Ruby Net/SSH exec - get pid of remote process

Question

I am launching a nohup remote script with Ruby Net/SSH.

Net::SSH.start(ip_address, user, options) do |ssh|
  script = File.join(remote_path, 'run_case.py')
  cmd = "nohup python #{script} #{args}  < /dev/null &"
  ssh.exec(cmd)
end

All stdout and stderr is saved to a file on the remote machine.

Is it possible to get the PID of the remote script so that I can kill it if needed?

EDIT 1: I have modified the script as suggested.

Net::SSH.start(ip_address, user, options) do |ssh|
  script = File.join(remote_path, 'run_case.py')
  cmd = "nohup python #{script} #{args}  < /dev/null & echo $! > save_pid.txt"
  ssh.exec(cmd)
  pid = ssh.exec!("cat save_pid.txt")
  puts mesh_pid
end

It complains that it cannot find the file. Is this because the file does not exist yet? I would prefer to avoid any sleep command if possible

EDIT 2: Maybe this is not elegant but it works. I have created a second ssh session and used pgrep.

Net::SSH.start(ip_address, user, options) do |ssh|
  script = File.join(remote_path, 'run_case.py')
  cmd = "nohup python #{script} #{args}  < /dev/null &"
  ssh.exec(cmd)
end

Net::SSH.start(ip_address, user, options) do |ssh|
  cmd = "python #{script} #{args}"
  mesh_pid = ssh.exec!("pgrep -f '#{cmd}'")
  puts mesh_pid
end

I've down-voted because your edit makes no sense. Did you looked even a second at the code before posting it back here? — hek2mgl
– hek2mgl, Commented Jun 28, 2017 at 10:07
I see. Yes, I had forgotten the change the first command. Damn copy & paste. — Rojj
– Rojj, Commented Jun 28, 2017 at 12:07
Yes. I had used the correct command in my program, but I pasted the old one here. I logged in the VM and the file is actually there. exec is non blocking so I thought that the second command (cat) is probably executed before the file is actually created. I could use a sleep, but it does not seem a robust approach. — Rojj
– Rojj, Commented Jun 28, 2017 at 12:26

Vamsi Krishna · Accepted Answer · 2017-06-28 07:10:13Z

2

You should be able to determine the PID (and store it in a file) as follows:

Net::SSH.start(ip_address, user, options) do |ssh|
  script = File.join(remote_path, 'run_case.py')
  cmd = "nohup python #{script} #{args}  < /dev/null & echo $! > save_pid.txt"
  ssh.exec(cmd)
end

In a script, $! represents the PID of the last process executed. If you need to kill the process, you can do it via:

kill -9 `cat save_pid.txt`
rm save_pid.txt

edited Jun 28, 2017 at 7:10

answered Jun 28, 2017 at 6:53

Vamsi Krishna

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1 Comment

hek2mgl Over a year ago

Ok, it will give you the pid now, but it will hang unless you close the connection manually. Try nohup sleep 1000 & in your terminal to see

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