Illustrates the use of 'trap' in Ruby for crude interprocess communication

trap_user_signals.rb

#!/usr/bin/env ruby

# Author: @keithrbennett (Github)

# Illustrates how to respond to Unix signals in a Ruby program, using SIGUSR1 and SIGUSR2
# for user-defined signals, and SIGINT for trapping Ctrl-C.

require 'awesome_print'
require 'json'
require 'yaml'

$stderr.puts  <<~GREETING
  Pid is #{Process.pid}.
  Standard output is a #{$stdout.tty? ? 'tty' : 'block'} device.
  
  You can send signals SIGUSR1 and SIGUSR2 (e.g. via `kill`) to poke this program
  to output its stats in human readable or YAML forms:
  
  kill -SIGUSR1 #{Process.pid}  # Human readable (Awesome/Amazing Print)
  kill -SIGUSR2 #{Process.pid}  # YAML
  
  ...and to terminate the program, outputting the final data observation:
  
  kill -SIGINT #{Process.pid}  # Terminate 
  
GREETING

SystemInfo = Struct.new(:pid, :uptime, :res_memory_mb)

# See this great article about monotonic time: https://blog.dnsimple.com/2018/03/elapsed-time-with-ruby-the-right-way/
def monotonic_time_now
  Process.clock_gettime(Process::CLOCK_MONOTONIC)
end

START_TIME = monotonic_time_now

def monotonic_uptime
  monotonic_time_now - START_TIME
end

def resident_memory_in_mb
  # -o = format with the following data
  # rss = resident memory set size, in ?
  # h = omit the header ("RSS")
  command = "ps -o rss h #{Process.pid}"
  output = `#{command}`
  kb = output.strip.to_i
  (kb / 1024.0).round(0)
end
  
def system_info
  SystemInfo.new(Process.pid, monotonic_uptime.round(2), resident_memory_in_mb)
end

def print_system_info
  ap system_info.to_h
end

Signal.trap('USR1') { print_system_info }
Signal.trap('USR2') { puts system_info.to_h.to_yaml }

Signal.trap('INT') do
  puts "\nEnding program, final stats are:"
  print_system_info
  exit
end

# See https://dsa.cs.tsinghua.edu.cn/oj/static/unix_signal.html for signal values.
#Signal.trap(30) { puts 30 }
#Signal.trap(10) { puts 10 }

memory_black_hole = []

loop do
  sleep 0.5
  memory_black_hole << Array.new(100_000) { Time.now }
  #puts "Time count: #{ObjectSpace.each_object(Time).count}"
  $stdout.flush
end