无法使用cron运行python脚本

Question

I am trying to run a simple python script with cron and I cannot for the life of me get it to work. I am running Ubuntu 18.04.

I know there are many questions (and answers) on this. I have looked at a number of them and I still cannot get it to work.

My python script is:

#!/usr/bin/env python

from tkinter import messagebox
messagebox.showinfo('MessageBox','Hello world!')

I have followed Unable to Run Python Script In Cron and changed the permissions as follows:

chmod +x /home/brb/bugs/projects/cron/hello.py

I can successfully run this from the command line as follows:

brb@bishop:~$ /home/brb/bugs/projects/cron/hello.py 

Inside crontab I have tried to execute the script in 2 different ways as follows (every 2 or 3 minutes to try and pin down which one works...)

*/2 * * * * /home/brb/bugs/projects/cron/hello.py
*/3 * * * * /usr/bin/env python /home/brb/bugs/projects/cron/hello.py

Nothing happens.

I followed http://www.jessicayung.com/automate-running-a-script-using-crontab/ and tried to inspect the error file as follows:

cd ../..
cd var/mail 
nano $brb

and I observe a message in nano that says

[ Directory '.' is not writable ]

I am not sure which directory '.' is, perhaps it is var/mail. In any case, not sure how to make it writable. I will google chmod some more to get this writable I guess, but that still doesn't solve why cron is not working when it looks like all the examples I've seen...

EDIT:

I followed How to run an X program from outside the X session (eg from the console or SSH) as per the comment from triplee and, not really knowing what I am doing, I attempted to change my display permissions as follows:

brb@bishop:~$ export XAUTHORITY=/home/brb/.Xauthority
brb@bishop:~$ export DISPLAY=':0'

My cron job is still not work...

Answer 1

Your problem is twofold:

• cronjob has does not know where it could be displaying a graphical app (which X server or DISPLAY to use.

  This can be addressed by making sure DISPLAY is set in the environment of you script. Run echo $DISPLAY in a terminal running in your X session. :0 is a very likely it.

• Even if it knows the correct DISPLAY , it does (should) not have access to it. If you are running the cronjob under the same user as the one running your target X session. If it is a different user, and it has access to your user's home, you could reuse it's authority file by setting XAUTHORITY to point to it.

  You can also export necessary information from your running session (renewed with each session), type this into a terminal there: xauth list $DISPLAY and add it for your cron running user: xauth add displayname protocolname hexkey (using those three values as returned by xauth list .