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/lib/partman/check.d/08biosgrub is in ubiquity 2.18.7.

This file is owned by root:root, with mode 0o755.

The actual contents of the file can be viewed below.

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#! /bin/sh

. /lib/partman/lib/base.sh

# Keep this in sync with grub-installer.
want_method=
ARCH="$(archdetect)"
case $ARCH in
    i386/mac|amd64/mac)
	if [ -d /sys/firmware/efi ]; then
	    want_method=efi
	else
	    want_method=biosgrub
	fi
	;;
    i386/efi|amd64/efi)
	want_method=efi
	;;
    i386/*|amd64/*|*-i386/*|*-amd64/*)
	want_method=biosgrub
	;;
    *)
	exit 0
	;;
esac

# Do all the disks on this system use GPT?
found_gpt=no
found_non_gpt=no
for dev in $DEVICES/*; do
	[ -d "$dev" ] || continue
	cd "$dev"
	open_dialog GET_LABEL_TYPE
	read_line label_type
	close_dialog
	if [ "$label_type" = gpt ]; then
		found_gpt=yes
	else
		found_non_gpt=yes
	fi
done
if [ "$found_gpt" = no ] || [ "$found_non_gpt" = yes ]; then
	# We might be intending to boot from one of the non-GPT disks.
	exit 0
fi

# Is there at least one EFI System Partition or BIOS Boot Partition, as
# appropriate?
have_bootable=no
for dev in $DEVICES/*; do
	[ -d "$dev" ] || continue
	cd "$dev"
	partitions=
	open_dialog PARTITIONS
	while { read_line num id size type fs path name; [ "$id" ]; }; do
		[ "$fs" != free ] || continue
		partitions="$partitions $id,$num"
	done
	close_dialog

	for part in $partitions; do
		id="${part%,*}"
		num="${part#*,}"
		[ -f "$id/method" ] || continue
		method="$(cat "$id/method")"
		if [ "$method" = "$want_method" ]; then
			have_bootable=yes
			break
		fi
	done
done

if [ "$have_bootable" = no ]; then
	template="partman-partitioning/no_bootable_gpt_$want_method"
	db_set "$template" true
	db_input critical "$template" || true
	db_go || true
	db_get "$template"
	if [ "$RET" = true ]; then
		exit 1
	fi
fi

exit 0