myguard.rb - a simple command line tool that acts like Guard

myguard.rb

#!/usr/bin/env ruby

#
# a simple command line tool that acts like Guard, but you can run any arbitrary command on any set of arbitrary files that change.
#
# Example usage:
#
#     # Run test when a certain 3 files are modified.
#     myguard.rb "ruby test_foo.rb" foo.rb foo_test.rb foo.json
#

require 'colorize'

class MyGuard

  attr_reader :files, :interval

  def initialize(*files, interval: 1)
    @files = [files].flatten
    @interval = interval
  end

  def print_start
    puts "#{'=' * `tput cols`.to_i}".blue
    puts "Starting at #{Time.now}".blue
  end

  def print_end
    puts "Finished at #{Time.now}".blue
  end

  def run_command(&block)
    print_start
    yield
    print_end
  end

  def watch(&block)
    raise ArgumentError unless block
    the_mtimes = mtimes(files)
    run_command &block
    loop do
      new_mtimes = mtimes(files)
      if new_mtimes != the_mtimes
        run_command &block
        the_mtimes = new_mtimes
      end
      sleep interval
    end
  end

  def mtimes(files)
    files.map { |f| [f, File.mtime(f)] }.to_h
  end

end

if $0 == __FILE__
  shell_cmd = ARGV[0]
  MyGuard.new(ARGV[1..-1]).watch { system shell_cmd }
end