Ruby script to generate a CHANGELOG from git commits

changelog_generator.rb

#!/usr/bin/env ruby

# Generate a changelog for a git repository
#
# Usage:
#
#   $ cd app_dir
#   $ ruby /path/to/changelog.rb

CHANGELOG_FILENAME = 'CHANGELOG.new.md'

if !Dir.exists?(File.join(Dir.pwd, '.git'))
  puts "FATAL: not a git repository"
  return
end


def project_name
  Dir.pwd.split('/').last.gsub(/[_-]/, ' ').split(' ').map { |word| word.capitalize }.join(' ')
end

def clean_tag(str)
  tag = str&.match(/tag: v?(\d+(\.\d+)*)/)
  if !tag.nil? && tag.size >= 2
    return tag[1] # return matched version
  end
  'x.x.x'
end

def clean_message(msg)
  return 'MERGE COMMIT' if msg.match(/\AMerge pull request/)

  # perform any other clean you find useful
  # msg.sub!(/\ACloses? #?(\d+)?/i, '')

  msg.sub!(/ *\(#\d+\)\z/, '')
  msg
end

#
# run the pipeline
#

Commit = Struct.new(:date, :tag, :message)
commits = []

# 3 passes to easily get the needed data...it's not so slow.
`git log --format='%cs'`.split("\n").each do |l|
  commit = Commit.new
  commit.date = l
  commits << commit
end
`git log --format='%D'`.split("\n").each_with_index do |l, i|
  commits[i].tag = l
end
`git log --format='%s'`.split("\n").each_with_index do |l, i|
  commits[i].message = l
end

# fix up the commits
commits.each do |c|
  c.tag = clean_tag(c.tag)
  c.message = clean_message(c.message)
end

# create the changelog
filename = "#{Dir.pwd}/#{CHANGELOG_FILENAME}"

File.open(filename, 'w') do |f|
  f.write "# #{project_name} changelog\n\n\n"
  f.write "## HEAD\n"

  commits.each do |c|
    next if c.message == 'MERGE COMMIT' && c.tag == 'x.x.x'

    f.write "\n\n"
    f.write "\n" unless c.tag == "x.x.x"
    f.write "## #{c.tag} (#{c.date})\n\n"
    f.write "* #{c.message}"
  end
  f.write "\n"
end