语法：
# .---------------- minute (0 - 59)
# | .------------- hour (0 - 23)
# | | .---------- day of month (1 - 31)
# | | | .------- month (1 - 12) OR jan,feb,mar,apr ...
# | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * * user-name command to be executed
用法：
# 指定编辑器
crawler$ echo 'export EDITOR="vi"' >> ~/.bashrc
crawler$ source ~/.bashrc
# 列出所有定时任务
crawler$ crontab -l [-u USER_NAME]
# 添加定时任务(文件名以 'cron' 结尾)
crawler$ cat crawlercron
# 每 30 分钟执行一次
# 30 * * * * source /path/to/project/env/bin/active && python /path/to/project/manage.py runcron
crawler$ crontab crawlercron
# 清空定时任务
crawler$ crontab -r [-u USER_NAME]
例子：
# minute hour day month dayofweek command
# 每分钟
* * * * * service nginx restart
*/1 * * * * service nginx restart
# 每小时
* */1 * * * service nginx restart
# 每天 23:00-03:00 的每小时
* 23-3/1 * * * service nginx restart
# 每天 10:24
24 10 * * * service nginx restart
# 每周末 10:24
24 10 * * 0 service nginx restart
# 每月 1 号的 10:24
24 10 1 * * service nginx restart
# 每年 1 月 1 号的 10:24
24 10 1 1 * service nginx restart
