为什么 Python 脚本可以从 CLI 运行，但从 cron 作业调用时却不能运行？

Question

I have created a Python script that I want to run daily via a cronjob on an Ubuntu server.

This is how this script would be run from the Command Line:

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

When called from the CLI, the script works fine.
However, when I attempt to run the script via cronjob, the script fails to run properly. The log files show that the script is being requested, but they are not showing why the script is not executing properly.

I added Python logging to the script. When the script is called from the CLI, the logging happens properly. When called from cron, python logging fails to write to it's log file.

I suspect the problem is the cronjob is not running the shell when it requests the script.

I can't seem to find anywhere that the cron daemon was logging it's errors.

I created /var/log/cron.log
Yet that doesn't seem to be updating.

Since I think the problem is stemming from the ENV variables, I tried to get the cronjob to display the ENV for itself.

Here's what my crontab looks like:
$ 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  

I thought it would output to the screen, but it didn't. Where would the ENV variables be outputting too?

Regardless, here are the log files:

# 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 <==  

What else should I try so I can determine why my scripts aren't running properly?

Answer 1

The crontab probably cannot find the python executable although it can on the CLI, so you need to write down the complete path to python. The same you get from

which python

Crontab supplies an environment for the scripts, which is not the same as the normal user environment.

Answer 2

3 17 * * * /bin/sh python /home/username/public_html/IDM_app/manage.py cleanUpPosts  

this line seems wrong. It seems you're running python as if it was a shell script.