How to check for a running process with Ruby?

Question

I use a scheduler (Rufus scheduler) to launch a process called "ar_sendmail" (from ARmailer), every minute.

The process should NOT be launched when there is already such a process running in order not to eat up memory.

How do I check to see if this process is already running? What goes after the unless below?

scheduler = Rufus::Scheduler.start_new

  scheduler.every '1m' do

    unless #[what goes here?]
      fork { exec "ar_sendmail -o" }
      Process.wait
    end

  end

end

Curtain

Answer 1

unless `ps aux | grep ar_sendmai[l]` != ""

Vizard

Thanks! 1 question back: Why the brackets around the "l" of "ar_sendmail"? – Curtain

it's to remove the calling process from the grep for a process that matches ar_sendmail - otherwise you'd get a result for 'grep ar_sendmail' – Vizard

And if you want to extract the pids: ps aux | grep ar_sendmai[l] | awk '{ print $2 }' – Navel

Hi! @stef, what is the approach if I'm using some Rails method (for example Email.send_notifications) instead of above process - fork { exec "ar_sendmail -o" } ? – Ama

Don't use unless with != as it results in mental anguish. Instead use if and ==: if something == something_else is more easily understood than unless something != something_else. – Rasia

For simpler checks use pidof ar_sendmail. – Arte

Answer 2

unless `pgrep -f ar_sendmail`.split("\n") != [Process.pid.to_s]

Brannon

I think this is a much better solution as you don't have to use the bracket thing to exclude the calling process. – Overflight

EDIT: Needed the split on \n because of the return structure of pgrep. – Brannon

Answer 3

This looks neater I think, and uses built-in Ruby module. Send a 0 kill signal (i.e. don't kill):

  # Check if a process is running
  def running?(pid)
    Process.kill(0, pid)
    true
  rescue Errno::ESRCH
    false
  rescue Errno::EPERM
    true
  end

Slightly amended from Quick dev tips You might not want to rescue EPERM, meaning "it's running, but you're not allowed to kill it".

Sackbut

it returns true, even if the process already in defunct state as reported by ps – Heraclid