Crontab not executing Python script on Ubuntu

Question

I have a Python script with the following code:

import subprocess
import sys
default = "Take a 20 second break - look at least 20 feet away!"
message = sys.argv[1] if len(sys.argv) > 1 else default
def sendmessage(message):
 subprocess.Popen(['notify-send', message])
 return
sendmessage(message)

called takebreak.py, which will send a system notification.

When I try to automate it using crontab like so:

* * * * * /usr/bin/python /home/polo/git-repositories/takebreak/takebreak.py

It doesn't work. Running the command

/usr/bin/python /home/polo/git-repositories/takebreak/takebreak.py

in the terminal does work, which means it's not a file location problem, but rather something to do with cron. Any ideas?

EDIT1:

After debugging and looking at the logs, I can verify that cron is actually executing the commmand export DISPLAY=:0; /usr/bin/python /home/polo/git-repositories/takebreak/takebreak.py every minute like I set it to do, but for some reason this command, while it should send a system notification, is not doing so. Any ideas?

EDIT2:

The solution was to add some address bus thing (forget the exact code) that I found in another post, that ended up fixing it. Unfortunately, none of the answers or comments here helped with solving the problem, but thanks regardless!

• Paulo Almeida: This may help: stackoverflow.com/questions/22743548/cronjob-not-running
• patrickstarpants: My crontab is running for sure, but for some reason the command in question will not work. typing just * * * * * export DISPLAY=:0; /usr/bin/notify-send blah does not display anything
• Paulo Almeida: unix.stackexchange.com/questions/111188/…
• patrickstarpants: Im having difficulty understand the solution in the code above. I'm supposed to add a DBUS_SESSION_BUS_ADDRESS to my main code? And what would the address be?
• Paulo Almeida: To begin with, my suggestion would be to run env|grep DBUS and hardcode it in the crontab, just like $PATH in Will's answer. That will tell you if it solves the problem. If it does, there are a couple of suggestions on how to do it, in that answer I linked to. It would be easier if it were a bash script, you'll have to figure out the best way to do it, either from Python or by dumping the environment variable somewhere and using it from the crontab.

Answer 1

Most likely, the problem is that notify-send is not in your $PATH when running from crontab. First, figure out where it's stored:

$ which notify-send
/usr/bin/notify-send

For me, it's in /usr/bin.

At the top of your crontab file (crontab -e), set $PATH:

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

If you want to include whatever $PATH may have already been set before (safer), do this instead:

PATH="${PATH}:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

Make sure this includes the directory where your command is installed, if it's not installed in /usr/bin.

Of course, the other option, is to simply specify the full command path in your Python script:

subprocess.Popen(['/usr/bin/notify-send', message])

• isedev: Wouldn't it be better to specify the absolute command path in the script? Setting the crontab PATH could have side-effects.
• Will: I'd say no--then the script is less portable as it depends on libnotify's tools being installed in the specified location. I added to my answer and put a safer way to modify $PATH, as well as changing the Python script to use the absolute path to the command.
• patrickstarpants: Both methods didn't work, cat /var/log/syslog | grep crontab returns (among edits and such) the following line: Mar 30 19:31:01 polo cron[634]: (polo) RELOAD (crontabs/polo)
• Will: Let's try writing our standard error output to a file and looking at it. Add >/tmp/cron-error.log 2>&1 to the end of your crontab line, then look at /tmp/cron-error.log and see what it says.
• patrickstarpants: I tried this before yet no log appeared after waiting several minutes (the crontab is set to run every minute right now)