Patching binary files with bash and xxd

tar-patch.sh

#!/bin/bash

export CHECKSUM_AT=148

function joinlines {
    while read line ; do
        echo -n "$line"
    done
}

function split-by {
    read input
    while [ "$input" ]; do
       echo ${input:0:$1}
       input=${input:$1}
    done
}

function strrep {
    read _where
    local _with=$1
    local _at=$2

    local _pre=${_where:0:$_at}
    local _post=${_where:$(($_at + ${#_with}))}
    echo "${_pre}${_with}${_post}"
}

function checksum {
    local sum=0
    read input
    echo "checksum input length is ${#input}"
    while [ "$input" ] ; do
        byte="0x${input:0:2}"
        sum=$(($sum + $byte))
        input=${input:2}
    done
    printf "0%o\n" $sum
}

function makedump {
    xxd -p $1 | joinlines
}

function fromdump {
    split-by 64 | xxd -r -p
}

function checksummable {
    cat | strrep "2020202020202020" $((2 * $CHECKSUM_AT))
}

function take {
    dd bs=1 count=$((2 * $1)) status=noxfer
}

function from_ascii {
    echo $1 | xxd -p
}

function patch-bin {
    makedump $1 | strrep "$2" $((2 * $3)) | fromdump
}

function tar-checksum {
    cat $1 | makedump | take 512 | checksummable | checksum
}