lib/capistrano/tasksにでも置いて使う

unicorn.cap

set :unicorn_config_path, -> { current_path.join('config', 'unicorn.rb') }
set :unicorn_pid_path,    -> { current_path.join('tmp', 'pids', 'unicorn.pid') }
set :unicorn_env,         -> { fetch(:rails_env) || fetch(:stage) }

SSHKit.config.command_map[:unicorn_stop]    = 'kill -QUIT'
SSHKit.config.command_map[:unicorn_restart] = 'kill -USR2'

namespace :deploy do

  def unicorn_running?
    test "[ -f #{fetch(:unicorn_pid_path)} ]"
  end

  def unicorn_pid
    capture "cat #{fetch(:unicorn_pid_path)}"
  end

  desc 'Start application'
  task :start do
    SSHKit.config.command_map.prefix[:unicorn_rails].tap do |prefix|
      rbenv_prefix = fetch(:rbenv_prefix, proc { "#{fetch(:rbenv_path)}/bin/rbenv exec" })
      prefix.unshift(rbenv_prefix)
      prefix.push('bundle exec')
    end

    on roles(:app), in: :parallel do
      within current_path do
        if unicorn_running?
          info 'unicorn is already running'
        else
          execute :unicorn_rails, '-c', fetch(:unicorn_config_path), '-E', fetch(:unicorn_env), '-D'
        end
      end
    end
  end

  desc 'Stop application'
  task :stop do
    on roles(:app), in: :parallel do
      if unicorn_running?
        execute :unicorn_stop, unicorn_pid
      else
        info 'unicorn is not running'
      end
    end
  end

  desc 'Restart application'
  task :restart do
    started = true

    on roles(:app), in: :parallel do
      if unicorn_running?
        execute :unicorn_restart, unicorn_pid
      else
        info 'unicorn is not running, so start unicorn'
        started = false
      end
    end

    invoke 'deploy:start' unless started
  end

end