Have you ever met a program that no matter how hard you hit Ctrl-C, it just wouldn’t stop?

I was a little annoyed by such behavior of the bc (basic calculator) program on Mac (or FreeBSD):

$ bc -q
(interrupt) use quit to exit.

In fact, when hitting Ctrl-C, we are just sending a SIGINT signal to the program. The program may trap the signal and probably ignore it.

To achieve that in Ruby is simple:

loop do
  input = gets.chomp

  exit if input == 'quit'

  # calculate(input)

  trap(:INT) do
    puts '(interrupt) use quit to exit.'
  end
end

Some processes even wouldn’t stop when we try to kill it:

$ kill <pid>

In such cases, the SIGTERM signal sent by kill is trapped.

We can force them to stop by sending the the SIGKILL signal, since it cannot be caught:

$ kill -s KILL <pid>