使用whenever gem代替 crontab !!!

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 在一个项目中，会有很多情况要用到crontab，最常见的是：

1. 定时备份数据库

2. 定时删除无用的日志。

使用 crontab 的话，需要这样写： 

0 * * * * /bin/bash -l -c 'cd /home/sg552/workspace/babble_porta  .....
可读性很差。

而如果使用whenever 的话，就可以弄的非常优雅： Whenever is a Ruby gem that provides a clear syntax for writing and deploying cron jobs.

1. 安装(Installation ) : $ gem install whenever 或者： Or with Bundler in your Gemfile. 
gem 'whenever', :require => false
2. 建立对应的文件config/schedule.rb ： ( create config file) 
$ cd /apps/my-great-project
$ wheneverize .
This will create an initial config/schedule.rb file for you. 
# config/schedule.rb 
every 5.minutes do
  runner "UserWatchedRepo.update"
end

# in another file, we defined the 'runner'. 
class UserWatchedRepo
  def update
    # do something
  end
end
3. 然后，我们可以查看他生成的crontab: ( let's check what content it generated ) 内容仅供参考 
sg552@siwei-moto:~/workspace/babble_portal$ whenever
0 0 1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31 * * /bin/bash -l -c 'cd /home/sg552/workspace/babble_portal && RAILS_ENV=production bundle exec rake 
....................................
## [message] Above is your schedule file converted to cron syntax; your crontab file was not updated.
## [message] Run `whenever --help' for more options.

4. 最后，让这个crontab 生效： (at last, let's make it to take effect ) 

sg552@siwei-moto:~/workspace/babble_portal$ whenever --update-crontab
[write] crontab file updated

末尾，语法参考，几个区别：   ( differences between command, runner and rake)

command : 用的是最纯粹的shell command. 例如 :   'touch /tmp/some_file"    ( a shell command)

runner :  用的是Ruby的类，例如：     SomeClass.some_method   ( a ruby's method call)

rake :  用的是rake 命令。   ( a rake task )

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