Script to extract useful information from FAT32 header

fat32info.sh

#!/usr/bin/env sh 

# Display information about internal layout of a FAT32 filesystem.
#
# Script directly reads data from FAT32 header. This might be useful
# for aligning fs structures to speed up flash drive access.
#
# http://kirelagin.ru/p/gist/3730254
#
# -- Kirill Elagin <[email protected]>
#    http://kirelagin.ru/


device="$1"

if [ -z "$device" ]; then
    echo "Usage: $0 <device>." >&2
    exit 1
fi
if [ ! -e "$device" ]; then
    echo "Error: '$device' does not exist." >&2
    exit 2
fi
if [ ! -b "$device" ]; then
    echo "Error: '$device' is not a block device." >&2
    exit 2
fi
if [ ! -r "$device" ]; then
    echo "Error: read on '$device' is not permitted. You probably want to run this script as root." >&2
    exit 2
fi


function readfat32() {
    offset="$2"
    len="$3"

    format="-tu$len"

    let $1=$(dd 2>/dev/null if="$device"  bs=1 count="$len" skip="$offset" | od -An $format | tr -d ' ')

    echo "$1 : ${!1}" >&2
}

readfat32 signature 66  1
if [ "$signature" != "41" -a "$signature" != "40" ]; then
    echo "Error: seems that '$device' has filesystem different from FAT32." >&2
    exit 3
fi


readfat32 sector_bytes      11  2
readfat32 cluster_sectors   13  1
readfat32 sectors_reserved  14  2
readfat32 fats_count        16  1
readfat32 fat_sectors       36  4
readfat32 fs_sectors        32  4

header_sectors=$((fat_sectors*fats_count + sectors_reserved))
echo "Header size is $header_sectors sectors ($((header_sectors * sector_bytes / 1024)) kbytes)"