watchlog.rb

require 'find'

class Watchlog
  class LogFile
    def initialize(path)
      @path = path
      @pid = nil
      @wth = nil
    end

    def perform
      unless @pid
        @pid = fork_and_exec('tail', '-f', @path)
        @wth = Thread.new(@pid) { |pid| Process.waitpid pid }
      end
    end

    def kill
      if @pid
        Process.kill :TERM, @pid
        @pid = nil
        @wth = nil
      end
    end

    def fork_and_exec(*command)
      pid = Process.fork { Process.exec(*command, out: :out, err: :err) }
      pid.nil? ? exit! : pid
    end
  end

  def initialize(dir, retains=nil, interval=3)
    @target = dir
    @retains = Array(retains).map { |f| File.expand_path f }
    @interval = interval
    @log_files = {}
    @create = []
    @delete = []
    @paths = []
    @first = true
    @break = false
  end

  def perform
    STDOUT.sync = true
    STDERR.sync = true
    sigtrap
    loop do
      find
      unwatch
      watch
      sleep @interval
      break if @break
    end
    unwatch!
  end

  def find
    paths = to_paths Find.find(@target)
    if @first
      @create = paths.dup
      @paths = paths
      @first = false
    else
      @create = paths - @paths
      @delete = @paths - paths
      @paths = paths
    end
  end

  def watch
    to_paths(@create | @retains).each do |path|
      log_file = @log_files[path] || LogFile.new(path)
      log_file.perform
      @log_files[path] = log_file
    end
    @create.clear
  end

  def unwatch
    @delete.each do |path|
      log_file = @log_files.delete path
      log_file.kill if log_file
    end
    @delete.clear
  end

  def unwatch!
    @log_files.each_value(&:kill)
    @log_files.clear
  end

  def sigtrap
    Signal.trap(:TERM) { @break = true }
    Signal.trap(:INT) { @break = true }
  end

  def to_paths(enum)
    enum.select { |f| File.file? f }.map { |f| File.expand_path f }.select { |f| File.extname(f).downcase == '.log' }
  end
end

if __FILE__ == $PROGRAM_NAME
  dir = ARGV.shift || '/path/to/log_dir'
  Watchlog.new(File.expand_path(dir), ARGV).perform
end