Cron，用于在指定时间执行指令。
crontab -e 编辑
crontab -l 查看
crontab -r 删除
Field Meaning
1 Minute (0-59)
2 Hour (2-24)
3 Day of month (1-31)
4 Month (1-12, Jan, Feb, etc)
5 Day of week (0-6) 0 = Sunday, 1 = Monday etc or Sun, Mon, etc)
6 User that the command will run as
7 Command to execute
例：
6,26,46 * * * * cd ~/some/dir; ./bin/paster serve --daemon development.ini
每小时的6，26，46分钟启动服务器
