Rubocop's --auto-gen-config changes your settings for certain cops. For example, if you have one line of code which has a LineLength of 150 characters then it is going to set your Metrics/LineLength config to Max: 150. This script instead generates a list of files to exclude only and does not lower your standards

generate-rubocop-excludes.rb

#!/usr/bin/env ruby

gem 'slop', '4.2.1'
require 'slop'

require 'pathname'
require 'json'
require 'yaml'


module SlopExt
  class PathOption < Slop::Option
    def call(value)
      Pathname(value).expand_path.freeze.tap do |path|
        fail "Invalid path #{path}" unless path.exist? && path.file?
      end
    end
  end
end

Slop.include(SlopExt)

module CLI
  def self.parse(args)
    options = Slop.parse(args) do |switch|
      # Print and display help
      switch.on('-h', '--help') { puts switch }

      # Accept configuration file
      switch.path '-c', '--config', 'Specify configuration file.'
    end

    fail "You must specify a config file" unless options.get(:config)

    options.to_hash
  end
end

SETTINGS = CLI.parse(ARGV)

# Note: comment out any existing excludes in your current config file
CONFIG_LOCATION = SETTINGS.fetch(:config)

json = `bundle exec rubocop -c #{CONFIG_LOCATION} -f json`

OUTPUT = JSON.parse(json, symbolize_names: true)

FILES = OUTPUT.fetch(:files)

OFFENSES = FILES.map do |file|
  file.fetch(:offenses).map do |offense|
    offense.merge(path: file.fetch(:path))
  end
end.flatten

GROUPS = OFFENSES.group_by { |item| item.fetch(:cop_name) }

CONF = GROUPS.each.with_object({}) do |(key, offense), config|
  config[key] ||= {}
  config[key]['Exclude'] ||= []

  offense.each { |o| config[key]['Exclude'] << o.fetch(:path) }
end

RUBOCOP_IGNORES = CONF.map do |key, settings|
  [key, { 'Exclude' => settings.fetch('Exclude').sort.uniq }]
end.to_h

modified_yaml = CONFIG_LOCATION.readlines.each.with_index.map do |line, index|
  line.gsub(/^#(.+)$/, ":yaml_comment#{index}:\\1")
end.join

CURRENT_CONFIG     = YAML.load(modified_yaml)
CONFIGURATION_KEYS = (CURRENT_CONFIG.keys + RUBOCOP_IGNORES.keys).uniq

MERGED = CONFIGURATION_KEYS.each.with_object({}) do |key, memo|
  if key.to_s.start_with?('yaml_comment')
    next memo[key] = CURRENT_CONFIG.fetch(key)
  end

  existing  = CURRENT_CONFIG.fetch(key, {})
  additions = RUBOCOP_IGNORES.fetch(key, {})

  excludes = (existing.fetch('Exclude', []) + additions.fetch('Exclude', [])).uniq
  merged = existing.merge(additions)
  merged = merged.merge('Exclude' => excludes) if excludes.any?

  memo[key] = merged
end.to_h

updated_yaml = YAML.dump(MERGED)
final_yaml = updated_yaml.gsub(/^:yaml_comment\d+:(.+)$/, '#\1')
CONFIG_LOCATION.write(final_yaml)