How to use
Try running ruby runner.rb and then go through its code. Notice the use of exec_stream method which is defined in exc_stream.rb
exec_stream(cmd) accepts a command and a block. cmd could be something like tail -f which generates output continuously into stdout.
Explanation
https://ruby-doc.org/stdlib-2.5.3/libdoc/pty/rdoc/PTY.html
PTY stands for pseudo terminal. It allows the ruby script to present itself to the subprocess (one that is spawned) as if it's a real user who has just typed/read into/from a shell so that output buffering etc can be overridden.