script to dump in a user's crontab to rotate the background image based off a random image in the ~/Pictures/ directory

rotate-background.sh

#!/bin/bash

CACHE_FILE=~/tmp/pic-list
PID=$(pgrep xfce4-session)
export DBUS_SESSION_BUS_ADDRESS=$(grep -z DBUS_SESSION_BUS_ADDRESS /proc/$PID/environ|cut -d= -f2-)

USE_CACHE=false
if [ -e $CACHE_FILE ]; then
    if [ $(date -r $CACHE_FILE +%Y/%m/%d) == $(date +%Y/%m/%d) ]; then
        USE_CACHE=true
    fi
fi

if [ "$USE_CACHE" = false ]; then
    find ~/Pictures/ -name '*' -exec file {} \; | grep -o -P '^.+: \w+ image' > $CACHE_FILE
fi

# < instead of cat for @tom :)
IMAGE=$(shuf < $CACHE_FILE | cut -d ':' -f 1 | head -n 1)
xfconf-query --channel xfce4-desktop --property /backdrop/screen0/monitor0/workspace0/last-image --set $IMAGE