A rake task for revving plist build version numbers...

Rakefile

def revBuild(plistFile)
  puts "Attempting to update #{plistFile} build version..."
  oldVersion = `/usr/libexec/PlistBuddy -c "Print CFBundleVersion" #{plistFile}`
  puts "The old version: #{oldVersion}"

  versionParts = oldVersion.split(".")
  previousDate = versionParts[2]
  newDate = Time.now.strftime("%Y%m%d")

  versionParts[2] = newDate
  versionParts[3] = previousDate == versionParts[2] ? (versionParts[3].to_i+1).to_s : "1"

  versionParts.each do |part|
    part.chomp!
  end

  newVersion = versionParts.join(".")
  
  `/usr/libexec/PlistBuddy -c "Set :CFBundleVersion #{newVersion}" #{plistFile}`

  puts "The new version: #{newVersion}"
end

desc "Rev the build numbers in a project's plist"
task :rev do
  revBuild './path/to/my.plist'  
end