为什么Python脚本可以在CLI中运行,但是从cron作业调用时却不行？

这是从命令行运行此脚本的方式：

python  /home/username/public_html/IDM_app/manage.py cleanUpPosts

从CLI调用时,脚本运行正常.
但是,当我尝试通过cronjob运行脚本时,脚本无法正常运行.
日志文件显示正在请求脚本,但它们未显示脚本未正确执行的原因.

我在脚本中添加了Python日志记录.从CLI调用脚本时,日志记录正确进行.从cron调用时,python日志记录无法写入其日志文件.

我怀疑问题是cronjob在请求脚本时没有运行shell.

我似乎无法找到cron守护进程记录错误的任何地方.

我创建了/var/log/cron.log
然而,这似乎没有更新.

由于我认为问题源于ENV变量,我试图让cronjob为自己显示ENV.

这是我的crontab的样子：
$crontab -u username -e

m h  dom mon dow   command  
43 17 * * * /bin/sh python /home/username/public_html/IDM_app/manage.py cleanUpPosts  
43 17 * * * python /home/username/public_html/IDM_app/manage.py cleanUpPosts  
45 21 * * * echo "-----------------"; echo "import os; print os.environ" | python  
47 21 * * * /bin/sh echo "------with shell-------"; echo "import os; printos.environ" | python  
MAILTO=bryanw@nowhere.com

我以为它会输出到屏幕,但它没有. ENV变量在哪里输出？

无论如何,这里是日志文件：

# tail -n 5 /var/log/*.log   
==> /var/log/auth.log <==  
Jan 13 17:43:01 servername CRON[7901]: pam_unix(cron:session): session opened for user username by (uid=0)  
Jan 13 17:43:01 servername CRON[7902]: pam_unix(cron:session): session opened for user username by (uid=0)  
Jan 13 17:44:48 servername su[7909]: Successful su for root by username  
Jan 13 17:44:48 servername su[7909]: + /dev/pts/0 username:root  
Jan 13 17:44:48 servername su[7909]: pam_unix(su:session): session opened for user root by username(uid=1000)  

==> /var/log/bootstrap.log <==  

==> /var/log/cron.log <==  

==> /var/log/daemon.log <==  

==> /var/log/dpkg.log <==  

==> /var/log/kern.log <==  

==> /var/log/lpr.log <==  

==> /var/log/mail.log <==  

==> /var/log/mysql.log <==  

==> /var/log/pycentral.log <==  

==> /var/log/user.log <==

我还应该尝试什么,以便确定我的脚本运行不正常的原因？