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/usr/lib/os-probes/mounted/20microsoft is in os-prober 1.76~deb9u1.

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#!/bin/sh
# Detects all Microsoft OSes on a collection of partitions.

. /usr/share/os-prober/common.sh

partition="$1"
mpoint="$2"
type="$3"

# This script looks for legacy BIOS bootloaders only. Skip if running UEFI
if [ -d /sys/firmware/efi ] && [ ! -f /var/lib/partman/ignore_uefi ]; then
	debug "Skipping legacy bootloaders on UEFI system"
	exit 1
fi

# Weed out stuff that doesn't apply to us
case "$type" in
	ntfs|ntfs-3g) debug "$1 is a NTFS partition" ;;
	vfat) debug "$1 is a FAT32 partition" ;;
	msdos) debug "$1 is a FAT16 partition" ;;
	fat) debug "$1 is a FAT partition (mounted by GRUB)" ;;
	fuse|fuseblk) debug "$1 is a FUSE partition" ;; # might be ntfs-3g
	*) debug "$1 is not a MS partition: exiting"; exit 1 ;;
esac

found=
# Vista (previously Longhorn)
if item_in_dir -q bootmgr "$2"; then
	# there might be different boot directories in different case as:
	# boot Boot BOOT
	for boot in $(item_in_dir boot "$2"); do
		bcd=$(item_in_dir bcd "$2/$boot")
		if [ -n "$bcd" ]; then
			if   grep -aqs "W.i.n.d.o.w.s. .1.0" "$2/$boot/$bcd"; then
				long="Windows 10"
			elif grep -aqs "W.i.n.d.o.w.s. .8" "$2/$boot/$bcd"; then
				long="Windows 8"
			elif grep -aqs "W.i.n.d.o.w.s. .7" "$2/$boot/$bcd"; then
				long="Windows 7"
			elif grep -aqs "W.i.n.d.o.w.s. .V.i.s.t.a" "$2/$boot/$bcd"; then
				long="Windows Vista"
			elif grep -aqs "W.i.n.d.o.w.s. .S.e.r.v.e.r. .2.0.0.8. .R.2." "$2/$boot/$bcd"; then
				long="Windows Server 2008 R2"
			elif grep -aqs "W.i.n.d.o.w.s. .S.e.r.v.e.r. .2.0.0.8." "$2/$boot/$bcd"; then
				long="Windows Server 2008"
			elif grep -aqs "W.i.n.d.o.w.s. .R.e.c.o.v.e.r.y. .E.n.v.i.r.o.n.m.e.n.t" "$2/$boot/$bcd"; then
				long="Windows Recovery Environment"
			elif grep -aqs "W.i.n.d.o.w.s. .S.e.t.u.p" "$2/$boot/$bcd"; then
				long="Windows Recovery Environment"
			else
				long="Windows Vista"
			fi
			short=Windows

			found=true

			break
		fi
	done
fi

# 2000/XP/NT4.0
if [ -z "$found" ] && item_in_dir -q ntldr "$2" && item_in_dir -q ntdetect.com "$2"; then
	long="Windows NT/2000/XP"
	short=Windows
	ini=$(item_in_dir boot.ini "$2")
	if [ -n "$ini" ]; then
		multicount="$(grep -e "^multi" "$2/$ini" | wc -l)"
		scsicount="$(grep -e "^scsi" "$2/$ini" | wc -l)"
		msoscount="$(expr "${multicount}" + "${scsicount}")"
		if [ "$msoscount" -eq 1 ]; then
			# We need to remove a Carriage Return at the end of
			# the line...
			defaultmspart="$(grep -e "^default=" "$2/$ini" | cut -d '=' -f2 | tr -d '\r')"
			# Escape any backslashes in defaultmspart
			grepexp="^$(echo "$defaultmspart" | sed -e 's/\\/\\\\/')="
			# Colons not allowed; replace by spaces
			# Accented characters (non UTF-8) cause debconf to
			# hang, so we fall back to the default if the name
			# contains any weird characters.
			long="$(grep -e "$grepexp" "$2/$ini" | cut -d '"' -f2 | \
				tr ':' ' ' | LC_ALL=C grep -v '[^a-zA-Z0-9 &()/_-]')"
			if [ -z "$long" ]; then
				long="Windows NT/2000/XP"
			fi
		else
			long="Windows NT/2000/XP"
		fi

		found=true
	fi
fi

# MS-DOS
if [ -z "$found" ] && item_in_dir -q dos "$2"; then
	long="MS-DOS 5.x/6.x/Win3.1"
	short=MS-DOS

	found=true
fi

# 95/98/Me
if [ -z "$found" ] && item_in_dir -q windows "$2" &&
     item_in_dir -q win.com "$2"/"$(item_in_dir windows "$2")"; then
	long="Windows 95/98/Me"
	short=Windows9xMe

	found=true
fi

if [ -z "$found" ]; then
	exit 1
fi

label="$(count_next_label "$short")"
result "${partition}:${long}:${label}:chain"
exit 0