per-chunk md5sum

#!/bin/bash

### HOW TO USE
# 1)
# set the $blocksize, $digits and $prefix variables.
# defaults are:
# blocksize=$((64*1024))
# digits=20
# prefix=FILE.part.
#
# 2)
# run/syntax:
# $ Chunkmd5sum FILE
#
# 3)
# check/read the FILE.parts.md5 file for a list of partial md5sums:
# $ more FILE.parts.md5
#
# 4)
# do something useful with this information. :-)
#
# limitations:
# FILE must be a fixed size regular file that can be "stat"'ed
# in order to get it's filesize.
# Chunkmd5sum currently does not support reading from stdin/streams.

function Split {
    split -a ${digits:-20} -d -b "$blocksize" "$infile" "${prefix:-$infile.part.}"
}

function Countblocks {
    NR=0
    stat "$infile" |
    while read -t 60 line; do
        ((NR += 1))
        if (( NR == 2 )); then
            line="${line##*Size: }"
            size="${line%% *}"
            if (( size >= 0 )) && (( blocksize >= 0 )); then
                blocks=$(( size/blocksize ))
                rest=$(( size - (blocks*blocksize) ))
                if (( blocks >= 0 )) && (( rest >= 0 )); then
                    echo "$blocks $rest"
                fi
            else
                return 1
            fi
            break
        fi
        if (( NR > 2 )); then
            # emergency handbrake
            return 1
        fi
    done
}

function Createfifos {
    noblocks=$(Countblocks)
    if [[ -z "$noblocks" ]]; then
        return 1
    fi
    blocks=${noblocks%% *}
    rest=${noblocks##* }
    if (( rest > 0 )); then
        (( blocks += 1 ))
    fi
    partno=0
    while (( partno < blocks )); do
        printf "%s%0${digits:-20}u\n" "${prefix:-$infile.part.}" "$partno"
        (( partno += 1 ))
    done | tee /tmp/fifos-$$.list | xargs -n 32 mkfifo
}

function Removefifos {
    xargs -a /tmp/fifos-$$.list -n 32 rm &&
    rm /tmp/fifos-$$.list
}

function Checksum {
    md5sum "${prefix:-$infile.part.}"* > "${prefix:-$infile.parts.}"md5
}

function Chunkmd5sum {
    infile="${infile:-$1}"
    blocksize=${blocksize:-$((64*1024))}
    digits=${digits:-20}
    if [[ -f "$infile" ]] && [[ -r "$infile" ]]; then
        Createfifos || return 1
        (Checksum &)
        Split &&
        Removefifos
    else
        return 1
    fi
}