prezto/plugins/capistrano/init.zsh

function _cap_does_task_list_need_generating() {
  if [[ ! -f .cap_tasks~ ]]; then
    return 0
  else
    accurate=$(stat -f%m .cap_tasks~)
    changed=$(stat -f%m config/deploy.rb)
    return $(expr $accurate '>=' $changed)
  fi
}

function _cap() {
  if [[ -f config/deploy.rb ]]; then
    if _cap_does_task_list_need_generating; then
      echo "\nGenerating .cap_tasks~..." > /dev/stderr
      cap show_tasks -q | cut -d " " -f 1 | sed -e '/^ *$/D' -e '1,2D'
      touch .cap_tasks~
    fi
    compadd $(cat .cap_tasks~)
  fi
}

compctl -K _cap cap