This is a commit-msg hook for git that will validate your commit message to conform to the rules outlined in http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html It's a ruby adaptation of the hook outlined in http://addamhardy.com/blog/2013/06/05/good-commit-messages-and-enforcing-them-with-git-hooks/ (Thanks Addam Hardy!)

commit-msg

#!/bin/sh

exec < /dev/tty
./.git/hooks/validate_commit.rb $1

validate_commit.rb

#!/usr/bin/ruby
# Encoding: utf-8

editor = ENV['EDITOR'] != 'none' ? ENV['EDITOR'] : 'vim'
message_file = ARGV[0]

def check_format_rules(line_number, line)
  real_line_number = line_number + 1
  return "Error #{real_line_number}: First line should be less than 50 characters in length." if line_number == 0 && line.length > 50
  return "Error #{real_line_number}: Second line should be empty." if line_number == 1 && line.length > 0
  (return "Error #{real_line_number}: No line should be over 72 characters long." if line.length > 72) unless line[0,1]  == '#'
  false
end

while true
  commit_msg = []
  errors = []

  File.open(message_file, 'r').each_with_index do |line, line_number|
    commit_msg.push line
    e = check_format_rules line_number, line.strip
    errors.push e if e
  end

  unless errors.empty?
    File.open(message_file, 'w') do |file|
      file.puts "\n# GIT COMMIT MESSAGE FORMAT ERRORS:"
      errors.each { |error| file.puts "#    #{error}" }
      file.puts "\n"
      commit_msg.each { |line| file.puts line }
    end
    puts 'Invalid git commit message format.  Press y to edit and n to cancel the commit. [y/n]'
    choice = $stdin.gets.chomp
    exit 1 if %w(no n).include?(choice.downcase)
    next if `#{editor} #{message_file}`
  end
  break
end

Is there a way to hook the messages passed from -m commit argument too?

I have message validation working locally via commit-msg. Thank you btw for demonstrating via example. However adding the script(s) to .git_template with the proper executable permissions to make it 'global' doesn't seem to work for me. What I really want is to turn this into a server side hook. I've read different documentation and examples of doing this with no success on my end so far. Any working use cases or an example of how to turn this into a server side hook, would help. :)