Category Archives: Software Development

Lately there has been some talking in Ruby-Talk on how to send files using scp. Some people are doing just fine by calling scp with a pre-shared password, but I don’t like that approach and it doesn’t work for my purpose, so I did a bit of experimentation and found this code works:

require 'net/ssh'
require 'net/sftp'

class SSHAgent
  def initialize
        @agent_env = Hash.new
        agenthandle = IO.popen("/usr/bin/ssh-agent -s", "r")
        agenthandle.each_line do |line|
          if line.index("echo") == nil
                  line = line.slice(0..(line.index(';')-1))
                  key, value = line.chomp.split(/=/)
                  puts "Key = #{key}, Value = #{value}"
                  @agent_env[key] = value
          end
        end
  end
  def [](key)
        return @agent_env[key]
  end
end

agent = SSHAgent.new
ENV["SSH_AUTH_SOCK"] = agent["SSH_AUTH_SOCK"]
ENV["SSH_AGENT_PID"] = agent["SSH_AGENT_PID"]
system("/usr/bin/ssh-add")

Net::SSH.start( '192.168.1.12',
                :username=>'pgquiles',
                :compression_level=>0,
                :compression=>'none'
              ) do |session|
                     session.sftp.connect do |sftp|
                     sftp.put_file("bigvideo.avi", "bigvideo.avi")
                end
end

I was using a passwordless private key for this experiment. Should you want to use a password-protected private key, you might want to set the DISPLAY and SSH_ASKPASS environment variables or use a smartcard reader and the ‘-s reader’ parameter.

The SSHAgent class comes from a public paste (I don’t know who its author is or the license that code is under).

Lastly, there is an undesired behaviour in Net::SFTP: when you use the put_file method it loads the whole file in memory, therefore you need loads of memory if you want to send big files. I’ve tried a naive fix (iterating writes) but it didn’t work (it complains about “no such file”, I think it’s a channels-related issue). Jamis Buck, the author of Net::SFTP, told me he currently has no time to fix this issue. In case you feel brave enough, the offending method starts in line 202 in

net-sftp-1.1.0/lib/net/sftp/session.rb.

Best introduction to ‘continuations’. Ever. Do not even try to understand this one.
Summarizing:

• Continuations are like bookmarks: "hey Joe, come here and start again from this point". The context in which you invoke the code after the continuation point is the context you are currently, not the context you had when you defined the continuation. It’s easy to see it with this code in Ruby:

  arr = [ "Freddie", "Herbie", "Ron", "Max", "Ringo" ]
  hello_var = "hello1"
  puts(hello_var)
  callcc{|$cc|}
  message = arr.shift
  hello_var = "hello2"
  puts(message)
  puts(hello_var)
  $cc.call unless message =~ /Max/

  The output is:

  hello1
  Freddie
  hello2
  Herbie
  hello2
  Ron
  hello2
  Max
  hello2
  

• Closures give you the lexical context you had when you defined the closure, including values of variables.