input.txt

pear
  d
   c
   b
  a
banana
  c
       a
  b
apple
  a
  f
  e

output.txt

apple
  a
  e
  f
banana
       a
  b
  c
pear
  a
   b
   c
  d

sort.rb

#!/usr/bin/env ruby

contents = {}
current = ""

# Parse string
ARGF.each do |line|
  if line =~ /^\s+/
    contents[current] << line
  else
    current = line.chop
    contents[current] = []
  end
end

# Sort
output = ""
Hash[contents.sort].each do |key, value|
  output << "#{key}\n"
  value.sort_by { |word| word.lstrip }.each do |child|
    output << child
  end
end

puts output