Python Script not running in cron

Question

I am trying to run a Python script from cron. I am using crontab to run the command as a user instead of root. My Python script has the shebang at the top #! /usr/bin/env python and I did chmod +x it to make the script executable.

The script does work when running it from a shell but not when using crontab. I did check the /var/log/cron file and saw that the script runs but that absolutely nothing from stdout or stderr prints anywhere.

Finally, I made a small script that prints the date and a string and called that every minute that worked but this other script does not. I am not sure why I am getting these variable results...

Here is my entry in crontab

SHELL=/bin/bash



#min    #hour   day-of-month    month   day-of-week     command
#-------------------------------------------------------------------------
*/5     *       *                *        *             /path/to/script/script.py

Here is the source code of my script that will not run from crontab but will run from the shell it is in when called like so ./script.py. The script is executable after I use the chmod +x command on it, by the way...:

#! /usr/bin/env python

#create a file and write to it
import time

def create_and_write():
    with open("py-write-out.out", "a+") as w:
        w.write("writing to file. hello from python. :D\n")
        w.write("the time is: " + str(time.asctime()) + "\n")
        w.write("--------------------------------\n")


def main():
    create_and_write()

if __name__ == "__main__":
    main()

EDIT I figured out what was going wrong. I needed to put the absolute file paths in the script otherwise it would write to some directory I had not planned for.

Answer 1

Ok, I guess this thread is still going to help googlers.
I use a workaround to run python scripts with cron jobs. In fact, python scripts need to be handled with delicate care with cron job.
So I let a bash script take care of all of it.
I create a bash script which has the command to run the python script. Then I schedule a cron job to run bash script. You can even use a redirector to log the output of bash command executing the python script.
For example

@reboot /home/user/auto_delete.sh

auto_delete.sh may contain following lines:-

#!/bin/sh
echo $(date) >> bash_cron_log.txt
/usr/bin/python /home/user/auto_delete.py >> bash_cron_log.txt

So I don' need to worry about Cron Jobs crashing on python scripts.

Answer 2

You can resolve this yourself by following these steps:

1. Modify the cron as /path/to/script/script.py > /tmp/log 2> &1
2. Now, let the cron run
3. Now read the file /tmp/log
4. You will find out the reason of the issue you are facing, so that you can fix it.

In my experience, the issue is mostly with the environment.
In cron, the env variables are not set. So you may have to explicitly set the env for your script in cron.