#!/bin/sh

print_padding() {
    [ $1 -eq 0 ] || dd if=/dev/zero bs=$1 count=1 2>/dev/null
}

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

    # 100 byte of padding bytes, using 0x01 since the shell does not tolate null bytes in strings
    local pad=$'\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1'

    # truncate string header values to their maximum length
    name=${name:0:100}
    link=${link:0:100}
    username=${username:0:32}
    groupname=${groupname:0:32}

    # construct header part before checksum field
    local header1="${name}${pad:0:$((100 - ${#name}))}"
    header1="${header1}$(printf '%07d\1' $mode)"
    header1="${header1}$(printf '%07o\1' $uid)"
    header1="${header1}$(printf '%07o\1' $gid)"
    header1="${header1}$(printf '%011o\1' $size)"
    header1="${header1}$(printf '%011o\1' $mtime)"

    # construct header part after checksum field
    local header2="$(printf '%d' $type)"
    header2="${header2}${link}${pad:0:$((100 - ${#link}))}"
    header2="${header2}ustar  ${pad:0:1}"
    header2="${header2}${username}${pad:0:$((32 - ${#username}))}"
    header2="${header2}${groupname}${pad:0:$((32 - ${#groupname}))}"

    # calculate checksum over header fields
    local checksum=0
    for byte in $(printf '%s%8s%s' "$header1" "" "$header2" | tr '\1' '\0' | hexdump -ve '1/1 "%u "'); do
        checksum=$((checksum + byte))
    done

    # print member header, padded to 512 byte
    printf '%s%06o\0 %s' "$header1" $checksum "$header2" | tr '\1' '\0'
    print_padding 183

    # print content data, padded to multiple of 512 byte
    printf "%s" "$content"
    print_padding $((512 - (size % 512)))
}


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

    # alternatively, instead of printing the trailing padding, run a `tar -c ...` here
    print_padding 1024
} | gzip > /tmp/test.tar.gz