git-exec-if-changed

#!/usr/bin/env ruby

require 'optparse'
require 'pathname'

paths = []
since = nil
since_last_exec = nil
verbose = false

parser = OptionParser.new do |opts|
  opts.banner = 'usage: git-exec-if-changed [<options>] [--] <cmd>...'

  opts.on('-p <path>', '--path=<path>', 'Path to test if changed (can be specified multiple times)') do |path|
    paths << path
  end

  opts.on('-s <commit>', '--since=<commit>', 'Find changes since <commit> (use empty string for the root commit)') do |commit|
    since = commit == '' ? :root : commit
  end

  opts.on('-S <history-file>', '--since-last-exec=<history-file>', 'Behave as --since=$(cat <history-file>) if it exists; write the current revision to <history-file> before exec') do |path|
    since_last_exec = Pathname(path)
  end

  opts.on('-v', '--verbose') do |v|
    verbose = v
  end
end

parser.parse!

if ARGV.empty?
  $stderr.puts parser.help
  exit 1
end

if paths.empty?
  $stderr.puts 'error: no paths specified'
  exit 1
end

if since.nil? && since_last_exec.nil?
  $stderr.puts 'error: neither --since nor --since-last-exec is specified'
  exit 1
end

if since.nil?
  if since_last_exec.file?
    since = since_last_exec.read.chomp
  else
    since = :root
  end
end

if since == :root || !system('git', 'diff', '--quiet', since, '--', *paths)
  $stderr.puts "exec-if-changed: changed since #{since == :root ? '(root)' : since}" if verbose

  since_last_exec.write(`git rev-parse HEAD`) if since_last_exec

  exec *ARGV
end

puts "exec-if-changed: not changed since #{since == :root ? '(root)' : since}" if verbose