/usr/lib/os-probes/init/10filesystems is in os-prober 1.74ubuntu1.
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 | #!/bin/sh
# Make sure filesystems are available.
set +e	# ignore errors from modprobe
FILESYSTEMS='ext2 ext3 ext4 xfs jfs msdos vfat ntfs minix hfs hfsplus qnx4 ufs btrfs'
# fuse is needed to make grub-mount work.
FILESYSTEMS="$FILESYSTEMS fuse"
# The Ubuntu kernel udebs put a number of filesystem modules in
# fs-{core,secondary}-modules. It's fairly cheap to check for these too.
FILESYSTEMS="$FILESYSTEMS fs-core fs-secondary"
if [ ! -e /var/lib/os-prober/modules ]; then
	# Check for anna-install to make it easier to use os-prober outside
	# d-i.
	if type anna-install >/dev/null 2>&1 && [ -d /lib/debian-installer ]; then
		for fs in $FILESYSTEMS; do
			ANNA_QUIET=1 DEBIAN_FRONTEND=none \
			log-output -t os-prober \
				anna-install "$fs-modules" || true
		done
		depmod -a >/dev/null 2>&1 || true
	fi
	for fs in $FILESYSTEMS; do
		case "$fs" in
			fs-*)
				;;
			*)
				modprobe "$fs" 2>/dev/null | logger -t os-prober
				;;
		esac
	done
	# We only want to keep this state inside d-i, so this is as good a
	# check as any.
	if type anna-install >/dev/null 2>&1 && [ -d /lib/debian-installer ]; then
		touch /var/lib/os-prober/modules
	fi
fi
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