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/lib/partman/check.d/10ext2_boot is in ubiquity 2.18.7.

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The actual contents of the file can be viewed below.

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#!/bin/sh
# Check if boot partition gets mounted on /boot/grub, /boot or root,
# is of type ext2 and is the first partition.

ARCH="$(archdetect)"

case $ARCH in
    powerpc/chrp_pegasos)
	;;
    *)
	exit 0
	;;
esac

. /lib/partman/lib/base.sh

for dev in $DEVICES/*; do
	[ -d "$dev" ] || continue
	cd $dev
	open_dialog PARTITIONS
	while { read_line num id size type fs path name; [ "$id" ]; }; do
		[ "$fs" != free ] || continue
		[ -f $id/method ] || continue
		[ -f $id/acting_filesystem ] || continue
		[ -f $id/mountpoint ] || continue
		mountpoint=$(cat $id/mountpoint)
		filesystem=$(cat $id/acting_filesystem)
		if [ "$mountpoint" = / ]; then
			root_fs=$filesystem
			root_type=$type
			root_path=$path
		elif [ "$mountpoint" = /boot ]; then
			boot_fs=$filesystem
			boot_type=$type
			boot_path=$path
		elif  [ "$mountpoint" = /boot/grub ]; then
			boot_grub_fs=$filesystem
			boot_grub_type=$type
			boot_grub_path=$path
		fi
	done
	close_dialog
done

# Check if separate /boot/grub partition exists 
if [ -n "$boot_grub_path" ]; then
	boot_fs=$boot_grub_fs
	boot_type=$boot_grub_type
	boot_path=$boot_grub_path
fi

# If no separate boot partition exists then root acts as boot
if [ -z "$boot_path" ]; then
	boot_fs=$root_fs
	boot_type=$root_type
	boot_path=$root_path
fi

# We need an ext2 filesystem to boot
if [ "$boot_fs" != ext2 ]; then
	db_set partman-basicfilesystems/boot_not_ext2 true
	db_input critical partman-basicfilesystems/boot_not_ext2 || true
	db_go || true
	db_get partman-basicfilesystems/boot_not_ext2
	if [ "$RET" = true ]; then
		exit 1
	fi
fi

# The boot file system has to be on the first partition
part_num=$(echo "$boot_path" | sed -e 's/.*[^0-9]\+//')
if [ "$part_num" -ne 1 ]; then
	db_set partman-basicfilesystems/boot_not_first_partition true
	db_input critical partman-basicfilesystems/boot_not_first_partition || true
	db_go || true
	db_get partman-basicfilesystems/boot_not_first_partition
	if [ "$RET" = true ]; then
		exit 1
	fi
fi