Command line backup failing silently - how can I learn why it's not working?

Hello

My enviornment:

PHP 5.5.9-1ubuntu4.11,
MySQLServer version: 5.5.44
Moodle: Moodle 2.9.1+ (Build: 20150730)
Running on a virtualized linux server (linode)
This is the results of free -h (I suspect I am running into a memory issue, but I am quite unsure).

                   total       used       free     shared    buffers     cached
Mem:          1.9G       1.9G        55M        51M       135M       1.3G



I'm running the following command

php backup.php --courseid=8 --destination=/home/bill/moodleback/

..and it is failing silently (I see "== Performing backup... ==", and then an exit to the command prompt). There are no files in the /home/bill/moodleback directory. 

How can I learn why it's not working? Is there a logfile I can poke through, or a debugger I can look at while executing the command to understand why these backups are failing? I've searched through these forums, and googled, but to no avail.

I built an introduction to computer science course on my personal moodle environment, and now I want to restore it to my school moodle's instance. Thank you in advance for your help!!