a script to change the default boot in grub 2. It parses the grub2 configuration to show you the options

gistfile1.rb

#!/usr/bin/ruby

GRUB_CFG = "/mnt/boot_loader/boot/grub/grub.cfg"

def read_menu_entries(cfg_file)
  menu_entries = []
  File.readlines(cfg_file).each do |line|
    next if !line.include?("menuentry")
    entry = line.match(/menuentry.*['"](.*)['"].*/)[1]  
    menu_entries << entry
  end
  menu_entries
end

def usage_and_exit(menu_entries)
  puts "Usage: set_default_boot N"
  puts " where N should be one of the numbers:"
  menu_entries.length.times do |i|
    puts "#{i}: #{menu_entries[i]}"
  end
  exit -1
end

if !File.exists?(GRUB_CFG)
  puts "I can't find configuration file for grub #{GRUB_CFG}"
  exit -1
end


option = ARGV[0].to_i
menu_entries = read_menu_entries(GRUB_CFG)
usage_and_exit(menu_entries) unless ARGV.length ==1
usage_and_exit(menu_entries) unless option < menu_entries.length
grub_cfg = File.read(GRUB_CFG)
grub_cfg.gsub!(/set default="\d"/, "set default=\"#{option}\"")
File.open(GRUB_CFG, "w") { |file| file.write(grub_cfg) }