What does Ruby's BEGIN do?

Question

What does BEGIN mean in Ruby, and how is it called? For example, given this code:

puts "This is sentence 1."

BEGIN {
  puts "This is sentence 2."
}

why is puts "This is sentence 2." executed first?

Answer 1

BEGIN and END set up blocks that are called before anything else gets executed, or after everything else, just before the interpreter quits.

For instance, running this:

END { puts 'END block' }

puts 'foobar'

BEGIN { puts 'BEGIN block' }

Outputs:

BEGIN block
foobar
END block

Normally we'd use a bit more logical order for the BEGIN and END blocks, but that demonstrates what they do.