Pcmanfm set wallpaper fails on Raspbian stretch in cron - cron

Question

I am running a nearly fresh image of Raspbian Stretch 4.9 with a desktop and have a program which creates a new image for the computer background every few minutes.
I am trying to create a cron job to properly update the background using pcmanfm and, having followed the suggestions here, have created the following script called update.sh to set the background:
!#/bin/bash
export DISPLAY=:0
export XAUTHORITY=/home/pi/.Xauthority
pcmanfm -w '/home/pi/folder/image.png'
The script is executable and when it is run from the terminal it functions as intended. I have created a crontab to have this run automatically as such:
* * * * * /home/pi/folder/update.sh > /home/pi/folder/log.txt 2>&1
When the cron job triggers every minute, a pop-up window appears with an error saying "Desktop manager is not active." with a button "OK" to dismiss it, and the log file reads:
** Message: x-terminal-emulator has very limited support, consider choose another terminal
I have tried the command directly in the crontab
* * * * * DISPLAY=:0 && pcmanfm -w '/home/pi/folder/image.png' > /home/pi/folder/log.txt 2>&1
And the error is different this time
Cannot open display:
I am not entirely sure what sense to make of this, though from looking around it seems cron jobs can be finnicky. I am not sure if it is a Path or environment problem because I do not know many details about these things, but I don't think it should be a problem as I am using the full path to the image and the scripts. It shouldn't be a permissions error, because I have tried this on both a user crontab and a system crontab, and both fail. (Besides, the default pi user has root permissions by default anyways.) I am not sure what else to search for or try so I am asking for help if someone could point me in the right direction or has encountered this problem before.

Answer 1

I had exactly the same issue, except I am running Lubuntu 17.04. It appeared lately after a recent update, though cannot pinpoint when. After lot's of research I became suspicious that one of the XDG enviromental variables must be exported too. Following some trial and error, I found that exporting XDG_RUNTIME_DIR solved the problem for me. You may want to give it a try.
To figure out the value run: echo $XDG_RUNTIME_DIR
The working wallpaper changer running from cron for me now looks like:
#!/bin/bash
...
export DISPLAY=:0
export XAUTHORITY=/home/krisz/.Xauthority
export XDG_RUNTIME_DIR=/run/user/1000
pcmanfm --set-wallpaper=${dir}/${file} --wallpaper-mode=crop
...