How do I get the STDOUT of a ruby system() call while it is being run?

Question

Similar to Getting output of system() calls in Ruby , I am running a system command, but in this case I need to output the STDOUT from the command as it runs.

Answer 1

As in the linked question, the answer is again not to use system at all as system does not support this.

However this time the solution isn't to use backticks, but IO.popen, which returns an IO object that you can use to read the input as it is being generated.

Answer 2

In case someone might want to read stdout and stderr:
It is important to read them in parallel, not first one then the other. Because programs are allowed to output to stdout and stderr by turns and even in parallel. So, you need threads. This fact isn't even Ruby-specific.

Stolen from here.

require 'open3'

cmd = './packer_mock.sh'
data = {:out => [], :err => []}

# see: http://stackoverflow.com/a/1162850/83386
Open3.popen3(cmd) do |stdin, stdout, stderr, thread|
  # read each stream from a new thread
  { :out => stdout, :err => stderr }.each do |key, stream|
    Thread.new do
      until (raw_line = stream.gets).nil? do
        parsed_line = Hash[:timestamp => Time.now, :line => "#{raw_line}"]
        # append new lines
        data[key].push parsed_line

        puts "#{key}: #{parsed_line}"
      end
    end
  end

  thread.join # don't exit until the external process is done
end

Answer 3

here is my solution

def io2stream(shell, &block)
  Open3.popen3(shell) do |_, stdout, stderr|
    while line = stdout.gets
      block.call(line)
    end

    while line = stderr.gets
      block.call(line)
    end
  end
end

io2stream("ls -la", &lambda { |str| puts str })

Answer 4

With following you can capture stdout of a system command:

output = capture(:stdout) do
  system("pwd") # your system command goes here
end

puts output

shortened version:

output = capture(:stdout) { system("pwd") }

Similarly we can also capture standard errors too with :stderr

capture method is provided by active_support/core_ext/kernel/reporting.rb

Looking at that library's code comments, capture is going to be deprecated, so not sure what is the current supported method name is.