git-post-checkout-submodules

#!/usr/bin/env ruby

# A git hook that automatically updates your submodules for you when you change branches or pull
#
# It works like this:
# - If before you changed branches, your submodule's revision differed from what the master repo specified, your submodule is left alone
# - If a submodule has been removed from the branch you move to, it alerts you
# - If a submodule has been added on the branch you move to, it alerts you
# - Otherwise, it checks out the revision for you
#
# Installation:
#
# - Install this script somewhere.  Make sure it's executable
# - In your git repository, create .git/hooks/post-checkout and .git/hooks/post-merge. Make them executable
# - Put the full path of this script in there

module GitMethods
  def chdir_parent
    Dir.chdir('..') until File.directory?('.git') || Dir.pwd == '/'
  end

  def list_submodules(ref)
    `git ls-tree --full-tree -r #{ref} | egrep '^160000'`.split("\n").inject({}) do |h, line|
      info, path = line.split("\t")
      filemode, object_type, ref = info.split(" ")
      h[path] = ref
      h
    end
  end

  def submodule_current_rev(path)
    return nil unless File.directory?(path)
    ref = nil
    Dir.chdir(path) do
      ref = `git rev-parse HEAD`.chomp
    end
    ref
  end

  def output_submodule_header(path)
    puts "\nSubmodule: #{path}\n#{'-' * 60}"
  end
end
include GitMethods

chdir_parent
current_submodules = list_submodules('HEAD')
previous_submodules = list_submodules('HEAD@{1}')

(current_submodules.keys + previous_submodules.keys).uniq.sort.each do |path|
  rev = submodule_current_rev(path)
  case
  when rev.nil?
    output_submodule_header(path)
    # it should be initialized / unstashed
    puts "Submodule is new and needs to be initialized"
  when rev == current_submodules[path]
    # do nothing
  when rev != previous_submodules[path]
    output_submodule_header(path)
    puts rev
    # it was modified before... don't touch it
    puts "Not updating '#{path}' because it's revision pointer isn't the same as the previous HEAD specified"
  when current_submodules[path].nil?
    output_submodule_header(path)
    # it should be stashed
    puts "Does not exist in this revision (you may wish to stash it with git submodule-helper stash)."
  when rev != current_submodules[path]
    output_submodule_header(path)
    # it should be updated to the latest
    # Fetch if it the change doesn't exist
    Dir.chdir(path) do
      system("(git show '#{current_submodules[path]}' 2> /dev/null 1> /dev/null) || git fetch")
      system("git checkout '#{current_submodules[path]}'")
    end
  end
end