script.rb

#!/usr/bin/env ruby

require 'media'

$0 = '0.0'

converter = 
  Media.convert do
    options y: true
    input '/path/to/input.mov'
    output '/path/to/output.png' do
      options ss: 0, 'frames:v' => 1
      graph do
        chain do
          filter 'thumbnail'
          filter 'scale' do
            expressions 'iw*sar', 'ih'
          end
        end
      end
    end
  end

result = converter.call {|progress| $0 = progress.to_s }

$stdout << result.out
$stderr << result.error

$0 = '1.0' if result.success?

subprocess.rb

class Subprocess
      extend Forwardable
  
      attr_reader :out, :error, :wait_thread
  
      def_delegators :status, :success?, :exitstatus
  
      def initialize(command)
        _, @stdout, @stderr, @wait_thread = Open3.popen3(*command)
        
        @out   = ''
        @error = '' 
      end
  
      def perform(timeout: 1)
        while wait_thread.alive?
          select(timeout: timeout)
          yield progress if block_given?
        end
        self
      end
  
      private

      def_delegators :wait_thread, :pid
  
      def progress
        ps[/(?<=\n).+/]
      end
  
      def ps
        `ps -p #{pid} -o comm`
      end
  
      def status
        wait_thread.value
      end
  
      def select(timeout:, buffer: 65536)
        if IO.select([@stdout, @stderr], nil, nil, timeout)
          out   << @stdout.read_nonblock(buffer)
          error << @stderr.read_nonblock(buffer)
        end
      rescue EOFError, IO::WaitReadable
        ''
      end
    end