#!/bin/sh

make_tar_member() {
    local name=$1
    local content=$2
    local mode=644
    local uid=1000
    local gid=1000
    local size=${#content}
    local mtime=$(date +%s)
    local type=0
    local link=""
    local username="jow"
    local groupname="jow"

    printf '%-100s%07d\0%07o\0%07o\0%011o\0%011o\0        %1d%-100sustar  \0%-32s%-32s%183s' \
        "$name" $mode $uid $gid $size $mtime $type \
        "$link" "$username" "$groupname" | tr ' ' '\0' > /tmp/header

    local checksum=$((8 * 32))

    for byte in $(hexdump -ve '1/1 "%u "' /tmp/header); do
        checksum=$((checksum + $byte))
    done

    printf '%06o\0 ' $checksum | dd bs=1 seek=148 count=8 conv=notrunc of=/tmp/header
    cat /tmp/header
    printf "%s" "$content"

    local pad=$((512 - (size % 512) ))
    [ $pad -eq 0 ] || dd if=/dev/zero bs=$pad count=1
}


{
    make_tar_member "example.txt" "Hello there"
    make_tar_member "example-2.txt" "Blah blah"

    # alternatively, instead of the dd, run a `tar -c ...` here
    dd if=/dev/zero bs=1024 count=1
} | gzip > /tmp/test.tar.gz