How to check to see if ruby child process has completed without waiting

Question

I'm aware that you can do something like this in ruby,

download_process = IO.popen "wget #{goodies}"
download_process.wait
puts "goodies gotten"

to spawn a subprocess and respond when it completes.

However, say I want to keep my script busy with other tasks whilst waiting for the child process to complete, then check back periodically whether any child processes have completed yet. How would I do this in ruby?

My goal is to have n simultaneous download threads, managed by a single ruby thread which also cleans up and processes the downloaded data files (so after processing a file it checks how many full downloaded files are waiting and decides how many download threads to spawn accordingly).

Answer 1

For this situation use IO.popen with block wrapped in a Thread. -q option added for this working example:

file1 = "http://www.vim.org/scripts/download_script.php?src_id=7701"
file2 = "http://www.openss7.org/repos/tarballs/strx25-0.9.2.1.tar.bz2"
threads = []

threads << Thread.new do
 IO.popen("wget -q   #{file1}"){ |io| io.read}
end

threads << Thread.new do
  IO.popen("wget -q  #{file2}"){ |io| io.read }
end

while true
  sleep(2)
  threads.each_with_index do |tr, index|
    if tr.alive?
      puts "Downloading in \##{index}"
    else
      puts "Downloaded in \##{index}"
      threads[index]  = Thread.new do
        IO.popen("wget -q   #{file1}"){ |io| io.read}
      end
    end
  end
end

Answer 2

"Without waiting" usualy means that you should run this in another thread. Try to read this: http://ruby-doc.org/core-2.0.0/Thread.html

Your code must be something like that:

download_process = Thread.new { IO.popen "wget #{goodies}" }
puts 'Goodies gotten'

When you want to check the thread state, use alive function:

download_process.alive? #=> true/false