Hmmmm .... let's see ... we have a 2.6.x and using a script that first came with 2.7.x. While one could type:
and see the options ... of which --courseshortname=STRING is an option displayed, it's still a script for a higher version of Moodle than what you are using on 2.6. Short answer (no pun intended): nope. I cannot tell you how to get that working with 2.6.x. But feel free to hack away! ;)
Did you try it? Bet 'STRING' needs to be enclosed in quotes as many folks just copy the title of the course into short name ... and don't think I'd like that cause that would mean they also have 'spaces' and other characters in name of the backup file that might actually be mis-interpreted by bash (like \) or I have to use quotes around those filenames when using those files with other file operations. In other words, I'd advise not using shortnames.
In the preferences for backups of a 2.6 (site admin), aren't there options for what to use in the backup filename? Not only the course id but also the short name in the backup file name? Check that.
We'll do two in one response ... to the best of my knowledge only moodledata/backup/temp/ is used when making backups. And if the destination option to the cli script is working and you see that /mnt/whereever filing up with .mbz files and no folders/directories remain in moodledata/backup/temp/ it's doing what it should.
Do you have automated backups turn on or off?
Are you running cron job often enough? (think the recommendation today is once every minute). The moodledata/trashdir/ might fill up for a while until cron runs to clean it up. Or, you need to break apart your courseidlist.txt file ... fewer course ID's listed ... fewer backups. That would give you time to go into moodledata/trashdir/ and manually remove all files/directories.
Cannot do Vulcan Mind Meld with your 2.6 from here ... but can Google ... and lookie what Google found for:
"linux finding largest files on a system". ;)
'spirit of sharing;, Ken