Why does STDERR output from Ruby come before earlier STDOUT output when redirecting?

Question

In bash, this gives the output in the expected order:

ruby -e "puts 'one'; raise 'two'"
one
-e:1:in `<main>': two (RuntimeError)

But if I redirect STDERR to STDOUT, I get the error before the output, which I do not want:

ruby -e "puts 'one'; raise 'two'" 2>&1 | cat
-e:1:in `<main>': two (RuntimeError)
one

I want to redirect the output to a text file (it behaves the same way as cat above) and get both output and exceptions, but in the same order as when looking at the output in my terminal. Can this be achieved?

Answer 1

It's because STDOUT doesn't always output right away, to force it to output you use IO#flush:

puts "one"
$>.flush

STDERR on the other hand always outputs immediately.

Answer 2

This happens because of line-buffering vs block-buffering. You can control the type of buffering, you can flush them at the point you want their output to be synced, or you can just wait until exit at which point everything gets flushed. Unless you force it one way or the other, buffering depends on whether the output is a tty-type¹ file descriptor, so redirection into a pipe changes the mode.

Specifically:

                 true          false 
              ------------- --------------
$stdout.tty?  line-buffered block-buffered
$stderr.tty?  line-buffered line-buffered

You can configure them both the same way with:

$stdout.sync = $stderr.sync = true # or false, of course

My test case:

$stdout.sync = $stderr.sync = true
$stdout.puts 'stdout a'
sleep 2
$stdout.puts 'stdout b'
sleep 2
$stderr.puts 'stderr a'
sleep 2
$stderr.puts 'stderr b'
sleep 2

---

^{1. See ttyname(3).}