$ cat /etc/crontab

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

# For details see man 4 crontabs

# Example of job definition:
# .---------------- 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

半小时就是:

*/30 * * * *　xx.py

感谢 @Drazzi

具体用法网上比较多，可以参考：http://www.cnblogs.com/peida/archive/2013/01/08/2850483.html

给你个关键词: cron

crontab写一个计划任务

python么
给个非正式的
写个无限循环，然后循环体最后sleep30分钟