Generate a .tar.gz archive with shell

make-tar.sh

#!/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"

	# 100 padding bytes, using 0x01 since the shell does not tolerate 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'
	dd if=/dev/zero bs=183 count=1 2>/dev/null

	# print content data, padded to multiple of 512 byte
	printf "%s" "$content"
	dd if=/dev/zero bs=1 count=$((512 - (size % 512))) 2>/dev/null
}


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

	# alternatively, instead of dd'ing the trailing padding, run a `tar -c ...` here
	dd if=/dev/zero bs=1024 count=1 2>/dev/null
} | gzip > /tmp/test.tar.gz