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Kim Burgestrand

Heartfelt software development. Sometimes other things.

Parallell remote execution with net/ssh in Ruby

~ 5 mins

Franks original question — and his example implementation — was a bit confusing. His question, coupled with code, was:

any suggestions on preserving the order of an array when using threads? i want it to print back the array in the order it was added in.

threads = []
IO.readlines('file.txt').each do |line|
  threads << Thread.new(line) do 
    puts line if line =~ /^test/

threads.each { |thread| thread.join }

# Output:
# test line 4
# test line 2
# test line 5
# test line 3
# test line 1

Frank wanted the output in order, meaning he wanted to print after the previously spawned thread had printed. After some delirium (I have the flu), I whopped up an example using #inject:

(1..4).to_a.inject(Thread.new {}) do |t, i|
  Thread.new do
    sleep(rand * 10 % 3) # simulate long-running operation
    puts "Lookup #{i} done"
    t.join # wait for previous thread to finish
    puts "Show result #{i}"

# Output:
# Lookup 3 done
# Lookup 2 done
# Lookup 1 done
# Show result 1
# Show result 2
# Show result 3
# Lookup 4 done
# Show result 4

Now that is nice; it works! Frank was happy, and quickly disappeared with a […] thanks! im going to try it in a few mins […].

Two hours later, Frank is back!

He’s very excited. He has now written a script that first reads a list of ip addresses from the user, and then goes on to visit each address and running a script on it using net/ssh. It looked like this:

@iplist = []
IO.readlines(ARGV[0]).inject(Thread.new {}) do |t, line|
  label, ip, pass, hostname, ip2 = line.split(/\s+/)
  Thread.new do
    Net::SSH.start("#{ip}", "root", :password => "#{pass}") do |ssh|
      @iplist << ssh.exec!("head -n1 /etc/ips|cut -d : -f1").match(/(\d{1,3}\.+\d{1,3}\.+\d{1,3}\.+\d{1,3})/)
    t.join # wait for previous thread to finish
puts @iplist

Now, this is quite different from what Frank originally asked! This is a script that will read a list of hosts from a file, and then visit each host executing the command within ssh.exec! on each host. In the end, it will print the results.

Me and Frank discussed his code and why he was doing this for a few minutes. He explained to me that he works as a system administrator, and does not really consider himself a programmer. Frank is too hard on himself.

Either way, I eventually began to see the code as a puzzle: could I improve the code, making it nicer and useful in other cases — and possibly even help Frank even more?

The answer to that is yes! Ten minutes later, I showed Frank what I came up with:

require 'net/ssh'

ips = $stdin.readlines.map do |line|
  label, host, user, command = line.split(' ', 4)
  [label, Thread.new do
    output = nil
    Net::SSH.start(host, user) { |ssh| output = ssh.exec!(command) }
  end, line]

ips.each do |(label, thread, _)|
  puts "[#{label}]"
  thread.value.each_line.map do |line|
    puts "  " + line

Executing this code like so ruby script.rb < instructions.txt, with instructions.txt containing this:

superman Kim whoami
database Kim sleep 1 && echo "Database says Hello!"
localhost Kim date

Will give this output:

  Database says Hello!
  Thu Mar 10 23:31:59 CET 2011

Frank was happy, I was happy. Me and Frank talked for another twenty minutes, and then parted ways. Goodbye Frank, it was nice talking to you today!

PS: Here’s the gist of it: https://gist.github.com/865129

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