Script to show how git creates objects

hash-object.rb

#!/usr/bin/env ruby
# Based on code from: https://git-scm.com/book/en/v2/Git-Internals-Git-Objects#Object-Storage

require 'digest/sha1'
require 'fileutils'
require 'zlib'
require 'optparse'
require 'ostruct'

def gen_header(content)
  "blob #{content.length}\0"
end

def calc_hash(str)
  Digest::SHA1.hexdigest(str)
end

def gen_path(repo, sha1)
  File.join(repo, '.git/objects', sha1[0,2], sha1[2,38])
end

# parse command-line options
class OptParser
  def self.parse(argv)
    options = OpenStruct.new
    opt_parser = OptionParser.new do |opts|
      opts.banner = 'Usage: hash-object.rb [options]'

      opts.on('-f', '--file FILENAME', String, 'File to read the object from') do |file|

        options[:file] = file
      end

      opts.on('-r', '--repo REPOSITORY', String, 'Path to the repository' ) do |repo|
        options[:repo] = repo
      end

      opts.on_tail('-h', '--help', 'Show this message') do
        puts opts
        exit
      end
    end
    begin
      opt_parser.parse!(argv)
      options
    rescue OptionParser::InvalidOption, OptionParser::MissingArgument => err
      puts err
      puts opt_parser
      exit 2
    end
  end
end

ARGV << '-h' if ARGV.empty?
options = OptParser.parse(ARGV)

if options.file.nil? || options.repo.nil?
  puts 'Required options missing'
  exit 2
end

unless Dir.exists?(File.join(options.repo, '.git'))
  puts "\"#{options.repo}\" not a git repository"
  exit 2
end

# read content from the given file
content = File.read(options.file)

# concatenate calculated header with actual file content
store = gen_header(content) + content

# calculate sha1 hash
sha1 = calc_hash(store)

# compress content
compressed_w_zlib = Zlib::Deflate.deflate(store)

# generate path where new blob object should be stored
path = gen_path(options.repo, sha1)

# create directory for path above
FileUtils.mkdir_p(File.dirname(path))

# write object to the disk
File.write(path, compressed_w_zlib)

# print sha1 hash to stdout
puts sha1