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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.

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  • Please include the relevant code - that will help us to answer. Commented Sep 25, 2014 at 23:01
  • There are often other differences between the shell environment which may affect your script: different PYTHONPATH is one possibility. You may want to run a script under cron which just prints its environment, and compare it with your expectations based on manual execution from the shell. Commented Sep 25, 2014 at 23:42
  • Are you redirecting stdout and stderr? Where do you expect to see them appear? Commented Sep 26, 2014 at 4:59
  • Please post how you made the entry in the crontab. Ex: 5 5 * * * sript_to_run. So It'll help us to figure out, is the problem with cron entry or the code. Most of the time it will be a problem with how made entry in cron. Commented Sep 26, 2014 at 6:10
  • 2
    I see you are using /usr/bin/env python. This assumes that python is in $PATH. Where is python added to the path? If in your .profile, then you might find that this is not run. Safer to use /usr/bin/python or whatever your path to python is. Commented Sep 26, 2014 at 15:38

5 Answers 5

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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.

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1 Comment

Nice! This is working for me - I just had to make sure that ALL of my path names were explicit in the python script and bash command.
3

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.

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3 Comments

I tried this to the best of my ability and it did not work. I created the file in /tmp/log but nothing was written there. Also, I think using &>> is a better option to write both stderr and stdout to the file. ;)
Is your script printing something on the console? I can't believe when you say that the log file is empty. There are 2 cases: 1. Either there was some error reported by shell while running your script. Then error should have been displayed in the file. 2. Your script should have displayed something, if at all any.
I made a similar script and it only writes to a log file but does not do what I initially wanted it to. It is supposed to call another program to run but it does not do this. It does not even print any error messages or anything... 0_o
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I would like to emphasise one more thing. I was trying to run a python script in cron using the same trick (using shell script) and this time it didn't run. It was @reboot cron. So I used redirection in crontab as I mentioned in one of the above comments. And understood the problem. I was using lot many file handlers in python script and cron runs the script from user's home directory. In that case python could not find the files used for file handlers and will crash.

What I used for troubleshooting was that I created a crontab as below. It would run the start.sh and throw all the stderror or stdoutput to file status.txt and I got error in status.txt saying that the file I used for file handler was not found. That was right because python script was executed by cron from user's home directory and then the script starts searching for files in home directory only. 

@reboot /var/www/html/start.sh > /cronStatus/status.txt 2>&1

This will write everything happening during cron execution to status.txt file. You can see the error over there. I will again advice running python scripts using bash scripts for cronjobs.

SOLUTION:- Either you use full path for all the files being used in script (which wasn't feasible for me, since I don't want script to be location dependent). Or you execute script from correct directory So I created my cron as below-

@reboot cd /var/www/html/ && /var/www/html/start.sh

This cron will first change directory to correct location and then start the script. Now I don't have to worry about hardcoding full path for all files in script. Yeah, it may sound being lazy though ;)
And my start.sh looks like-

#!/bin/sh
/usr/bin/python /var/www/html/script.py

Hope it helps
Regards,
Kriss

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1

I had a similar issue but with a little bit different scenario: I had a bash script (run.sh) and a Python script (hello.py). When I typed 

sh run.sh

from the script directory, it worked; if I added the sh run.sh command in my crontab, it did not work.

To troubleshoot the issue I added the following line in my run.sh script:

printenv > env.txt

In this way you are able to see the environment variable used when you (or the crontab) run the script. By analyzing the differences between the env.txt generated from the manual run and the crontab run I noticed that the PWD variable was different. In my case I was able to resolve by adding the following line at the top of my .sh script:

cd /PYTHON_SCRIPT_ABSOLUTE_PATH/

Hope this could help you!

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0

Another reason may be that the way we judged executed or not executed is wrong.

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