preinst is in virtualbox-guest-utils 4.1.12-dfsg-2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
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 | #!/bin/sh
set -e
# Remove a no-longer used conffile
rm_conffile() {
	PKGNAME="$1"
	CONFFILE="$2"
	if [ -e "$CONFFILE" ]; then
		md5sum="`md5sum \"$CONFFILE\" | sed -e \"s/ .*//\"`"
		old_md5sum="`dpkg-query -W -f='${Conffiles}' $PKGNAME | sed -n -e \"\\\\' $CONFFILE '{s/ obsolete$//;s/.* //p}\"`"
		if [ "$md5sum" != "$old_md5sum" ]; then
			echo "Obsolete conffile $CONFFILE has been modified by you."
			echo "Saving as $CONFFILE.dpkg-bak ..."
			mv -f "$CONFFILE" "$CONFFILE".dpkg-bak
		else
			echo "Removing obsolete conffile $CONFFILE ..."
			rm -f "$CONFFILE"
		fi
	fi
}
if [ "$1" = install ]; then
	# virtualbox-ose-guest-utils << 4.0.6-dfsg-1~
	if [ -e /etc/init.d/virtualbox-ose-guest-utils ]; then
		rm_conffile virtualbox-ose-guest-utils /etc/init.d/virtualbox-ose-guest-utils
		update-rc.d virtualbox-ose-guest-utils remove >/dev/null
	fi
fi
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