bc: command not found

[fergbrain]

Problem:
When running the script, it will exit with the following:

---------[ Destination Disk Use Information ]--------
./duplicity-backup.sh: line 727: bc: command not found
	Backblaze B2 type backend

Solution:
The likely issue is that bc ("An arbitrary precision calculator language") is not installed. You will need to install it: sudo apt-get install bc