How to print stdout immediately?

Question

How can I immediately output stdout? stdout is going to print after all input is complete.

require \'open3\'
def run(cmd)
    Open3         


        

Answer 1

Ruby is buffering output until the output buffer is full. To change the behavior so it automatically writes use sync and sync=:

old_sync = $stdout.sync
$stdout.sync = true

puts "immediately output lotsa stuff"
puts "immediately output lotsa stuff"
puts "immediately output lotsa stuff"
puts "immediately output lotsa stuff"

# reenable the default behavior
$stdout.sync = old_sync

From the documentation for sync=:

Sets the “sync mode” to true or false. When sync mode is true, all output is immediately flushed to the underlying operating system and is not buffered internally.

It's important to understand that enabling automatic flushing of the buffer can actually slow the overall execution speed of your code, especially if you're writing to a file or device that wants to receive its data in chunks. Use sync or flushing carefully.

Answer 2

Flush the output

What you are looking for is to flush the io stream by using the flush method.

Try the following after each iteration or each puts:

stdout.flush

If you had multiple puts in a row, I would suggest doing a flush after the last one, so you do not do it too often. Example:

stdout.puts "Hello"
stdout.puts "Mate"
stdout.flush