parse_commented_lines.rb

Augeas.open do |aug|
  aug.match('/files/etc/default//#comment').each_with_index do |p, i|
    aug.set("/comments/raw/#{i}", "#{aug.get(p)}\n")
    aug.text_store('Shellvars.lns', "/comments/raw/#{i}", "/comments/tree/#{i}")
  end
end

uncomment_value.rb

def uncomment(variable)
  Augeas.open do |aug|
    matches = aug.match("/files/etc/default/grub/#comment[.=~regexp('.*#{variable}.*')]")
    unless matches.empty?
      aug.insert(matches.first, variable, false)
      aug.set('/comment/raw', "#{aug.get(matches.first)}\n")
      aug.text_store('Shellvars.lns', '/comment/raw', '/comment/tree')
      aug.mv("/comment/tree/#{variable}", "/files/etc/default/grub/#{variable}")
      aug.save!
    end
  end
end

uncomment 'GRUB_GFXMODE'

Augeas.open do |aug|
  value = aug.get("/files/etc/default/grub/GRUB_GFXMODE")
  puts "value: #{value}"
end