Stream output from a Ruby script

Set STDOUT.sync = true once, at the top of your script, before you print anything. Ruby then writes every puts and print straight to stdout instead of collecting them in a buffer.

websocketd --port=8080 ruby ./count.rb

STDOUT.sync = true

(1..5).each do |count|
  puts count
  sleep(0.5)
end

Connect a client and the five numbers arrive half a second apart. Remove the sync line and all five arrive together when the script exits. Output buffering explains why.

There is no per-call flush to remember afterwards. Setting sync changes the stream for the life of the process, so every later write goes out immediately without any further work.

If you cannot edit the top of the script

Ruby has no command-line switch that turns buffering off, so the change has to happen in Ruby code. Where the script is not yours to edit, put the one line in a small wrapper that loads it:

STDOUT.sync = true
load File.expand_path('vendored_script.rb', __dir__)

Wrap the wrapper, not the original:

websocketd --port=8080 ruby ./wrapper.rb

Flushing selectively instead

If you have a reason to keep buffering on, STDOUT.flush empties the buffer at a point you choose:

(1..5).each do |count|
  puts count
  STDOUT.flush
  sleep(0.5)
end

This is more code and one more thing to forget. Use it only when you write enough output for the buffering to pay for itself.

Next

• Output buffering is the reason all of this is necessary.
• Message framing covers the other requirement: each message needs a trailing newline, which puts adds for you and print does not.
• Debug a script shows the timing of what arrives.