Question

I have a snippet of code, simply trying to execute a script on a remote server, in the event that it fails, I’d like to make a follow-up call, imagine this:

require 'rubygems'
require 'net/ssh'
require 'etc'

server = 'localhost'

Net::SSH.start(server, Etc.getlogin) do |ssh|
  puts (ssh.exec("true")  ? 'Exit Success' : "Exit Failure")
  puts (ssh.exec("false") ? 'Exit Success' : "Exit Failure")  
end

I would expect (ignoring that stdout and stderr are printed in my contrived example) – but first line should exit with 0 which I would expect Ruby would interperate as false and display “Exit Failure” (sure, so the logic is wrong, the ternary needs to be flipped) – but the second line should exit with the opposite status, and it doesn’t.

I can’t even find anything in the documentation about how to do this, and I’m a little worried that I might be doing it wrong?!

Answer 1

I find the following way of running processes with Net::SSH much more useful. It provides you with distinct stdout and stderr, exit code and exit signal.

require 'rubygems'
require 'net/ssh'
require 'etc'

server = 'localhost'

def ssh_exec!(ssh, command)
  stdout_data = ""
  stderr_data = ""
  exit_code = nil
  exit_signal = nil
  ssh.open_channel do |channel|
    channel.exec(command) do |ch, success|
      unless success
        abort "FAILED: couldn't execute command (ssh.channel.exec)"
      end
      channel.on_data do |ch,data|
        stdout_data+=data
      end

      channel.on_extended_data do |ch,type,data|
        stderr_data+=data
      end

      channel.on_request("exit-status") do |ch,data|
        exit_code = data.read_long
      end

      channel.on_request("exit-signal") do |ch, data|
        exit_signal = data.read_long
      end
    end
  end
  ssh.loop
  [stdout_data, stderr_data, exit_code, exit_signal]
end

Net::SSH.start(server, Etc.getlogin) do |ssh|
  puts ssh_exec!(ssh, "true").inspect
  # => ["", "", 0, nil]

  puts ssh_exec!(ssh, "false").inspect  
  # => ["", "", 1, nil]

end

Hope this helps.