使用crontab运行时，matplotlib savefig不起作用

Question

I'm trying to run a python script with crontab that uses matplotlib to save a plot png. The script runs fine when run normally/not by cron.

My crontab file is:

SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
* * * * * /home/matthew/Programs/random_walk/random_walk.py >/dev/null 2>&1

Permissions seem to be set correctly and pythonpath seems right; I added a couple lines to random_walk.py that write a file with the sys.path output, and that worked when run by cron.

Several blog posts and mailing lists suggested the problem would be resolved by having

import matplotlib
matplotlib.use('Agg')

at the top of my python file (after the shebang), but that doesn't help.

My savefig code is

if len(sys.argv) > 1:
    save_path = sys.argv[1].rstrip('/')+'/random_walk.png'
else:
    save_path = 'random_walk.png'
plt.savefig(save_path)

How can I get cron and matplotlib to play nicely together?

Answer 1

Thanks to @tcaswell, I figured out that the problem was a paths issue.

My crontab file was not passing the image file location to random_walk.py, so the image was being saved in the same directory as my crontab file- not the same directory as random_walk.py where I was expecting it.

Solution 1

Pass the path to the script in crontab like I meant to.

SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
* * * * * /home/matthew/Programs/random_walk/random_walk.py /home/matthew/Programs/random_walk >/dev/null 2>&1

Solution 2

Alternatively, I could just make the python script always write its output file where I want it to go. In this case, I want the output file to be in the same directory as the random_walk.py, so I'm using the following code:

save_path = ''
if len(sys.argv) > 1:
    save_path = sys.argv[1].rstrip('/')+'/random_walk.png'
else:
    save_path = os.path.dirname(os.path.realpath(__file__))+'/random_walk.png'
plt.savefig(save_path)

Note

Just to be clear, having

import matplotlib
matplotlib.use('Agg')

was also necessary to get matplotlib to work with cron.