How to get exit status with Ruby's Net::SSH library? - ruby

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.

Answer 2

Building on the answer by flitzwald - I've monkey patched my version of this into Net::SSH (Ruby 1.9+)
class Net::SSH::Connection::Session
class CommandFailed < StandardError
end
class CommandExecutionFailed < StandardError
end
def exec_sc!(command)
stdout_data,stderr_data = "",""
exit_code,exit_signal = nil,nil
self.open_channel do |channel|
channel.exec(command) do |_, success|
raise CommandExecutionFailed, "Command \"#{command}\" was unable to execute" unless success
channel.on_data do |_,data|
stdout_data += data
end
channel.on_extended_data do |_,_,data|
stderr_data += data
end
channel.on_request("exit-status") do |_,data|
exit_code = data.read_long
end
channel.on_request("exit-signal") do |_, data|
exit_signal = data.read_long
end
end
end
self.loop
raise CommandFailed, "Command \"#{command}\" returned exit code #{exit_code}" unless exit_code == 0
{
stdout:stdout_data,
stderr:stderr_data,
exit_code:exit_code,
exit_signal:exit_signal
}
end
end

Answer 3

For newer versions of Net::SSH, you can just pass a status hash to Net::SSH::Connection::Session#exec:
status = {}
Net::SSH.start(hostname, user, options) do |ssh|
channel = ssh.exec(command, status: status)
channel.wait # wait for the command to actually be executed
end
puts status.inspect
# {:exit_code=>0}
By default, exec streams its output to $stdout and $stderr. You can pass a block to exec to do something different, a la:
ssh.exec(command, status: status) do |ch, stream, data|
if stream == :stdout
do_something_with_stdout(data)
else
do_something_with_stderr(data)
end
end
This works on 6.1.0 - not sure about availability for older versions. See http://net-ssh.github.io/net-ssh/Net/SSH/Connection/Session.html#method-i-exec for more details.