disp
$kind $*
$verbose
/
xauth list $disp | sed 's/^/AUTH X11 /'
echo ""
) |  $host $name rstartd
 |